Upgrade GDB from 7.4.1 to 7.6.1 on the vendor branch
[dragonfly.git] / contrib / gdb-7 / gdb / doc / stabs.texinfo
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1\input texinfo
2@setfilename stabs.info
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3@setchapternewpage odd
4@settitle STABS
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5
6@c @finalout
7
8@c This is a dir.info fragment to support semi-automated addition of
9@c manuals to an info tree.
10@dircategory Software development
11@direntry
12* Stabs: (stabs). The "stabs" debugging information format.
13@end direntry
14
15@copying
ef5ccd6c 16Copyright @copyright{} 1992-2013 Free Software Foundation, Inc.
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17Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon,
18and David MacKenzie.
19
20Permission is granted to copy, distribute and/or modify this document
cf7f2e2d 21under the terms of the GNU Free Documentation License, Version 1.3 or
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22any later version published by the Free Software Foundation; with no
23Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
24Texts. A copy of the license is included in the section entitled ``GNU
25Free Documentation License''.
26@end copying
27
28@ifnottex
29This document describes the stabs debugging symbol tables.
30
31@insertcopying
32@end ifnottex
33
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34@titlepage
35@title The ``stabs'' debug format
36@author Julia Menapace, Jim Kingdon, David MacKenzie
37@author Cygnus Support
38@page
39@tex
40\def\$#1${{#1}} % Kluge: collect RCS revision info without $...$
41\xdef\manvers{\$Revision: 2.130 $} % For use in headers, footers too
42{\parskip=0pt
43\hfill Cygnus Support\par
44\hfill \manvers\par
45\hfill \TeX{}info \texinfoversion\par
46}
47@end tex
48
49@vskip 0pt plus 1filll
50@insertcopying
51@end titlepage
52
53@ifnottex
54@node Top
55@top The "stabs" representation of debugging information
56
57This document describes the stabs debugging format.
58
59@menu
60* Overview:: Overview of stabs
61* Program Structure:: Encoding of the structure of the program
62* Constants:: Constants
63* Variables::
64* Types:: Type definitions
65* Macro define and undefine:: Representation of #define and #undef
66* Symbol Tables:: Symbol information in symbol tables
67* Cplusplus:: Stabs specific to C++
68* Stab Types:: Symbol types in a.out files
69* Symbol Descriptors:: Table of symbol descriptors
70* Type Descriptors:: Table of type descriptors
71* Expanded Reference:: Reference information by stab type
72* Questions:: Questions and anomalies
73* Stab Sections:: In some object file formats, stabs are
74 in sections.
5796c8dc 75* GNU Free Documentation License:: The license for this documentation
ef5ccd6c 76* Symbol Types Index:: Index of symbolic stab symbol type names.
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77@end menu
78@end ifnottex
79
80@contents
81
82@node Overview
83@chapter Overview of Stabs
84
85@dfn{Stabs} refers to a format for information that describes a program
86to a debugger. This format was apparently invented by
87Peter Kessler at
88the University of California at Berkeley, for the @code{pdx} Pascal
89debugger; the format has spread widely since then.
90
91This document is one of the few published sources of documentation on
92stabs. It is believed to be comprehensive for stabs used by C. The
93lists of symbol descriptors (@pxref{Symbol Descriptors}) and type
94descriptors (@pxref{Type Descriptors}) are believed to be completely
95comprehensive. Stabs for COBOL-specific features and for variant
96records (used by Pascal and Modula-2) are poorly documented here.
97
98@c FIXME: Need to document all OS9000 stuff in GDB; see all references
99@c to os9k_stabs in stabsread.c.
100
101Other sources of information on stabs are @cite{Dbx and Dbxtool
102Interfaces}, 2nd edition, by Sun, 1988, and @cite{AIX Version 3.2 Files
103Reference}, Fourth Edition, September 1992, "dbx Stabstring Grammar" in
104the a.out section, page 2-31. This document is believed to incorporate
105the information from those two sources except where it explicitly directs
106you to them for more information.
107
108@menu
109* Flow:: Overview of debugging information flow
110* Stabs Format:: Overview of stab format
111* String Field:: The string field
112* C Example:: A simple example in C source
113* Assembly Code:: The simple example at the assembly level
114@end menu
115
116@node Flow
117@section Overview of Debugging Information Flow
118
119The GNU C compiler compiles C source in a @file{.c} file into assembly
120language in a @file{.s} file, which the assembler translates into
121a @file{.o} file, which the linker combines with other @file{.o} files and
122libraries to produce an executable file.
123
124With the @samp{-g} option, GCC puts in the @file{.s} file additional
125debugging information, which is slightly transformed by the assembler
126and linker, and carried through into the final executable. This
127debugging information describes features of the source file like line
128numbers, the types and scopes of variables, and function names,
129parameters, and scopes.
130
131For some object file formats, the debugging information is encapsulated
132in assembler directives known collectively as @dfn{stab} (symbol table)
133directives, which are interspersed with the generated code. Stabs are
134the native format for debugging information in the a.out and XCOFF
135object file formats. The GNU tools can also emit stabs in the COFF and
136ECOFF object file formats.
137
138The assembler adds the information from stabs to the symbol information
139it places by default in the symbol table and the string table of the
140@file{.o} file it is building. The linker consolidates the @file{.o}
141files into one executable file, with one symbol table and one string
142table. Debuggers use the symbol and string tables in the executable as
143a source of debugging information about the program.
144
145@node Stabs Format
146@section Overview of Stab Format
147
148There are three overall formats for stab assembler directives,
149differentiated by the first word of the stab. The name of the directive
150describes which combination of four possible data fields follows. It is
151either @code{.stabs} (string), @code{.stabn} (number), or @code{.stabd}
152(dot). IBM's XCOFF assembler uses @code{.stabx} (and some other
153directives such as @code{.file} and @code{.bi}) instead of
154@code{.stabs}, @code{.stabn} or @code{.stabd}.
155
156The overall format of each class of stab is:
157
158@example
159.stabs "@var{string}",@var{type},@var{other},@var{desc},@var{value}
160.stabn @var{type},@var{other},@var{desc},@var{value}
161.stabd @var{type},@var{other},@var{desc}
162.stabx "@var{string}",@var{value},@var{type},@var{sdb-type}
163@end example
164
165@c what is the correct term for "current file location"? My AIX
166@c assembler manual calls it "the value of the current location counter".
167For @code{.stabn} and @code{.stabd}, there is no @var{string} (the
168@code{n_strx} field is zero; see @ref{Symbol Tables}). For
169@code{.stabd}, the @var{value} field is implicit and has the value of
170the current file location. For @code{.stabx}, the @var{sdb-type} field
171is unused for stabs and can always be set to zero. The @var{other}
172field is almost always unused and can be set to zero.
173
174The number in the @var{type} field gives some basic information about
175which type of stab this is (or whether it @emph{is} a stab, as opposed
176to an ordinary symbol). Each valid type number defines a different stab
177type; further, the stab type defines the exact interpretation of, and
178possible values for, any remaining @var{string}, @var{desc}, or
179@var{value} fields present in the stab. @xref{Stab Types}, for a list
180in numeric order of the valid @var{type} field values for stab directives.
181
182@node String Field
183@section The String Field
184
185For most stabs the string field holds the meat of the
186debugging information. The flexible nature of this field
187is what makes stabs extensible. For some stab types the string field
188contains only a name. For other stab types the contents can be a great
189deal more complex.
190
191The overall format of the string field for most stab types is:
192
193@example
194"@var{name}:@var{symbol-descriptor} @var{type-information}"
195@end example
196
197@var{name} is the name of the symbol represented by the stab; it can
198contain a pair of colons (@pxref{Nested Symbols}). @var{name} can be
199omitted, which means the stab represents an unnamed object. For
200example, @samp{:t10=*2} defines type 10 as a pointer to type 2, but does
201not give the type a name. Omitting the @var{name} field is supported by
202AIX dbx and GDB after about version 4.8, but not other debuggers. GCC
203sometimes uses a single space as the name instead of omitting the name
204altogether; apparently that is supported by most debuggers.
205
206The @var{symbol-descriptor} following the @samp{:} is an alphabetic
207character that tells more specifically what kind of symbol the stab
208represents. If the @var{symbol-descriptor} is omitted, but type
209information follows, then the stab represents a local variable. For a
210list of symbol descriptors, see @ref{Symbol Descriptors}. The @samp{c}
211symbol descriptor is an exception in that it is not followed by type
212information. @xref{Constants}.
213
214@var{type-information} is either a @var{type-number}, or
215@samp{@var{type-number}=}. A @var{type-number} alone is a type
216reference, referring directly to a type that has already been defined.
217
218The @samp{@var{type-number}=} form is a type definition, where the
219number represents a new type which is about to be defined. The type
220definition may refer to other types by number, and those type numbers
221may be followed by @samp{=} and nested definitions. Also, the Lucid
222compiler will repeat @samp{@var{type-number}=} more than once if it
223wants to define several type numbers at once.
224
225In a type definition, if the character that follows the equals sign is
226non-numeric then it is a @var{type-descriptor}, and tells what kind of
227type is about to be defined. Any other values following the
228@var{type-descriptor} vary, depending on the @var{type-descriptor}.
229@xref{Type Descriptors}, for a list of @var{type-descriptor} values. If
230a number follows the @samp{=} then the number is a @var{type-reference}.
231For a full description of types, @ref{Types}.
232
233A @var{type-number} is often a single number. The GNU and Sun tools
234additionally permit a @var{type-number} to be a pair
235(@var{file-number},@var{filetype-number}) (the parentheses appear in the
236string, and serve to distinguish the two cases). The @var{file-number}
237is 0 for the base source file, 1 for the first included file, 2 for the
238next, and so on. The @var{filetype-number} is a number starting with
2391 which is incremented for each new type defined in the file.
240(Separating the file number and the type number permits the
241@code{N_BINCL} optimization to succeed more often; see @ref{Include
242Files}).
243
244There is an AIX extension for type attributes. Following the @samp{=}
245are any number of type attributes. Each one starts with @samp{@@} and
246ends with @samp{;}. Debuggers, including AIX's dbx and GDB 4.10, skip
247any type attributes they do not recognize. GDB 4.9 and other versions
248of dbx may not do this. Because of a conflict with C@t{++}
249(@pxref{Cplusplus}), new attributes should not be defined which begin
250with a digit, @samp{(}, or @samp{-}; GDB may be unable to distinguish
251those from the C@t{++} type descriptor @samp{@@}. The attributes are:
252
253@table @code
254@item a@var{boundary}
255@var{boundary} is an integer specifying the alignment. I assume it
256applies to all variables of this type.
257
258@item p@var{integer}
259Pointer class (for checking). Not sure what this means, or how
260@var{integer} is interpreted.
261
262@item P
263Indicate this is a packed type, meaning that structure fields or array
264elements are placed more closely in memory, to save memory at the
265expense of speed.
266
267@item s@var{size}
268Size in bits of a variable of this type. This is fully supported by GDB
2694.11 and later.
270
271@item S
272Indicate that this type is a string instead of an array of characters,
273or a bitstring instead of a set. It doesn't change the layout of the
274data being represented, but does enable the debugger to know which type
275it is.
276
277@item V
278Indicate that this type is a vector instead of an array. The only
279major difference between vectors and arrays is that vectors are
280passed by value instead of by reference (vector coprocessor extension).
281
282@end table
283
284All of this can make the string field quite long. All versions of GDB,
285and some versions of dbx, can handle arbitrarily long strings. But many
286versions of dbx (or assemblers or linkers, I'm not sure which)
287cretinously limit the strings to about 80 characters, so compilers which
288must work with such systems need to split the @code{.stabs} directive
289into several @code{.stabs} directives. Each stab duplicates every field
290except the string field. The string field of every stab except the last
291is marked as continued with a backslash at the end (in the assembly code
292this may be written as a double backslash, depending on the assembler).
293Removing the backslashes and concatenating the string fields of each
294stab produces the original, long string. Just to be incompatible (or so
295they don't have to worry about what the assembler does with
296backslashes), AIX can use @samp{?} instead of backslash.
297
298@node C Example
299@section A Simple Example in C Source
300
301To get the flavor of how stabs describe source information for a C
302program, let's look at the simple program:
303
304@example
305main()
306@{
307 printf("Hello world");
308@}
309@end example
310
311When compiled with @samp{-g}, the program above yields the following
312@file{.s} file. Line numbers have been added to make it easier to refer
313to parts of the @file{.s} file in the description of the stabs that
314follows.
315
316@node Assembly Code
317@section The Simple Example at the Assembly Level
318
319This simple ``hello world'' example demonstrates several of the stab
320types used to describe C language source files.
321
322@example
3231 gcc2_compiled.:
3242 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
3253 .stabs "hello.c",100,0,0,Ltext0
3264 .text
3275 Ltext0:
3286 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
3297 .stabs "char:t2=r2;0;127;",128,0,0,0
3308 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
3319 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
33210 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
33311 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
33412 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
33513 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
33614 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
33715 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
33816 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
33917 .stabs "float:t12=r1;4;0;",128,0,0,0
34018 .stabs "double:t13=r1;8;0;",128,0,0,0
34119 .stabs "long double:t14=r1;8;0;",128,0,0,0
34220 .stabs "void:t15=15",128,0,0,0
34321 .align 4
34422 LC0:
34523 .ascii "Hello, world!\12\0"
34624 .align 4
34725 .global _main
34826 .proc 1
34927 _main:
35028 .stabn 68,0,4,LM1
35129 LM1:
35230 !#PROLOGUE# 0
35331 save %sp,-136,%sp
35432 !#PROLOGUE# 1
35533 call ___main,0
35634 nop
35735 .stabn 68,0,5,LM2
35836 LM2:
35937 LBB2:
36038 sethi %hi(LC0),%o1
36139 or %o1,%lo(LC0),%o0
36240 call _printf,0
36341 nop
36442 .stabn 68,0,6,LM3
36543 LM3:
36644 LBE2:
36745 .stabn 68,0,6,LM4
36846 LM4:
36947 L1:
37048 ret
37149 restore
37250 .stabs "main:F1",36,0,0,_main
37351 .stabn 192,0,0,LBB2
37452 .stabn 224,0,0,LBE2
375@end example
376
377@node Program Structure
378@chapter Encoding the Structure of the Program
379
380The elements of the program structure that stabs encode include the name
381of the main function, the names of the source and include files, the
382line numbers, procedure names and types, and the beginnings and ends of
383blocks of code.
384
385@menu
386* Main Program:: Indicate what the main program is
387* Source Files:: The path and name of the source file
388* Include Files:: Names of include files
389* Line Numbers::
390* Procedures::
391* Nested Procedures::
392* Block Structure::
393* Alternate Entry Points:: Entering procedures except at the beginning.
394@end menu
395
396@node Main Program
397@section Main Program
398
399@findex N_MAIN
400Most languages allow the main program to have any name. The
401@code{N_MAIN} stab type tells the debugger the name that is used in this
402program. Only the string field is significant; it is the name of
403a function which is the main program. Most C compilers do not use this
404stab (they expect the debugger to assume that the name is @code{main}),
405but some C compilers emit an @code{N_MAIN} stab for the @code{main}
406function. I'm not sure how XCOFF handles this.
407
408@node Source Files
409@section Paths and Names of the Source Files
410
411@findex N_SO
412Before any other stabs occur, there must be a stab specifying the source
413file. This information is contained in a symbol of stab type
414@code{N_SO}; the string field contains the name of the file. The
415value of the symbol is the start address of the portion of the
416text section corresponding to that file.
417
418Some compilers use the desc field to indicate the language of the
419source file. Sun's compilers started this usage, and the first
420constants are derived from their documentation. Languages added
421by gcc/gdb start at 0x32 to avoid conflict with languages Sun may
422add in the future. A desc field with a value 0 indicates that no
423language has been specified via this mechanism.
424
425@table @asis
426@item @code{N_SO_AS} (0x1)
427Assembly language
428@item @code{N_SO_C} (0x2)
429K&R traditional C
430@item @code{N_SO_ANSI_C} (0x3)
431ANSI C
432@item @code{N_SO_CC} (0x4)
433C++
434@item @code{N_SO_FORTRAN} (0x5)
435Fortran
436@item @code{N_SO_PASCAL} (0x6)
437Pascal
438@item @code{N_SO_FORTRAN90} (0x7)
439Fortran90
440@item @code{N_SO_OBJC} (0x32)
441Objective-C
442@item @code{N_SO_OBJCPLUS} (0x33)
443Objective-C++
444@end table
445
446Some compilers (for example, GCC2 and SunOS4 @file{/bin/cc}) also
447include the directory in which the source was compiled, in a second
448@code{N_SO} symbol preceding the one containing the file name. This
449symbol can be distinguished by the fact that it ends in a slash. Code
450from the @code{cfront} C@t{++} compiler can have additional @code{N_SO} symbols for
451nonexistent source files after the @code{N_SO} for the real source file;
452these are believed to contain no useful information.
453
454For example:
455
456@example
457.stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 # @r{100 is N_SO}
458.stabs "hello.c",100,0,0,Ltext0
459 .text
460Ltext0:
461@end example
462
463@findex C_FILE
464Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
465directive which assembles to a @code{C_FILE} symbol; explaining this in
466detail is outside the scope of this document.
467
468@c FIXME: Exactly when should the empty N_SO be used? Why?
469If it is useful to indicate the end of a source file, this is done with
470an @code{N_SO} symbol with an empty string for the name. The value is
471the address of the end of the text section for the file. For some
472systems, there is no indication of the end of a source file, and you
473just need to figure it ended when you see an @code{N_SO} for a different
474source file, or a symbol ending in @code{.o} (which at least some
475linkers insert to mark the start of a new @code{.o} file).
476
477@node Include Files
478@section Names of Include Files
479
480There are several schemes for dealing with include files: the
481traditional @code{N_SOL} approach, Sun's @code{N_BINCL} approach, and the
482XCOFF @code{C_BINCL} approach (which despite the similar name has little in
483common with @code{N_BINCL}).
484
485@findex N_SOL
486An @code{N_SOL} symbol specifies which include file subsequent symbols
487refer to. The string field is the name of the file and the value is the
488text address corresponding to the end of the previous include file and
489the start of this one. To specify the main source file again, use an
490@code{N_SOL} symbol with the name of the main source file.
491
492@findex N_BINCL
493@findex N_EINCL
494@findex N_EXCL
495The @code{N_BINCL} approach works as follows. An @code{N_BINCL} symbol
496specifies the start of an include file. In an object file, only the
497string is significant; the linker puts data into some of the other
498fields. The end of the include file is marked by an @code{N_EINCL}
499symbol (which has no string field). In an object file, there is no
500significant data in the @code{N_EINCL} symbol. @code{N_BINCL} and
501@code{N_EINCL} can be nested.
502
503If the linker detects that two source files have identical stabs between
504an @code{N_BINCL} and @code{N_EINCL} pair (as will generally be the case
505for a header file), then it only puts out the stabs once. Each
506additional occurrence is replaced by an @code{N_EXCL} symbol. I believe
507the GNU linker and the Sun (both SunOS4 and Solaris) linker are the only
508ones which supports this feature.
509
510A linker which supports this feature will set the value of a
511@code{N_BINCL} symbol to the total of all the characters in the stabs
512strings included in the header file, omitting any file numbers. The
513value of an @code{N_EXCL} symbol is the same as the value of the
514@code{N_BINCL} symbol it replaces. This information can be used to
515match up @code{N_EXCL} and @code{N_BINCL} symbols which have the same
516filename. The @code{N_EINCL} value, and the values of the other and
517description fields for all three, appear to always be zero.
518
519@findex C_BINCL
520@findex C_EINCL
521For the start of an include file in XCOFF, use the @file{.bi} assembler
522directive, which generates a @code{C_BINCL} symbol. A @file{.ei}
523directive, which generates a @code{C_EINCL} symbol, denotes the end of
524the include file. Both directives are followed by the name of the
525source file in quotes, which becomes the string for the symbol.
526The value of each symbol, produced automatically by the assembler
527and linker, is the offset into the executable of the beginning
528(inclusive, as you'd expect) or end (inclusive, as you would not expect)
529of the portion of the COFF line table that corresponds to this include
530file. @code{C_BINCL} and @code{C_EINCL} do not nest.
531
532@node Line Numbers
533@section Line Numbers
534
535@findex N_SLINE
536An @code{N_SLINE} symbol represents the start of a source line. The
537desc field contains the line number and the value contains the code
538address for the start of that source line. On most machines the address
539is absolute; for stabs in sections (@pxref{Stab Sections}), it is
540relative to the function in which the @code{N_SLINE} symbol occurs.
541
542@findex N_DSLINE
543@findex N_BSLINE
544GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
545numbers in the data or bss segments, respectively. They are identical
546to @code{N_SLINE} but are relocated differently by the linker. They
547were intended to be used to describe the source location of a variable
548declaration, but I believe that GCC2 actually puts the line number in
549the desc field of the stab for the variable itself. GDB has been
550ignoring these symbols (unless they contain a string field) since
551at least GDB 3.5.
552
553For single source lines that generate discontiguous code, such as flow
554of control statements, there may be more than one line number entry for
555the same source line. In this case there is a line number entry at the
556start of each code range, each with the same line number.
557
558XCOFF does not use stabs for line numbers. Instead, it uses COFF line
559numbers (which are outside the scope of this document). Standard COFF
560line numbers cannot deal with include files, but in XCOFF this is fixed
561with the @code{C_BINCL} method of marking include files (@pxref{Include
562Files}).
563
564@node Procedures
565@section Procedures
566
567@findex N_FUN, for functions
568@findex N_FNAME
569@findex N_STSYM, for functions (Sun acc)
570@findex N_GSYM, for functions (Sun acc)
571All of the following stabs normally use the @code{N_FUN} symbol type.
572However, Sun's @code{acc} compiler on SunOS4 uses @code{N_GSYM} and
573@code{N_STSYM}, which means that the value of the stab for the function
574is useless and the debugger must get the address of the function from
575the non-stab symbols instead. On systems where non-stab symbols have
576leading underscores, the stabs will lack underscores and the debugger
577needs to know about the leading underscore to match up the stab and the
578non-stab symbol. BSD Fortran is said to use @code{N_FNAME} with the
579same restriction; the value of the symbol is not useful (I'm not sure it
580really does use this, because GDB doesn't handle this and no one has
581complained).
582
583@findex C_FUN
584A function is represented by an @samp{F} symbol descriptor for a global
585(extern) function, and @samp{f} for a static (local) function. For
586a.out, the value of the symbol is the address of the start of the
587function; it is already relocated. For stabs in ELF, the SunPRO
588compiler version 2.0.1 and GCC put out an address which gets relocated
589by the linker. In a future release SunPRO is planning to put out zero,
590in which case the address can be found from the ELF (non-stab) symbol.
591Because looking things up in the ELF symbols would probably be slow, I'm
592not sure how to find which symbol of that name is the right one, and
593this doesn't provide any way to deal with nested functions, it would
594probably be better to make the value of the stab an address relative to
595the start of the file, or just absolute. See @ref{ELF Linker
596Relocation} for more information on linker relocation of stabs in ELF
597files. For XCOFF, the stab uses the @code{C_FUN} storage class and the
598value of the stab is meaningless; the address of the function can be
599found from the csect symbol (XTY_LD/XMC_PR).
600
601The type information of the stab represents the return type of the
602function; thus @samp{foo:f5} means that foo is a function returning type
6035. There is no need to try to get the line number of the start of the
604function from the stab for the function; it is in the next
605@code{N_SLINE} symbol.
606
607@c FIXME: verify whether the "I suspect" below is true or not.
608Some compilers (such as Sun's Solaris compiler) support an extension for
609specifying the types of the arguments. I suspect this extension is not
610used for old (non-prototyped) function definitions in C. If the
611extension is in use, the type information of the stab for the function
612is followed by type information for each argument, with each argument
613preceded by @samp{;}. An argument type of 0 means that additional
614arguments are being passed, whose types and number may vary (@samp{...}
615in ANSI C). GDB has tolerated this extension (parsed the syntax, if not
616necessarily used the information) since at least version 4.8; I don't
617know whether all versions of dbx tolerate it. The argument types given
618here are not redundant with the symbols for the formal parameters
619(@pxref{Parameters}); they are the types of the arguments as they are
620passed, before any conversions might take place. For example, if a C
621function which is declared without a prototype takes a @code{float}
622argument, the value is passed as a @code{double} but then converted to a
623@code{float}. Debuggers need to use the types given in the arguments
624when printing values, but when calling the function they need to use the
625types given in the symbol defining the function.
626
627If the return type and types of arguments of a function which is defined
628in another source file are specified (i.e., a function prototype in ANSI
629C), traditionally compilers emit no stab; the only way for the debugger
630to find the information is if the source file where the function is
631defined was also compiled with debugging symbols. As an extension the
632Solaris compiler uses symbol descriptor @samp{P} followed by the return
633type of the function, followed by the arguments, each preceded by
634@samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
635This use of symbol descriptor @samp{P} can be distinguished from its use
636for register parameters (@pxref{Register Parameters}) by the fact that it has
637symbol type @code{N_FUN}.
638
639The AIX documentation also defines symbol descriptor @samp{J} as an
640internal function. I assume this means a function nested within another
641function. It also says symbol descriptor @samp{m} is a module in
642Modula-2 or extended Pascal.
643
644Procedures (functions which do not return values) are represented as
645functions returning the @code{void} type in C. I don't see why this couldn't
646be used for all languages (inventing a @code{void} type for this purpose if
647necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
648@samp{Q} for internal, global, and static procedures, respectively.
649These symbol descriptors are unusual in that they are not followed by
650type information.
651
652The following example shows a stab for a function @code{main} which
653returns type number @code{1}. The @code{_main} specified for the value
654is a reference to an assembler label which is used to fill in the start
655address of the function.
656
657@example
658.stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
659@end example
660
661The stab representing a procedure is located immediately following the
662code of the procedure. This stab is in turn directly followed by a
663group of other stabs describing elements of the procedure. These other
664stabs describe the procedure's parameters, its block local variables, and
665its block structure.
666
667If functions can appear in different sections, then the debugger may not
668be able to find the end of a function. Recent versions of GCC will mark
669the end of a function with an @code{N_FUN} symbol with an empty string
670for the name. The value is the address of the end of the current
671function. Without such a symbol, there is no indication of the address
672of the end of a function, and you must assume that it ended at the
673starting address of the next function or at the end of the text section
674for the program.
675
676@node Nested Procedures
677@section Nested Procedures
678
679For any of the symbol descriptors representing procedures, after the
680symbol descriptor and the type information is optionally a scope
681specifier. This consists of a comma, the name of the procedure, another
682comma, and the name of the enclosing procedure. The first name is local
683to the scope specified, and seems to be redundant with the name of the
684symbol (before the @samp{:}). This feature is used by GCC, and
685presumably Pascal, Modula-2, etc., compilers, for nested functions.
686
687If procedures are nested more than one level deep, only the immediately
688containing scope is specified. For example, this code:
689
690@example
691int
692foo (int x)
693@{
694 int bar (int y)
695 @{
696 int baz (int z)
697 @{
698 return x + y + z;
699 @}
700 return baz (x + 2 * y);
701 @}
702 return x + bar (3 * x);
703@}
704@end example
705
706@noindent
707produces the stabs:
708
709@example
710.stabs "baz:f1,baz,bar",36,0,0,_baz.15 # @r{36 is N_FUN}
711.stabs "bar:f1,bar,foo",36,0,0,_bar.12
712.stabs "foo:F1",36,0,0,_foo
713@end example
714
715@node Block Structure
716@section Block Structure
717
718@findex N_LBRAC
719@findex N_RBRAC
720@c For GCC 2.5.8 or so stabs-in-coff, these are absolute instead of
721@c function relative (as documented below). But GDB has never been able
722@c to deal with that (it had wanted them to be relative to the file, but
723@c I just fixed that (between GDB 4.12 and 4.13)), so it is function
724@c relative just like ELF and SOM and the below documentation.
725The program's block structure is represented by the @code{N_LBRAC} (left
726brace) and the @code{N_RBRAC} (right brace) stab types. The variables
727defined inside a block precede the @code{N_LBRAC} symbol for most
728compilers, including GCC. Other compilers, such as the Convex, Acorn
729RISC machine, and Sun @code{acc} compilers, put the variables after the
730@code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
731@code{N_RBRAC} symbols are the start and end addresses of the code of
732the block, respectively. For most machines, they are relative to the
733starting address of this source file. For the Gould NP1, they are
734absolute. For stabs in sections (@pxref{Stab Sections}), they are
735relative to the function in which they occur.
736
737The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
738scope of a procedure are located after the @code{N_FUN} stab that
739represents the procedure itself.
740
741Sun documents the desc field of @code{N_LBRAC} and
742@code{N_RBRAC} symbols as containing the nesting level of the block.
743However, dbx seems to not care, and GCC always sets desc to
744zero.
745
746@findex .bb
747@findex .be
748@findex C_BLOCK
749For XCOFF, block scope is indicated with @code{C_BLOCK} symbols. If the
750name of the symbol is @samp{.bb}, then it is the beginning of the block;
751if the name of the symbol is @samp{.be}; it is the end of the block.
752
753@node Alternate Entry Points
754@section Alternate Entry Points
755
756@findex N_ENTRY
757@findex C_ENTRY
758Some languages, like Fortran, have the ability to enter procedures at
759some place other than the beginning. One can declare an alternate entry
760point. The @code{N_ENTRY} stab is for this; however, the Sun FORTRAN
761compiler doesn't use it. According to AIX documentation, only the name
762of a @code{C_ENTRY} stab is significant; the address of the alternate
763entry point comes from the corresponding external symbol. A previous
764revision of this document said that the value of an @code{N_ENTRY} stab
765was the address of the alternate entry point, but I don't know the
766source for that information.
767
768@node Constants
769@chapter Constants
770
771The @samp{c} symbol descriptor indicates that this stab represents a
772constant. This symbol descriptor is an exception to the general rule
773that symbol descriptors are followed by type information. Instead, it
774is followed by @samp{=} and one of the following:
775
776@table @code
777@item b @var{value}
778Boolean constant. @var{value} is a numeric value; I assume it is 0 for
779false or 1 for true.
780
781@item c @var{value}
782Character constant. @var{value} is the numeric value of the constant.
783
784@item e @var{type-information} , @var{value}
785Constant whose value can be represented as integral.
786@var{type-information} is the type of the constant, as it would appear
787after a symbol descriptor (@pxref{String Field}). @var{value} is the
788numeric value of the constant. GDB 4.9 does not actually get the right
789value if @var{value} does not fit in a host @code{int}, but it does not
790do anything violent, and future debuggers could be extended to accept
791integers of any size (whether unsigned or not). This constant type is
792usually documented as being only for enumeration constants, but GDB has
793never imposed that restriction; I don't know about other debuggers.
794
795@item i @var{value}
796Integer constant. @var{value} is the numeric value. The type is some
797sort of generic integer type (for GDB, a host @code{int}); to specify
798the type explicitly, use @samp{e} instead.
799
800@item r @var{value}
801Real constant. @var{value} is the real value, which can be @samp{INF}
802(optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
803NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
804normal number the format is that accepted by the C library function
805@code{atof}.
806
807@item s @var{string}
808String constant. @var{string} is a string enclosed in either @samp{'}
809(in which case @samp{'} characters within the string are represented as
810@samp{\'} or @samp{"} (in which case @samp{"} characters within the
811string are represented as @samp{\"}).
812
813@item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
814Set constant. @var{type-information} is the type of the constant, as it
815would appear after a symbol descriptor (@pxref{String Field}).
816@var{elements} is the number of elements in the set (does this means
817how many bits of @var{pattern} are actually used, which would be
818redundant with the type, or perhaps the number of bits set in
819@var{pattern}? I don't get it), @var{bits} is the number of bits in the
820constant (meaning it specifies the length of @var{pattern}, I think),
821and @var{pattern} is a hexadecimal representation of the set. AIX
822documentation refers to a limit of 32 bytes, but I see no reason why
823this limit should exist. This form could probably be used for arbitrary
824constants, not just sets; the only catch is that @var{pattern} should be
825understood to be target, not host, byte order and format.
826@end table
827
828The boolean, character, string, and set constants are not supported by
829GDB 4.9, but it ignores them. GDB 4.8 and earlier gave an error
830message and refused to read symbols from the file containing the
831constants.
832
833The above information is followed by @samp{;}.
834
835@node Variables
836@chapter Variables
837
838Different types of stabs describe the various ways that variables can be
839allocated: on the stack, globally, in registers, in common blocks,
840statically, or as arguments to a function.
841
842@menu
843* Stack Variables:: Variables allocated on the stack.
844* Global Variables:: Variables used by more than one source file.
845* Register Variables:: Variables in registers.
846* Common Blocks:: Variables statically allocated together.
847* Statics:: Variables local to one source file.
848* Based Variables:: Fortran pointer based variables.
849* Parameters:: Variables for arguments to functions.
850@end menu
851
852@node Stack Variables
853@section Automatic Variables Allocated on the Stack
854
855If a variable's scope is local to a function and its lifetime is only as
856long as that function executes (C calls such variables
857@dfn{automatic}), it can be allocated in a register (@pxref{Register
858Variables}) or on the stack.
859
860@findex N_LSYM, for stack variables
861@findex C_LSYM
862Each variable allocated on the stack has a stab with the symbol
863descriptor omitted. Since type information should begin with a digit,
864@samp{-}, or @samp{(}, only those characters precluded from being used
865for symbol descriptors. However, the Acorn RISC machine (ARM) is said
866to get this wrong: it puts out a mere type definition here, without the
867preceding @samp{@var{type-number}=}. This is a bad idea; there is no
868guarantee that type descriptors are distinct from symbol descriptors.
869Stabs for stack variables use the @code{N_LSYM} stab type, or
870@code{C_LSYM} for XCOFF.
871
872The value of the stab is the offset of the variable within the
873local variables. On most machines this is an offset from the frame
874pointer and is negative. The location of the stab specifies which block
875it is defined in; see @ref{Block Structure}.
876
877For example, the following C code:
878
879@example
880int
881main ()
882@{
883 int x;
884@}
885@end example
886
887produces the following stabs:
888
889@example
890.stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
891.stabs "x:1",128,0,0,-12 # @r{128 is N_LSYM}
892.stabn 192,0,0,LBB2 # @r{192 is N_LBRAC}
893.stabn 224,0,0,LBE2 # @r{224 is N_RBRAC}
894@end example
895
896See @ref{Procedures} for more information on the @code{N_FUN} stab, and
897@ref{Block Structure} for more information on the @code{N_LBRAC} and
898@code{N_RBRAC} stabs.
899
900@node Global Variables
901@section Global Variables
902
903@findex N_GSYM
904@findex C_GSYM
905@c FIXME: verify for sure that it really is C_GSYM on XCOFF
906A variable whose scope is not specific to just one source file is
907represented by the @samp{G} symbol descriptor. These stabs use the
908@code{N_GSYM} stab type (C_GSYM for XCOFF). The type information for
909the stab (@pxref{String Field}) gives the type of the variable.
910
911For example, the following source code:
912
913@example
914char g_foo = 'c';
915@end example
916
917@noindent
918yields the following assembly code:
919
920@example
921.stabs "g_foo:G2",32,0,0,0 # @r{32 is N_GSYM}
922 .global _g_foo
923 .data
924_g_foo:
925 .byte 99
926@end example
927
928The address of the variable represented by the @code{N_GSYM} is not
929contained in the @code{N_GSYM} stab. The debugger gets this information
930from the external symbol for the global variable. In the example above,
931the @code{.global _g_foo} and @code{_g_foo:} lines tell the assembler to
932produce an external symbol.
933
934Some compilers, like GCC, output @code{N_GSYM} stabs only once, where
935the variable is defined. Other compilers, like SunOS4 /bin/cc, output a
936@code{N_GSYM} stab for each compilation unit which references the
937variable.
938
939@node Register Variables
940@section Register Variables
941
942@findex N_RSYM
943@findex C_RSYM
944@c According to an old version of this manual, AIX uses C_RPSYM instead
945@c of C_RSYM. I am skeptical; this should be verified.
946Register variables have their own stab type, @code{N_RSYM}
947(@code{C_RSYM} for XCOFF), and their own symbol descriptor, @samp{r}.
948The stab's value is the number of the register where the variable data
949will be stored.
950@c .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
951
952AIX defines a separate symbol descriptor @samp{d} for floating point
953registers. This seems unnecessary; why not just just give floating
954point registers different register numbers? I have not verified whether
955the compiler actually uses @samp{d}.
956
957If the register is explicitly allocated to a global variable, but not
958initialized, as in:
959
960@example
961register int g_bar asm ("%g5");
962@end example
963
964@noindent
965then the stab may be emitted at the end of the object file, with
966the other bss symbols.
967
968@node Common Blocks
969@section Common Blocks
970
971A common block is a statically allocated section of memory which can be
972referred to by several source files. It may contain several variables.
973I believe Fortran is the only language with this feature.
974
975@findex N_BCOMM
976@findex N_ECOMM
977@findex C_BCOMM
978@findex C_ECOMM
979A @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
980ends it. The only field that is significant in these two stabs is the
981string, which names a normal (non-debugging) symbol that gives the
982address of the common block. According to IBM documentation, only the
983@code{N_BCOMM} has the name of the common block (even though their
984compiler actually puts it both places).
985
986@findex N_ECOML
987@findex C_ECOML
988The stabs for the members of the common block are between the
989@code{N_BCOMM} and the @code{N_ECOMM}; the value of each stab is the
990offset within the common block of that variable. IBM uses the
991@code{C_ECOML} stab type, and there is a corresponding @code{N_ECOML}
992stab type, but Sun's Fortran compiler uses @code{N_GSYM} instead. The
993variables within a common block use the @samp{V} symbol descriptor (I
994believe this is true of all Fortran variables). Other stabs (at least
995type declarations using @code{C_DECL}) can also be between the
996@code{N_BCOMM} and the @code{N_ECOMM}.
997
998@node Statics
999@section Static Variables
1000
1001Initialized static variables are represented by the @samp{S} and
1002@samp{V} symbol descriptors. @samp{S} means file scope static, and
1003@samp{V} means procedure scope static. One exception: in XCOFF, IBM's
1004xlc compiler always uses @samp{V}, and whether it is file scope or not
1005is distinguished by whether the stab is located within a function.
1006
1007@c This is probably not worth mentioning; it is only true on the sparc
1008@c for `double' variables which although declared const are actually in
1009@c the data segment (the text segment can't guarantee 8 byte alignment).
1010@c (although GCC
1011@c 2.4.5 has a bug in that it uses @code{N_FUN}, so neither dbx nor GDB can
1012@c find the variables)
1013@findex N_STSYM
1014@findex N_LCSYM
1015@findex N_FUN, for variables
1016@findex N_ROSYM
1017In a.out files, @code{N_STSYM} means the data section, @code{N_FUN}
1018means the text section, and @code{N_LCSYM} means the bss section. For
1019those systems with a read-only data section separate from the text
1020section (Solaris), @code{N_ROSYM} means the read-only data section.
1021
1022For example, the source lines:
1023
1024@example
1025static const int var_const = 5;
1026static int var_init = 2;
1027static int var_noinit;
1028@end example
1029
1030@noindent
1031yield the following stabs:
1032
1033@example
1034.stabs "var_const:S1",36,0,0,_var_const # @r{36 is N_FUN}
1035@dots{}
1036.stabs "var_init:S1",38,0,0,_var_init # @r{38 is N_STSYM}
1037@dots{}
1038.stabs "var_noinit:S1",40,0,0,_var_noinit # @r{40 is N_LCSYM}
1039@end example
1040
1041@findex C_STSYM
1042@findex C_BSTAT
1043@findex C_ESTAT
1044In XCOFF files, the stab type need not indicate the section;
1045@code{C_STSYM} can be used for all statics. Also, each static variable
1046is enclosed in a static block. A @code{C_BSTAT} (emitted with a
1047@samp{.bs} assembler directive) symbol begins the static block; its
1048value is the symbol number of the csect symbol whose value is the
1049address of the static block, its section is the section of the variables
1050in that static block, and its name is @samp{.bs}. A @code{C_ESTAT}
1051(emitted with a @samp{.es} assembler directive) symbol ends the static
1052block; its name is @samp{.es} and its value and section are ignored.
1053
1054In ECOFF files, the storage class is used to specify the section, so the
1055stab type need not indicate the section.
1056
1057In ELF files, for the SunPRO compiler version 2.0.1, symbol descriptor
1058@samp{S} means that the address is absolute (the linker relocates it)
1059and symbol descriptor @samp{V} means that the address is relative to the
1060start of the relevant section for that compilation unit. SunPRO has
1061plans to have the linker stop relocating stabs; I suspect that their the
1062debugger gets the address from the corresponding ELF (not stab) symbol.
1063I'm not sure how to find which symbol of that name is the right one.
1064The clean way to do all this would be to have the value of a symbol
1065descriptor @samp{S} symbol be an offset relative to the start of the
1066file, just like everything else, but that introduces obvious
1067compatibility problems. For more information on linker stab relocation,
1068@xref{ELF Linker Relocation}.
1069
1070@node Based Variables
1071@section Fortran Based Variables
1072
1073Fortran (at least, the Sun and SGI dialects of FORTRAN-77) has a feature
1074which allows allocating arrays with @code{malloc}, but which avoids
1075blurring the line between arrays and pointers the way that C does. In
1076stabs such a variable uses the @samp{b} symbol descriptor.
1077
1078For example, the Fortran declarations
1079
1080@example
1081real foo, foo10(10), foo10_5(10,5)
1082pointer (foop, foo)
1083pointer (foo10p, foo10)
1084pointer (foo105p, foo10_5)
1085@end example
1086
1087produce the stabs
1088
1089@example
1090foo:b6
1091foo10:bar3;1;10;6
1092foo10_5:bar3;1;5;ar3;1;10;6
1093@end example
1094
1095In this example, @code{real} is type 6 and type 3 is an integral type
1096which is the type of the subscripts of the array (probably
1097@code{integer}).
1098
1099The @samp{b} symbol descriptor is like @samp{V} in that it denotes a
1100statically allocated symbol whose scope is local to a function; see
1101@xref{Statics}. The value of the symbol, instead of being the address
1102of the variable itself, is the address of a pointer to that variable.
1103So in the above example, the value of the @code{foo} stab is the address
1104of a pointer to a real, the value of the @code{foo10} stab is the
1105address of a pointer to a 10-element array of reals, and the value of
1106the @code{foo10_5} stab is the address of a pointer to a 5-element array
1107of 10-element arrays of reals.
1108
1109@node Parameters
1110@section Parameters
1111
1112Formal parameters to a function are represented by a stab (or sometimes
1113two; see below) for each parameter. The stabs are in the order in which
1114the debugger should print the parameters (i.e., the order in which the
1115parameters are declared in the source file). The exact form of the stab
1116depends on how the parameter is being passed.
1117
1118@findex N_PSYM
1119@findex C_PSYM
1120Parameters passed on the stack use the symbol descriptor @samp{p} and
1121the @code{N_PSYM} symbol type (or @code{C_PSYM} for XCOFF). The value
1122of the symbol is an offset used to locate the parameter on the stack;
1123its exact meaning is machine-dependent, but on most machines it is an
1124offset from the frame pointer.
1125
1126As a simple example, the code:
1127
1128@example
1129main (argc, argv)
1130 int argc;
1131 char **argv;
1132@end example
1133
1134produces the stabs:
1135
1136@example
1137.stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
1138.stabs "argc:p1",160,0,0,68 # @r{160 is N_PSYM}
1139.stabs "argv:p20=*21=*2",160,0,0,72
1140@end example
1141
1142The type definition of @code{argv} is interesting because it contains
1143several type definitions. Type 21 is pointer to type 2 (char) and
1144@code{argv} (type 20) is pointer to type 21.
1145
1146@c FIXME: figure out what these mean and describe them coherently.
1147The following symbol descriptors are also said to go with @code{N_PSYM}.
1148The value of the symbol is said to be an offset from the argument
1149pointer (I'm not sure whether this is true or not).
1150
1151@example
1152pP (<<??>>)
1153pF Fortran function parameter
1154X (function result variable)
1155@end example
1156
1157@menu
1158* Register Parameters::
1159* Local Variable Parameters::
1160* Reference Parameters::
1161* Conformant Arrays::
1162@end menu
1163
1164@node Register Parameters
1165@subsection Passing Parameters in Registers
1166
1167If the parameter is passed in a register, then traditionally there are
1168two symbols for each argument:
1169
1170@example
1171.stabs "arg:p1" . . . ; N_PSYM
1172.stabs "arg:r1" . . . ; N_RSYM
1173@end example
1174
1175Debuggers use the second one to find the value, and the first one to
1176know that it is an argument.
1177
1178@findex C_RPSYM
1179@findex N_RSYM, for parameters
1180Because that approach is kind of ugly, some compilers use symbol
1181descriptor @samp{P} or @samp{R} to indicate an argument which is in a
1182register. Symbol type @code{C_RPSYM} is used in XCOFF and @code{N_RSYM}
1183is used otherwise. The symbol's value is the register number. @samp{P}
1184and @samp{R} mean the same thing; the difference is that @samp{P} is a
1185GNU invention and @samp{R} is an IBM (XCOFF) invention. As of version
11864.9, GDB should handle either one.
1187
1188There is at least one case where GCC uses a @samp{p} and @samp{r} pair
1189rather than @samp{P}; this is where the argument is passed in the
1190argument list and then loaded into a register.
1191
1192According to the AIX documentation, symbol descriptor @samp{D} is for a
1193parameter passed in a floating point register. This seems
1194unnecessary---why not just use @samp{R} with a register number which
1195indicates that it's a floating point register? I haven't verified
1196whether the system actually does what the documentation indicates.
1197
1198@c FIXME: On the hppa this is for any type > 8 bytes, I think, and not
1199@c for small structures (investigate).
1200On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1201or union, the register contains the address of the structure. On the
1202sparc, this is also true of a @samp{p} and @samp{r} pair (using Sun
1203@code{cc}) or a @samp{p} symbol. However, if a (small) structure is
1204really in a register, @samp{r} is used. And, to top it all off, on the
1205hppa it might be a structure which was passed on the stack and loaded
1206into a register and for which there is a @samp{p} and @samp{r} pair! I
1207believe that symbol descriptor @samp{i} is supposed to deal with this
1208case (it is said to mean "value parameter by reference, indirect
1209access"; I don't know the source for this information), but I don't know
1210details or what compilers or debuggers use it, if any (not GDB or GCC).
1211It is not clear to me whether this case needs to be dealt with
1212differently than parameters passed by reference (@pxref{Reference Parameters}).
1213
1214@node Local Variable Parameters
1215@subsection Storing Parameters as Local Variables
1216
1217There is a case similar to an argument in a register, which is an
1218argument that is actually stored as a local variable. Sometimes this
1219happens when the argument was passed in a register and then the compiler
1220stores it as a local variable. If possible, the compiler should claim
1221that it's in a register, but this isn't always done.
1222
1223If a parameter is passed as one type and converted to a smaller type by
1224the prologue (for example, the parameter is declared as a @code{float},
1225but the calling conventions specify that it is passed as a
1226@code{double}), then GCC2 (sometimes) uses a pair of symbols. The first
1227symbol uses symbol descriptor @samp{p} and the type which is passed.
1228The second symbol has the type and location which the parameter actually
1229has after the prologue. For example, suppose the following C code
1230appears with no prototypes involved:
1231
1232@example
1233void
1234subr (f)
1235 float f;
1236@{
1237@end example
1238
1239if @code{f} is passed as a double at stack offset 8, and the prologue
1240converts it to a float in register number 0, then the stabs look like:
1241
1242@example
1243.stabs "f:p13",160,0,3,8 # @r{160 is @code{N_PSYM}, here 13 is @code{double}}
1244.stabs "f:r12",64,0,3,0 # @r{64 is @code{N_RSYM}, here 12 is @code{float}}
1245@end example
1246
1247In both stabs 3 is the line number where @code{f} is declared
1248(@pxref{Line Numbers}).
1249
1250@findex N_LSYM, for parameter
1251GCC, at least on the 960, has another solution to the same problem. It
1252uses a single @samp{p} symbol descriptor for an argument which is stored
1253as a local variable but uses @code{N_LSYM} instead of @code{N_PSYM}. In
1254this case, the value of the symbol is an offset relative to the local
1255variables for that function, not relative to the arguments; on some
1256machines those are the same thing, but not on all.
1257
1258@c This is mostly just background info; the part that logically belongs
1259@c here is the last sentence.
1260On the VAX or on other machines in which the calling convention includes
1261the number of words of arguments actually passed, the debugger (GDB at
1262least) uses the parameter symbols to keep track of whether it needs to
1263print nameless arguments in addition to the formal parameters which it
1264has printed because each one has a stab. For example, in
1265
1266@example
1267extern int fprintf (FILE *stream, char *format, @dots{});
1268@dots{}
1269fprintf (stdout, "%d\n", x);
1270@end example
1271
1272there are stabs for @code{stream} and @code{format}. On most machines,
1273the debugger can only print those two arguments (because it has no way
1274of knowing that additional arguments were passed), but on the VAX or
1275other machines with a calling convention which indicates the number of
1276words of arguments, the debugger can print all three arguments. To do
1277so, the parameter symbol (symbol descriptor @samp{p}) (not necessarily
1278@samp{r} or symbol descriptor omitted symbols) needs to contain the
1279actual type as passed (for example, @code{double} not @code{float} if it
1280is passed as a double and converted to a float).
1281
1282@node Reference Parameters
1283@subsection Passing Parameters by Reference
1284
1285If the parameter is passed by reference (e.g., Pascal @code{VAR}
1286parameters), then the symbol descriptor is @samp{v} if it is in the
1287argument list, or @samp{a} if it in a register. Other than the fact
1288that these contain the address of the parameter rather than the
1289parameter itself, they are identical to @samp{p} and @samp{R},
1290respectively. I believe @samp{a} is an AIX invention; @samp{v} is
1291supported by all stabs-using systems as far as I know.
1292
1293@node Conformant Arrays
1294@subsection Passing Conformant Array Parameters
1295
1296@c Is this paragraph correct? It is based on piecing together patchy
1297@c information and some guesswork
1298Conformant arrays are a feature of Modula-2, and perhaps other
1299languages, in which the size of an array parameter is not known to the
1300called function until run-time. Such parameters have two stabs: a
1301@samp{x} for the array itself, and a @samp{C}, which represents the size
1302of the array. The value of the @samp{x} stab is the offset in the
1303argument list where the address of the array is stored (it this right?
1304it is a guess); the value of the @samp{C} stab is the offset in the
1305argument list where the size of the array (in elements? in bytes?) is
1306stored.
1307
1308@node Types
1309@chapter Defining Types
1310
1311The examples so far have described types as references to previously
1312defined types, or defined in terms of subranges of or pointers to
1313previously defined types. This chapter describes the other type
1314descriptors that may follow the @samp{=} in a type definition.
1315
1316@menu
1317* Builtin Types:: Integers, floating point, void, etc.
1318* Miscellaneous Types:: Pointers, sets, files, etc.
1319* Cross-References:: Referring to a type not yet defined.
1320* Subranges:: A type with a specific range.
1321* Arrays:: An aggregate type of same-typed elements.
1322* Strings:: Like an array but also has a length.
1323* Enumerations:: Like an integer but the values have names.
1324* Structures:: An aggregate type of different-typed elements.
1325* Typedefs:: Giving a type a name.
1326* Unions:: Different types sharing storage.
1327* Function Types::
1328@end menu
1329
1330@node Builtin Types
1331@section Builtin Types
1332
1333Certain types are built in (@code{int}, @code{short}, @code{void},
1334@code{float}, etc.); the debugger recognizes these types and knows how
1335to handle them. Thus, don't be surprised if some of the following ways
1336of specifying builtin types do not specify everything that a debugger
1337would need to know about the type---in some cases they merely specify
1338enough information to distinguish the type from other types.
1339
1340The traditional way to define builtin types is convoluted, so new ways
1341have been invented to describe them. Sun's @code{acc} uses special
1342builtin type descriptors (@samp{b} and @samp{R}), and IBM uses negative
1343type numbers. GDB accepts all three ways, as of version 4.8; dbx just
1344accepts the traditional builtin types and perhaps one of the other two
1345formats. The following sections describe each of these formats.
1346
1347@menu
1348* Traditional Builtin Types:: Put on your seat belts and prepare for kludgery
1349* Builtin Type Descriptors:: Builtin types with special type descriptors
1350* Negative Type Numbers:: Builtin types using negative type numbers
1351@end menu
1352
1353@node Traditional Builtin Types
1354@subsection Traditional Builtin Types
1355
1356This is the traditional, convoluted method for defining builtin types.
1357There are several classes of such type definitions: integer, floating
1358point, and @code{void}.
1359
1360@menu
1361* Traditional Integer Types::
1362* Traditional Other Types::
1363@end menu
1364
1365@node Traditional Integer Types
1366@subsubsection Traditional Integer Types
1367
1368Often types are defined as subranges of themselves. If the bounding values
1369fit within an @code{int}, then they are given normally. For example:
1370
1371@example
1372.stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 # @r{128 is N_LSYM}
1373.stabs "char:t2=r2;0;127;",128,0,0,0
1374@end example
1375
1376Builtin types can also be described as subranges of @code{int}:
1377
1378@example
1379.stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1380@end example
1381
1382If the lower bound of a subrange is 0 and the upper bound is -1,
1383the type is an unsigned integral type whose bounds are too
1384big to describe in an @code{int}. Traditionally this is only used for
1385@code{unsigned int} and @code{unsigned long}:
1386
1387@example
1388.stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1389@end example
1390
1391For larger types, GCC 2.4.5 puts out bounds in octal, with one or more
1392leading zeroes. In this case a negative bound consists of a number
1393which is a 1 bit (for the sign bit) followed by a 0 bit for each bit in
1394the number (except the sign bit), and a positive bound is one which is a
13951 bit for each bit in the number (except possibly the sign bit). All
1396known versions of dbx and GDB version 4 accept this (at least in the
1397sense of not refusing to process the file), but GDB 3.5 refuses to read
1398the whole file containing such symbols. So GCC 2.3.3 did not output the
1399proper size for these types. As an example of octal bounds, the string
1400fields of the stabs for 64 bit integer types look like:
1401
1402@c .stabs directives, etc., omitted to make it fit on the page.
1403@example
1404long int:t3=r1;001000000000000000000000;000777777777777777777777;
1405long unsigned int:t5=r1;000000000000000000000000;001777777777777777777777;
1406@end example
1407
1408If the lower bound of a subrange is 0 and the upper bound is negative,
1409the type is an unsigned integral type whose size in bytes is the
1410absolute value of the upper bound. I believe this is a Convex
1411convention for @code{unsigned long long}.
1412
1413If the lower bound of a subrange is negative and the upper bound is 0,
1414the type is a signed integral type whose size in bytes is
1415the absolute value of the lower bound. I believe this is a Convex
1416convention for @code{long long}. To distinguish this from a legitimate
1417subrange, the type should be a subrange of itself. I'm not sure whether
1418this is the case for Convex.
1419
1420@node Traditional Other Types
1421@subsubsection Traditional Other Types
1422
1423If the upper bound of a subrange is 0 and the lower bound is positive,
1424the type is a floating point type, and the lower bound of the subrange
1425indicates the number of bytes in the type:
1426
1427@example
1428.stabs "float:t12=r1;4;0;",128,0,0,0
1429.stabs "double:t13=r1;8;0;",128,0,0,0
1430@end example
1431
1432However, GCC writes @code{long double} the same way it writes
1433@code{double}, so there is no way to distinguish.
1434
1435@example
1436.stabs "long double:t14=r1;8;0;",128,0,0,0
1437@end example
1438
1439Complex types are defined the same way as floating-point types; there is
1440no way to distinguish a single-precision complex from a double-precision
1441floating-point type.
1442
1443The C @code{void} type is defined as itself:
1444
1445@example
1446.stabs "void:t15=15",128,0,0,0
1447@end example
1448
1449I'm not sure how a boolean type is represented.
1450
1451@node Builtin Type Descriptors
1452@subsection Defining Builtin Types Using Builtin Type Descriptors
1453
1454This is the method used by Sun's @code{acc} for defining builtin types.
1455These are the type descriptors to define builtin types:
1456
1457@table @code
1458@c FIXME: clean up description of width and offset, once we figure out
1459@c what they mean
1460@item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1461Define an integral type. @var{signed} is @samp{u} for unsigned or
1462@samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1463is a character type, or is omitted. I assume this is to distinguish an
1464integral type from a character type of the same size, for example it
1465might make sense to set it for the C type @code{wchar_t} so the debugger
1466can print such variables differently (Solaris does not do this). Sun
1467sets it on the C types @code{signed char} and @code{unsigned char} which
1468arguably is wrong. @var{width} and @var{offset} appear to be for small
1469objects stored in larger ones, for example a @code{short} in an
1470@code{int} register. @var{width} is normally the number of bytes in the
1471type. @var{offset} seems to always be zero. @var{nbits} is the number
1472of bits in the type.
1473
1474Note that type descriptor @samp{b} used for builtin types conflicts with
1475its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1476be distinguished because the character following the type descriptor
1477will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1478@samp{u} or @samp{s} for a builtin type.
1479
1480@item w
1481Documented by AIX to define a wide character type, but their compiler
1482actually uses negative type numbers (@pxref{Negative Type Numbers}).
1483
1484@item R @var{fp-type} ; @var{bytes} ;
1485Define a floating point type. @var{fp-type} has one of the following values:
1486
1487@table @code
1488@item 1 (NF_SINGLE)
1489IEEE 32-bit (single precision) floating point format.
1490
1491@item 2 (NF_DOUBLE)
1492IEEE 64-bit (double precision) floating point format.
1493
1494@item 3 (NF_COMPLEX)
1495@item 4 (NF_COMPLEX16)
1496@item 5 (NF_COMPLEX32)
1497@c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1498@c to put that here got an overfull hbox.
1499These are for complex numbers. A comment in the GDB source describes
1500them as Fortran @code{complex}, @code{double complex}, and
1501@code{complex*16}, respectively, but what does that mean? (i.e., Single
1502precision? Double precision?).
1503
1504@item 6 (NF_LDOUBLE)
1505Long double. This should probably only be used for Sun format
1506@code{long double}, and new codes should be used for other floating
1507point formats (@code{NF_DOUBLE} can be used if a @code{long double} is
1508really just an IEEE double, of course).
1509@end table
1510
1511@var{bytes} is the number of bytes occupied by the type. This allows a
1512debugger to perform some operations with the type even if it doesn't
1513understand @var{fp-type}.
1514
1515@item g @var{type-information} ; @var{nbits}
1516Documented by AIX to define a floating type, but their compiler actually
1517uses negative type numbers (@pxref{Negative Type Numbers}).
1518
1519@item c @var{type-information} ; @var{nbits}
1520Documented by AIX to define a complex type, but their compiler actually
1521uses negative type numbers (@pxref{Negative Type Numbers}).
1522@end table
1523
1524The C @code{void} type is defined as a signed integral type 0 bits long:
1525@example
1526.stabs "void:t19=bs0;0;0",128,0,0,0
1527@end example
1528The Solaris compiler seems to omit the trailing semicolon in this case.
1529Getting sloppy in this way is not a swift move because if a type is
1530embedded in a more complex expression it is necessary to be able to tell
1531where it ends.
1532
1533I'm not sure how a boolean type is represented.
1534
1535@node Negative Type Numbers
1536@subsection Negative Type Numbers
1537
1538This is the method used in XCOFF for defining builtin types.
1539Since the debugger knows about the builtin types anyway, the idea of
1540negative type numbers is simply to give a special type number which
1541indicates the builtin type. There is no stab defining these types.
1542
1543There are several subtle issues with negative type numbers.
1544
1545One is the size of the type. A builtin type (for example the C types
1546@code{int} or @code{long}) might have different sizes depending on
1547compiler options, the target architecture, the ABI, etc. This issue
1548doesn't come up for IBM tools since (so far) they just target the
1549RS/6000; the sizes indicated below for each size are what the IBM
1550RS/6000 tools use. To deal with differing sizes, either define separate
1551negative type numbers for each size (which works but requires changing
1552the debugger, and, unless you get both AIX dbx and GDB to accept the
1553change, introduces an incompatibility), or use a type attribute
1554(@pxref{String Field}) to define a new type with the appropriate size
1555(which merely requires a debugger which understands type attributes,
1556like AIX dbx or GDB). For example,
1557
1558@example
1559.stabs "boolean:t10=@@s8;-16",128,0,0,0
1560@end example
1561
1562defines an 8-bit boolean type, and
1563
1564@example
1565.stabs "boolean:t10=@@s64;-16",128,0,0,0
1566@end example
1567
1568defines a 64-bit boolean type.
1569
1570A similar issue is the format of the type. This comes up most often for
1571floating-point types, which could have various formats (particularly
1572extended doubles, which vary quite a bit even among IEEE systems).
1573Again, it is best to define a new negative type number for each
1574different format; changing the format based on the target system has
1575various problems. One such problem is that the Alpha has both VAX and
1576IEEE floating types. One can easily imagine one library using the VAX
1577types and another library in the same executable using the IEEE types.
1578Another example is that the interpretation of whether a boolean is true
1579or false can be based on the least significant bit, most significant
1580bit, whether it is zero, etc., and different compilers (or different
1581options to the same compiler) might provide different kinds of boolean.
1582
1583The last major issue is the names of the types. The name of a given
1584type depends @emph{only} on the negative type number given; these do not
1585vary depending on the language, the target system, or anything else.
1586One can always define separate type numbers---in the following list you
1587will see for example separate @code{int} and @code{integer*4} types
1588which are identical except for the name. But compatibility can be
1589maintained by not inventing new negative type numbers and instead just
1590defining a new type with a new name. For example:
1591
1592@example
1593.stabs "CARDINAL:t10=-8",128,0,0,0
1594@end example
1595
1596Here is the list of negative type numbers. The phrase @dfn{integral
1597type} is used to mean twos-complement (I strongly suspect that all
1598machines which use stabs use twos-complement; most machines use
1599twos-complement these days).
1600
1601@table @code
1602@item -1
1603@code{int}, 32 bit signed integral type.
1604
1605@item -2
1606@code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1607treat this as signed. GCC uses this type whether @code{char} is signed
1608or not, which seems like a bad idea. The AIX compiler (@code{xlc}) seems to
1609avoid this type; it uses -5 instead for @code{char}.
1610
1611@item -3
1612@code{short}, 16 bit signed integral type.
1613
1614@item -4
1615@code{long}, 32 bit signed integral type.
1616
1617@item -5
1618@code{unsigned char}, 8 bit unsigned integral type.
1619
1620@item -6
1621@code{signed char}, 8 bit signed integral type.
1622
1623@item -7
1624@code{unsigned short}, 16 bit unsigned integral type.
1625
1626@item -8
1627@code{unsigned int}, 32 bit unsigned integral type.
1628
1629@item -9
1630@code{unsigned}, 32 bit unsigned integral type.
1631
1632@item -10
1633@code{unsigned long}, 32 bit unsigned integral type.
1634
1635@item -11
1636@code{void}, type indicating the lack of a value.
1637
1638@item -12
1639@code{float}, IEEE single precision.
1640
1641@item -13
1642@code{double}, IEEE double precision.
1643
1644@item -14
1645@code{long double}, IEEE double precision. The compiler claims the size
1646will increase in a future release, and for binary compatibility you have
1647to avoid using @code{long double}. I hope when they increase it they
1648use a new negative type number.
1649
1650@item -15
1651@code{integer}. 32 bit signed integral type.
1652
1653@item -16
1654@code{boolean}. 32 bit type. GDB and GCC assume that zero is false,
1655one is true, and other values have unspecified meaning. I hope this
1656agrees with how the IBM tools use the type.
1657
1658@item -17
1659@code{short real}. IEEE single precision.
1660
1661@item -18
1662@code{real}. IEEE double precision.
1663
1664@item -19
1665@code{stringptr}. @xref{Strings}.
1666
1667@item -20
1668@code{character}, 8 bit unsigned character type.
1669
1670@item -21
1671@code{logical*1}, 8 bit type. This Fortran type has a split
1672personality in that it is used for boolean variables, but can also be
1673used for unsigned integers. 0 is false, 1 is true, and other values are
1674non-boolean.
1675
1676@item -22
1677@code{logical*2}, 16 bit type. This Fortran type has a split
1678personality in that it is used for boolean variables, but can also be
1679used for unsigned integers. 0 is false, 1 is true, and other values are
1680non-boolean.
1681
1682@item -23
1683@code{logical*4}, 32 bit type. This Fortran type has a split
1684personality in that it is used for boolean variables, but can also be
1685used for unsigned integers. 0 is false, 1 is true, and other values are
1686non-boolean.
1687
1688@item -24
1689@code{logical}, 32 bit type. This Fortran type has a split
1690personality in that it is used for boolean variables, but can also be
1691used for unsigned integers. 0 is false, 1 is true, and other values are
1692non-boolean.
1693
1694@item -25
1695@code{complex}. A complex type consisting of two IEEE single-precision
1696floating point values.
1697
1698@item -26
1699@code{complex}. A complex type consisting of two IEEE double-precision
1700floating point values.
1701
1702@item -27
1703@code{integer*1}, 8 bit signed integral type.
1704
1705@item -28
1706@code{integer*2}, 16 bit signed integral type.
1707
1708@item -29
1709@code{integer*4}, 32 bit signed integral type.
1710
1711@item -30
1712@code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1713Unicode?).
1714
1715@item -31
1716@code{long long}, 64 bit signed integral type.
1717
1718@item -32
1719@code{unsigned long long}, 64 bit unsigned integral type.
1720
1721@item -33
1722@code{logical*8}, 64 bit unsigned integral type.
1723
1724@item -34
1725@code{integer*8}, 64 bit signed integral type.
1726@end table
1727
1728@node Miscellaneous Types
1729@section Miscellaneous Types
1730
1731@table @code
1732@item b @var{type-information} ; @var{bytes}
1733Pascal space type. This is documented by IBM; what does it mean?
1734
1735This use of the @samp{b} type descriptor can be distinguished
1736from its use for builtin integral types (@pxref{Builtin Type
1737Descriptors}) because the character following the type descriptor is
1738always a digit, @samp{(}, or @samp{-}.
1739
1740@item B @var{type-information}
1741A volatile-qualified version of @var{type-information}. This is
1742a Sun extension. References and stores to a variable with a
1743volatile-qualified type must not be optimized or cached; they
1744must occur as the user specifies them.
1745
1746@item d @var{type-information}
1747File of type @var{type-information}. As far as I know this is only used
1748by Pascal.
1749
1750@item k @var{type-information}
1751A const-qualified version of @var{type-information}. This is a Sun
1752extension. A variable with a const-qualified type cannot be modified.
1753
1754@item M @var{type-information} ; @var{length}
1755Multiple instance type. The type seems to composed of @var{length}
1756repetitions of @var{type-information}, for example @code{character*3} is
1757represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1758character type (@pxref{Negative Type Numbers}). I'm not sure how this
1759differs from an array. This appears to be a Fortran feature.
1760@var{length} is a bound, like those in range types; see @ref{Subranges}.
1761
1762@item S @var{type-information}
1763Pascal set type. @var{type-information} must be a small type such as an
1764enumeration or a subrange, and the type is a bitmask whose length is
1765specified by the number of elements in @var{type-information}.
1766
1767In CHILL, if it is a bitstring instead of a set, also use the @samp{S}
1768type attribute (@pxref{String Field}).
1769
1770@item * @var{type-information}
1771Pointer to @var{type-information}.
1772@end table
1773
1774@node Cross-References
1775@section Cross-References to Other Types
1776
1777A type can be used before it is defined; one common way to deal with
1778that situation is just to use a type reference to a type which has not
1779yet been defined.
1780
1781Another way is with the @samp{x} type descriptor, which is followed by
1782@samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1783a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1784If the name contains @samp{::} between a @samp{<} and @samp{>} pair (for
1785C@t{++} templates), such a @samp{::} does not end the name---only a single
1786@samp{:} ends the name; see @ref{Nested Symbols}.
1787
1788For example, the following C declarations:
1789
1790@example
1791struct foo;
1792struct foo *bar;
1793@end example
1794
1795@noindent
1796produce:
1797
1798@example
1799.stabs "bar:G16=*17=xsfoo:",32,0,0,0
1800@end example
1801
1802Not all debuggers support the @samp{x} type descriptor, so on some
1803machines GCC does not use it. I believe that for the above example it
1804would just emit a reference to type 17 and never define it, but I
1805haven't verified that.
1806
1807Modula-2 imported types, at least on AIX, use the @samp{i} type
1808descriptor, which is followed by the name of the module from which the
1809type is imported, followed by @samp{:}, followed by the name of the
1810type. There is then optionally a comma followed by type information for
1811the type. This differs from merely naming the type (@pxref{Typedefs}) in
1812that it identifies the module; I don't understand whether the name of
1813the type given here is always just the same as the name we are giving
1814it, or whether this type descriptor is used with a nameless stab
1815(@pxref{String Field}), or what. The symbol ends with @samp{;}.
1816
1817@node Subranges
1818@section Subrange Types
1819
1820The @samp{r} type descriptor defines a type as a subrange of another
1821type. It is followed by type information for the type of which it is a
1822subrange, a semicolon, an integral lower bound, a semicolon, an
1823integral upper bound, and a semicolon. The AIX documentation does not
1824specify the trailing semicolon, in an effort to specify array indexes
1825more cleanly, but a subrange which is not an array index has always
1826included a trailing semicolon (@pxref{Arrays}).
1827
1828Instead of an integer, either bound can be one of the following:
1829
1830@table @code
1831@item A @var{offset}
1832The bound is passed by reference on the stack at offset @var{offset}
1833from the argument list. @xref{Parameters}, for more information on such
1834offsets.
1835
1836@item T @var{offset}
1837The bound is passed by value on the stack at offset @var{offset} from
1838the argument list.
1839
1840@item a @var{register-number}
1841The bound is passed by reference in register number
1842@var{register-number}.
1843
1844@item t @var{register-number}
1845The bound is passed by value in register number @var{register-number}.
1846
1847@item J
1848There is no bound.
1849@end table
1850
1851Subranges are also used for builtin types; see @ref{Traditional Builtin Types}.
1852
1853@node Arrays
1854@section Array Types
1855
1856Arrays use the @samp{a} type descriptor. Following the type descriptor
1857is the type of the index and the type of the array elements. If the
1858index type is a range type, it ends in a semicolon; otherwise
1859(for example, if it is a type reference), there does not
1860appear to be any way to tell where the types are separated. In an
1861effort to clean up this mess, IBM documents the two types as being
1862separated by a semicolon, and a range type as not ending in a semicolon
1863(but this is not right for range types which are not array indexes,
1864@pxref{Subranges}). I think probably the best solution is to specify
1865that a semicolon ends a range type, and that the index type and element
1866type of an array are separated by a semicolon, but that if the index
1867type is a range type, the extra semicolon can be omitted. GDB (at least
1868through version 4.9) doesn't support any kind of index type other than a
1869range anyway; I'm not sure about dbx.
1870
1871It is well established, and widely used, that the type of the index,
1872unlike most types found in the stabs, is merely a type definition, not
1873type information (@pxref{String Field}) (that is, it need not start with
1874@samp{@var{type-number}=} if it is defining a new type). According to a
1875comment in GDB, this is also true of the type of the array elements; it
1876gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1877dimensional array. According to AIX documentation, the element type
1878must be type information. GDB accepts either.
1879
1880The type of the index is often a range type, expressed as the type
1881descriptor @samp{r} and some parameters. It defines the size of the
1882array. In the example below, the range @samp{r1;0;2;} defines an index
1883type which is a subrange of type 1 (integer), with a lower bound of 0
1884and an upper bound of 2. This defines the valid range of subscripts of
1885a three-element C array.
1886
1887For example, the definition:
1888
1889@example
1890char char_vec[3] = @{'a','b','c'@};
1891@end example
1892
1893@noindent
1894produces the output:
1895
1896@example
1897.stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1898 .global _char_vec
1899 .align 4
1900_char_vec:
1901 .byte 97
1902 .byte 98
1903 .byte 99
1904@end example
1905
1906If an array is @dfn{packed}, the elements are spaced more
1907closely than normal, saving memory at the expense of speed. For
1908example, an array of 3-byte objects might, if unpacked, have each
1909element aligned on a 4-byte boundary, but if packed, have no padding.
1910One way to specify that something is packed is with type attributes
1911(@pxref{String Field}). In the case of arrays, another is to use the
1912@samp{P} type descriptor instead of @samp{a}. Other than specifying a
1913packed array, @samp{P} is identical to @samp{a}.
1914
1915@c FIXME-what is it? A pointer?
1916An open array is represented by the @samp{A} type descriptor followed by
1917type information specifying the type of the array elements.
1918
1919@c FIXME: what is the format of this type? A pointer to a vector of pointers?
1920An N-dimensional dynamic array is represented by
1921
1922@example
1923D @var{dimensions} ; @var{type-information}
1924@end example
1925
1926@c Does dimensions really have this meaning? The AIX documentation
1927@c doesn't say.
1928@var{dimensions} is the number of dimensions; @var{type-information}
1929specifies the type of the array elements.
1930
1931@c FIXME: what is the format of this type? A pointer to some offsets in
1932@c another array?
1933A subarray of an N-dimensional array is represented by
1934
1935@example
1936E @var{dimensions} ; @var{type-information}
1937@end example
1938
1939@c Does dimensions really have this meaning? The AIX documentation
1940@c doesn't say.
1941@var{dimensions} is the number of dimensions; @var{type-information}
1942specifies the type of the array elements.
1943
1944@node Strings
1945@section Strings
1946
1947Some languages, like C or the original Pascal, do not have string types,
1948they just have related things like arrays of characters. But most
1949Pascals and various other languages have string types, which are
1950indicated as follows:
1951
1952@table @code
1953@item n @var{type-information} ; @var{bytes}
1954@var{bytes} is the maximum length. I'm not sure what
1955@var{type-information} is; I suspect that it means that this is a string
1956of @var{type-information} (thus allowing a string of integers, a string
1957of wide characters, etc., as well as a string of characters). Not sure
1958what the format of this type is. This is an AIX feature.
1959
1960@item z @var{type-information} ; @var{bytes}
1961Just like @samp{n} except that this is a gstring, not an ordinary
1962string. I don't know the difference.
1963
1964@item N
1965Pascal Stringptr. What is this? This is an AIX feature.
1966@end table
1967
1968Languages, such as CHILL which have a string type which is basically
1969just an array of characters use the @samp{S} type attribute
1970(@pxref{String Field}).
1971
1972@node Enumerations
1973@section Enumerations
1974
1975Enumerations are defined with the @samp{e} type descriptor.
1976
1977@c FIXME: Where does this information properly go? Perhaps it is
1978@c redundant with something we already explain.
1979The source line below declares an enumeration type at file scope.
1980The type definition is located after the @code{N_RBRAC} that marks the end of
1981the previous procedure's block scope, and before the @code{N_FUN} that marks
1982the beginning of the next procedure's block scope. Therefore it does not
1983describe a block local symbol, but a file local one.
1984
1985The source line:
1986
1987@example
1988enum e_places @{first,second=3,last@};
1989@end example
1990
1991@noindent
1992generates the following stab:
1993
1994@example
1995.stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1996@end example
1997
1998The symbol descriptor (@samp{T}) says that the stab describes a
1999structure, enumeration, or union tag. The type descriptor @samp{e},
2000following the @samp{22=} of the type definition narrows it down to an
2001enumeration type. Following the @samp{e} is a list of the elements of
2002the enumeration. The format is @samp{@var{name}:@var{value},}. The
2003list of elements ends with @samp{;}. The fact that @var{value} is
2004specified as an integer can cause problems if the value is large. GCC
20052.5.2 tries to output it in octal in that case with a leading zero,
2006which is probably a good thing, although GDB 4.11 supports octal only in
2007cases where decimal is perfectly good. Negative decimal values are
2008supported by both GDB and dbx.
2009
2010There is no standard way to specify the size of an enumeration type; it
2011is determined by the architecture (normally all enumerations types are
201232 bits). Type attributes can be used to specify an enumeration type of
2013another size for debuggers which support them; see @ref{String Field}.
2014
2015Enumeration types are unusual in that they define symbols for the
2016enumeration values (@code{first}, @code{second}, and @code{third} in the
2017above example), and even though these symbols are visible in the file as
2018a whole (rather than being in a more local namespace like structure
2019member names), they are defined in the type definition for the
2020enumeration type rather than each having their own symbol. In order to
2021be fast, GDB will only get symbols from such types (in its initial scan
2022of the stabs) if the type is the first thing defined after a @samp{T} or
2023@samp{t} symbol descriptor (the above example fulfills this
2024requirement). If the type does not have a name, the compiler should
2025emit it in a nameless stab (@pxref{String Field}); GCC does this.
2026
2027@node Structures
2028@section Structures
2029
2030The encoding of structures in stabs can be shown with an example.
2031
2032The following source code declares a structure tag and defines an
2033instance of the structure in global scope. Then a @code{typedef} equates the
2034structure tag with a new type. Separate stabs are generated for the
2035structure tag, the structure @code{typedef}, and the structure instance. The
2036stabs for the tag and the @code{typedef} are emitted when the definitions are
2037encountered. Since the structure elements are not initialized, the
2038stab and code for the structure variable itself is located at the end
2039of the program in the bss section.
2040
2041@example
2042struct s_tag @{
2043 int s_int;
2044 float s_float;
2045 char s_char_vec[8];
2046 struct s_tag* s_next;
2047@} g_an_s;
2048
2049typedef struct s_tag s_typedef;
2050@end example
2051
2052The structure tag has an @code{N_LSYM} stab type because, like the
2053enumeration, the symbol has file scope. Like the enumeration, the
2054symbol descriptor is @samp{T}, for enumeration, structure, or tag type.
2055The type descriptor @samp{s} following the @samp{16=} of the type
2056definition narrows the symbol type to structure.
2057
2058Following the @samp{s} type descriptor is the number of bytes the
2059structure occupies, followed by a description of each structure element.
2060The structure element descriptions are of the form
2061@samp{@var{name}:@var{type}, @var{bit offset from the start of the
2062struct}, @var{number of bits in the element}}.
2063
2064@c FIXME: phony line break. Can probably be fixed by using an example
2065@c with fewer fields.
2066@example
2067# @r{128 is N_LSYM}
2068.stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
2069 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2070@end example
2071
2072In this example, the first two structure elements are previously defined
2073types. For these, the type following the @samp{@var{name}:} part of the
2074element description is a simple type reference. The other two structure
2075elements are new types. In this case there is a type definition
2076embedded after the @samp{@var{name}:}. The type definition for the
2077array element looks just like a type definition for a stand-alone array.
2078The @code{s_next} field is a pointer to the same kind of structure that
2079the field is an element of. So the definition of structure type 16
2080contains a type definition for an element which is a pointer to type 16.
2081
2082If a field is a static member (this is a C@t{++} feature in which a single
2083variable appears to be a field of every structure of a given type) it
2084still starts out with the field name, a colon, and the type, but then
2085instead of a comma, bit position, comma, and bit size, there is a colon
2086followed by the name of the variable which each such field refers to.
2087
2088If the structure has methods (a C@t{++} feature), they follow the non-method
2089fields; see @ref{Cplusplus}.
2090
2091@node Typedefs
2092@section Giving a Type a Name
2093
2094@findex N_LSYM, for types
2095@findex C_DECL, for types
2096To give a type a name, use the @samp{t} symbol descriptor. The type
2097is specified by the type information (@pxref{String Field}) for the stab.
2098For example,
2099
2100@example
2101.stabs "s_typedef:t16",128,0,0,0 # @r{128 is N_LSYM}
2102@end example
2103
2104specifies that @code{s_typedef} refers to type number 16. Such stabs
2105have symbol type @code{N_LSYM} (or @code{C_DECL} for XCOFF). (The Sun
2106documentation mentions using @code{N_GSYM} in some cases).
2107
2108If you are specifying the tag name for a structure, union, or
2109enumeration, use the @samp{T} symbol descriptor instead. I believe C is
2110the only language with this feature.
2111
2112If the type is an opaque type (I believe this is a Modula-2 feature),
2113AIX provides a type descriptor to specify it. The type descriptor is
2114@samp{o} and is followed by a name. I don't know what the name
2115means---is it always the same as the name of the type, or is this type
2116descriptor used with a nameless stab (@pxref{String Field})? There
2117optionally follows a comma followed by type information which defines
2118the type of this type. If omitted, a semicolon is used in place of the
2119comma and the type information, and the type is much like a generic
2120pointer type---it has a known size but little else about it is
2121specified.
2122
2123@node Unions
2124@section Unions
2125
2126@example
2127union u_tag @{
2128 int u_int;
2129 float u_float;
2130 char* u_char;
2131@} an_u;
2132@end example
2133
2134This code generates a stab for a union tag and a stab for a union
2135variable. Both use the @code{N_LSYM} stab type. If a union variable is
2136scoped locally to the procedure in which it is defined, its stab is
2137located immediately preceding the @code{N_LBRAC} for the procedure's block
2138start.
2139
2140The stab for the union tag, however, is located preceding the code for
2141the procedure in which it is defined. The stab type is @code{N_LSYM}. This
2142would seem to imply that the union type is file scope, like the struct
2143type @code{s_tag}. This is not true. The contents and position of the stab
2144for @code{u_type} do not convey any information about its procedure local
2145scope.
2146
2147@c FIXME: phony line break. Can probably be fixed by using an example
2148@c with fewer fields.
2149@smallexample
2150# @r{128 is N_LSYM}
2151.stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2152 128,0,0,0
2153@end smallexample
2154
2155The symbol descriptor @samp{T}, following the @samp{name:} means that
2156the stab describes an enumeration, structure, or union tag. The type
2157descriptor @samp{u}, following the @samp{23=} of the type definition,
2158narrows it down to a union type definition. Following the @samp{u} is
2159the number of bytes in the union. After that is a list of union element
2160descriptions. Their format is @samp{@var{name}:@var{type}, @var{bit
2161offset into the union}, @var{number of bytes for the element};}.
2162
2163The stab for the union variable is:
2164
2165@example
2166.stabs "an_u:23",128,0,0,-20 # @r{128 is N_LSYM}
2167@end example
2168
2169@samp{-20} specifies where the variable is stored (@pxref{Stack
2170Variables}).
2171
2172@node Function Types
2173@section Function Types
2174
2175Various types can be defined for function variables. These types are
2176not used in defining functions (@pxref{Procedures}); they are used for
2177things like pointers to functions.
2178
2179The simple, traditional, type is type descriptor @samp{f} is followed by
2180type information for the return type of the function, followed by a
2181semicolon.
2182
2183This does not deal with functions for which the number and types of the
2184parameters are part of the type, as in Modula-2 or ANSI C. AIX provides
2185extensions to specify these, using the @samp{f}, @samp{F}, @samp{p}, and
2186@samp{R} type descriptors.
2187
2188First comes the type descriptor. If it is @samp{f} or @samp{F}, this
2189type involves a function rather than a procedure, and the type
2190information for the return type of the function follows, followed by a
2191comma. Then comes the number of parameters to the function and a
2192semicolon. Then, for each parameter, there is the name of the parameter
2193followed by a colon (this is only present for type descriptors @samp{R}
2194and @samp{F} which represent Pascal function or procedure parameters),
2195type information for the parameter, a comma, 0 if passed by reference or
21961 if passed by value, and a semicolon. The type definition ends with a
2197semicolon.
2198
2199For example, this variable definition:
2200
2201@example
2202int (*g_pf)();
2203@end example
2204
2205@noindent
2206generates the following code:
2207
2208@example
2209.stabs "g_pf:G24=*25=f1",32,0,0,0
2210 .common _g_pf,4,"bss"
2211@end example
2212
2213The variable defines a new type, 24, which is a pointer to another new
2214type, 25, which is a function returning @code{int}.
2215
2216@node Macro define and undefine
2217@chapter Representation of #define and #undef
2218
2219This section describes the stabs support for macro define and undefine
2220information, supported on some systems. (e.g., with @option{-g3}
2221@option{-gstabs} when using GCC).
2222
2223A @code{#define @var{macro-name} @var{macro-body}} is represented with
2224an @code{N_MAC_DEFINE} stab with a string field of
2225@code{@var{macro-name} @var{macro-body}}.
2226@findex N_MAC_DEFINE
2227
2228An @code{#undef @var{macro-name}} is represented with an
2229@code{N_MAC_UNDEF} stabs with a string field of simply
2230@code{@var{macro-name}}.
2231@findex N_MAC_UNDEF
2232
2233For both @code{N_MAC_DEFINE} and @code{N_MAC_UNDEF}, the desc field is
2234the line number within the file where the corresponding @code{#define}
2235or @code{#undef} occurred.
2236
2237For example, the following C code:
2238
2239@example
2240 #define NONE 42
2241 #define TWO(a, b) (a + (a) + 2 * b)
2242 #define ONE(c) (c + 19)
2243
2244 main(int argc, char *argv[])
2245 @{
2246 func(NONE, TWO(10, 11));
2247 func(NONE, ONE(23));
2248
2249 #undef ONE
2250 #define ONE(c) (c + 23)
2251
2252 func(NONE, ONE(-23));
2253
2254 return (0);
2255 @}
2256
2257 int global;
2258
2259 func(int arg1, int arg2)
2260 @{
2261 global = arg1 + arg2;
2262 @}
2263@end example
2264
2265@noindent
2266produces the following stabs (as well as many others):
2267
2268@example
2269 .stabs "NONE 42",54,0,1,0
2270 .stabs "TWO(a,b) (a + (a) + 2 * b)",54,0,2,0
2271 .stabs "ONE(c) (c + 19)",54,0,3,0
2272 .stabs "ONE",58,0,10,0
2273 .stabs "ONE(c) (c + 23)",54,0,11,0
2274@end example
2275
2276@noindent
2277NOTE: In the above example, @code{54} is @code{N_MAC_DEFINE} and
2278@code{58} is @code{N_MAC_UNDEF}.
2279
2280@node Symbol Tables
2281@chapter Symbol Information in Symbol Tables
2282
2283This chapter describes the format of symbol table entries
2284and how stab assembler directives map to them. It also describes the
2285transformations that the assembler and linker make on data from stabs.
2286
2287@menu
2288* Symbol Table Format::
2289* Transformations On Symbol Tables::
2290@end menu
2291
2292@node Symbol Table Format
2293@section Symbol Table Format
2294
2295Each time the assembler encounters a stab directive, it puts
2296each field of the stab into a corresponding field in a symbol table
2297entry of its output file. If the stab contains a string field, the
2298symbol table entry for that stab points to a string table entry
2299containing the string data from the stab. Assembler labels become
2300relocatable addresses. Symbol table entries in a.out have the format:
2301
2302@c FIXME: should refer to external, not internal.
2303@example
2304struct internal_nlist @{
2305 unsigned long n_strx; /* index into string table of name */
2306 unsigned char n_type; /* type of symbol */
2307 unsigned char n_other; /* misc info (usually empty) */
2308 unsigned short n_desc; /* description field */
2309 bfd_vma n_value; /* value of symbol */
2310@};
2311@end example
2312
2313If the stab has a string, the @code{n_strx} field holds the offset in
2314bytes of the string within the string table. The string is terminated
2315by a NUL character. If the stab lacks a string (for example, it was
2316produced by a @code{.stabn} or @code{.stabd} directive), the
2317@code{n_strx} field is zero.
2318
2319Symbol table entries with @code{n_type} field values greater than 0x1f
2320originated as stabs generated by the compiler (with one random
2321exception). The other entries were placed in the symbol table of the
2322executable by the assembler or the linker.
2323
2324@node Transformations On Symbol Tables
2325@section Transformations on Symbol Tables
2326
2327The linker concatenates object files and does fixups of externally
2328defined symbols.
2329
2330You can see the transformations made on stab data by the assembler and
2331linker by examining the symbol table after each pass of the build. To
2332do this, use @samp{nm -ap}, which dumps the symbol table, including
2333debugging information, unsorted. For stab entries the columns are:
2334@var{value}, @var{other}, @var{desc}, @var{type}, @var{string}. For
2335assembler and linker symbols, the columns are: @var{value}, @var{type},
2336@var{string}.
2337
2338The low 5 bits of the stab type tell the linker how to relocate the
2339value of the stab. Thus for stab types like @code{N_RSYM} and
2340@code{N_LSYM}, where the value is an offset or a register number, the
2341low 5 bits are @code{N_ABS}, which tells the linker not to relocate the
2342value.
2343
2344Where the value of a stab contains an assembly language label,
2345it is transformed by each build step. The assembler turns it into a
2346relocatable address and the linker turns it into an absolute address.
2347
2348@menu
2349* Transformations On Static Variables::
2350* Transformations On Global Variables::
2351* Stab Section Transformations:: For some object file formats,
2352 things are a bit different.
2353@end menu
2354
2355@node Transformations On Static Variables
2356@subsection Transformations on Static Variables
2357
2358This source line defines a static variable at file scope:
2359
2360@example
2361static int s_g_repeat
2362@end example
2363
2364@noindent
2365The following stab describes the symbol:
2366
2367@example
2368.stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2369@end example
2370
2371@noindent
2372The assembler transforms the stab into this symbol table entry in the
2373@file{.o} file. The location is expressed as a data segment offset.
2374
2375@example
237600000084 - 00 0000 STSYM s_g_repeat:S1
2377@end example
2378
2379@noindent
2380In the symbol table entry from the executable, the linker has made the
2381relocatable address absolute.
2382
2383@example
23840000e00c - 00 0000 STSYM s_g_repeat:S1
2385@end example
2386
2387@node Transformations On Global Variables
2388@subsection Transformations on Global Variables
2389
2390Stabs for global variables do not contain location information. In
2391this case, the debugger finds location information in the assembler or
2392linker symbol table entry describing the variable. The source line:
2393
2394@example
2395char g_foo = 'c';
2396@end example
2397
2398@noindent
2399generates the stab:
2400
2401@example
2402.stabs "g_foo:G2",32,0,0,0
2403@end example
2404
2405The variable is represented by two symbol table entries in the object
2406file (see below). The first one originated as a stab. The second one
2407is an external symbol. The upper case @samp{D} signifies that the
2408@code{n_type} field of the symbol table contains 7, @code{N_DATA} with
2409local linkage. The stab's value is zero since the value is not used for
2410@code{N_GSYM} stabs. The value of the linker symbol is the relocatable
2411address corresponding to the variable.
2412
2413@example
241400000000 - 00 0000 GSYM g_foo:G2
241500000080 D _g_foo
2416@end example
2417
2418@noindent
2419These entries as transformed by the linker. The linker symbol table
2420entry now holds an absolute address:
2421
2422@example
242300000000 - 00 0000 GSYM g_foo:G2
2424@dots{}
24250000e008 D _g_foo
2426@end example
2427
2428@node Stab Section Transformations
2429@subsection Transformations of Stabs in separate sections
2430
2431For object file formats using stabs in separate sections (@pxref{Stab
2432Sections}), use @code{objdump --stabs} instead of @code{nm} to show the
2433stabs in an object or executable file. @code{objdump} is a GNU utility;
2434Sun does not provide any equivalent.
2435
2436The following example is for a stab whose value is an address is
2437relative to the compilation unit (@pxref{ELF Linker Relocation}). For
2438example, if the source line
2439
2440@example
2441static int ld = 5;
2442@end example
2443
2444appears within a function, then the assembly language output from the
2445compiler contains:
2446
2447@example
2448.Ddata.data:
2449@dots{}
2450 .stabs "ld:V(0,3)",0x26,0,4,.L18-Ddata.data # @r{0x26 is N_STSYM}
2451@dots{}
2452.L18:
2453 .align 4
2454 .word 0x5
2455@end example
2456
2457Because the value is formed by subtracting one symbol from another, the
2458value is absolute, not relocatable, and so the object file contains
2459
2460@example
2461Symnum n_type n_othr n_desc n_value n_strx String
246231 STSYM 0 4 00000004 680 ld:V(0,3)
2463@end example
2464
2465without any relocations, and the executable file also contains
2466
2467@example
2468Symnum n_type n_othr n_desc n_value n_strx String
246931 STSYM 0 4 00000004 680 ld:V(0,3)
2470@end example
2471
2472@node Cplusplus
2473@chapter GNU C@t{++} Stabs
2474
2475@menu
2476* Class Names:: C++ class names are both tags and typedefs.
2477* Nested Symbols:: C++ symbol names can be within other types.
2478* Basic Cplusplus Types::
2479* Simple Classes::
2480* Class Instance::
2481* Methods:: Method definition
2482* Method Type Descriptor:: The @samp{#} type descriptor
2483* Member Type Descriptor:: The @samp{@@} type descriptor
2484* Protections::
2485* Method Modifiers::
2486* Virtual Methods::
2487* Inheritance::
2488* Virtual Base Classes::
2489* Static Members::
2490@end menu
2491
2492@node Class Names
2493@section C@t{++} Class Names
2494
2495In C@t{++}, a class name which is declared with @code{class}, @code{struct},
2496or @code{union}, is not only a tag, as in C, but also a type name. Thus
2497there should be stabs with both @samp{t} and @samp{T} symbol descriptors
2498(@pxref{Typedefs}).
2499
2500To save space, there is a special abbreviation for this case. If the
2501@samp{T} symbol descriptor is followed by @samp{t}, then the stab
2502defines both a type name and a tag.
2503
2504For example, the C@t{++} code
2505
2506@example
2507struct foo @{int x;@};
2508@end example
2509
2510can be represented as either
2511
2512@example
2513.stabs "foo:T19=s4x:1,0,32;;",128,0,0,0 # @r{128 is N_LSYM}
2514.stabs "foo:t19",128,0,0,0
2515@end example
2516
2517or
2518
2519@example
2520.stabs "foo:Tt19=s4x:1,0,32;;",128,0,0,0
2521@end example
2522
2523@node Nested Symbols
2524@section Defining a Symbol Within Another Type
2525
2526In C@t{++}, a symbol (such as a type name) can be defined within another type.
2527@c FIXME: Needs example.
2528
2529In stabs, this is sometimes represented by making the name of a symbol
2530which contains @samp{::}. Such a pair of colons does not end the name
2531of the symbol, the way a single colon would (@pxref{String Field}). I'm
2532not sure how consistently used or well thought out this mechanism is.
2533So that a pair of colons in this position always has this meaning,
2534@samp{:} cannot be used as a symbol descriptor.
2535
2536For example, if the string for a stab is @samp{foo::bar::baz:t5=*6},
2537then @code{foo::bar::baz} is the name of the symbol, @samp{t} is the
2538symbol descriptor, and @samp{5=*6} is the type information.
2539
2540@node Basic Cplusplus Types
2541@section Basic Types For C@t{++}
2542
2543<< the examples that follow are based on a01.C >>
2544
2545
2546C@t{++} adds two more builtin types to the set defined for C. These are
2547the unknown type and the vtable record type. The unknown type, type
254816, is defined in terms of itself like the void type.
2549
2550The vtable record type, type 17, is defined as a structure type and
2551then as a structure tag. The structure has four fields: delta, index,
2552pfn, and delta2. pfn is the function pointer.
2553
2554<< In boilerplate $vtbl_ptr_type, what are the fields delta,
2555index, and delta2 used for? >>
2556
2557This basic type is present in all C@t{++} programs even if there are no
2558virtual methods defined.
2559
2560@display
2561.stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2562 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2563 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2564 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2565 bit_offset(32),field_bits(32);
2566 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2567 N_LSYM, NIL, NIL
2568@end display
2569
2570@smallexample
2571.stabs "$vtbl_ptr_type:t17=s8
2572 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2573 ,128,0,0,0
2574@end smallexample
2575
2576@display
2577.stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2578@end display
2579
2580@example
2581.stabs "$vtbl_ptr_type:T17",128,0,0,0
2582@end example
2583
2584@node Simple Classes
2585@section Simple Class Definition
2586
2587The stabs describing C@t{++} language features are an extension of the
2588stabs describing C. Stabs representing C@t{++} class types elaborate
2589extensively on the stab format used to describe structure types in C.
2590Stabs representing class type variables look just like stabs
2591representing C language variables.
2592
2593Consider the following very simple class definition.
2594
2595@example
2596class baseA @{
2597public:
2598 int Adat;
2599 int Ameth(int in, char other);
2600@};
2601@end example
2602
2603The class @code{baseA} is represented by two stabs. The first stab describes
2604the class as a structure type. The second stab describes a structure
2605tag of the class type. Both stabs are of stab type @code{N_LSYM}. Since the
2606stab is not located between an @code{N_FUN} and an @code{N_LBRAC} stab this indicates
2607that the class is defined at file scope. If it were, then the @code{N_LSYM}
2608would signify a local variable.
2609
2610A stab describing a C@t{++} class type is similar in format to a stab
2611describing a C struct, with each class member shown as a field in the
2612structure. The part of the struct format describing fields is
2613expanded to include extra information relevant to C@t{++} class members.
2614In addition, if the class has multiple base classes or virtual
2615functions the struct format outside of the field parts is also
2616augmented.
2617
2618In this simple example the field part of the C@t{++} class stab
2619representing member data looks just like the field part of a C struct
2620stab. The section on protections describes how its format is
2621sometimes extended for member data.
2622
2623The field part of a C@t{++} class stab representing a member function
2624differs substantially from the field part of a C struct stab. It
2625still begins with @samp{name:} but then goes on to define a new type number
2626for the member function, describe its return type, its argument types,
2627its protection level, any qualifiers applied to the method definition,
2628and whether the method is virtual or not. If the method is virtual
2629then the method description goes on to give the vtable index of the
2630method, and the type number of the first base class defining the
2631method.
2632
2633When the field name is a method name it is followed by two colons rather
2634than one. This is followed by a new type definition for the method.
2635This is a number followed by an equal sign and the type of the method.
2636Normally this will be a type declared using the @samp{#} type
2637descriptor; see @ref{Method Type Descriptor}; static member functions
2638are declared using the @samp{f} type descriptor instead; see
2639@ref{Function Types}.
2640
2641The format of an overloaded operator method name differs from that of
2642other methods. It is @samp{op$::@var{operator-name}.} where
2643@var{operator-name} is the operator name such as @samp{+} or @samp{+=}.
2644The name ends with a period, and any characters except the period can
2645occur in the @var{operator-name} string.
2646
2647The next part of the method description represents the arguments to the
2648method, preceded by a colon and ending with a semi-colon. The types of
2649the arguments are expressed in the same way argument types are expressed
2650in C@t{++} name mangling. In this example an @code{int} and a @code{char}
2651map to @samp{ic}.
2652
2653This is followed by a number, a letter, and an asterisk or period,
2654followed by another semicolon. The number indicates the protections
2655that apply to the member function. Here the 2 means public. The
2656letter encodes any qualifier applied to the method definition. In
2657this case, @samp{A} means that it is a normal function definition. The dot
2658shows that the method is not virtual. The sections that follow
2659elaborate further on these fields and describe the additional
2660information present for virtual methods.
2661
2662
2663@display
2664.stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2665 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2666
2667 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2668 :arg_types(int char);
2669 protection(public)qualifier(normal)virtual(no);;"
2670 N_LSYM,NIL,NIL,NIL
2671@end display
2672
2673@smallexample
2674.stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2675
2676.stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2677
2678.stabs "baseA:T20",128,0,0,0
2679@end smallexample
2680
2681@node Class Instance
2682@section Class Instance
2683
2684As shown above, describing even a simple C@t{++} class definition is
2685accomplished by massively extending the stab format used in C to
2686describe structure types. However, once the class is defined, C stabs
2687with no modifications can be used to describe class instances. The
2688following source:
2689
2690@example
2691main () @{
2692 baseA AbaseA;
2693@}
2694@end example
2695
2696@noindent
2697yields the following stab describing the class instance. It looks no
2698different from a standard C stab describing a local variable.
2699
2700@display
2701.stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2702@end display
2703
2704@example
2705.stabs "AbaseA:20",128,0,0,-20
2706@end example
2707
2708@node Methods
2709@section Method Definition
2710
2711The class definition shown above declares Ameth. The C@t{++} source below
2712defines Ameth:
2713
2714@example
2715int
2716baseA::Ameth(int in, char other)
2717@{
2718 return in;
2719@};
2720@end example
2721
2722
2723This method definition yields three stabs following the code of the
2724method. One stab describes the method itself and following two describe
2725its parameters. Although there is only one formal argument all methods
2726have an implicit argument which is the @code{this} pointer. The @code{this}
2727pointer is a pointer to the object on which the method was called. Note
2728that the method name is mangled to encode the class name and argument
2729types. Name mangling is described in the @sc{arm} (@cite{The Annotated
2730C++ Reference Manual}, by Ellis and Stroustrup, @sc{isbn}
27310-201-51459-1); @file{gpcompare.texi} in Cygnus GCC distributions
2732describes the differences between GNU mangling and @sc{arm}
2733mangling.
2734@c FIXME: Use @xref, especially if this is generally installed in the
2735@c info tree.
2736@c FIXME: This information should be in a net release, either of GCC or
2737@c GDB. But gpcompare.texi doesn't seem to be in the FSF GCC.
2738
2739@example
2740.stabs "name:symbol_descriptor(global function)return_type(int)",
2741 N_FUN, NIL, NIL, code_addr_of_method_start
2742
2743.stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2744@end example
2745
2746Here is the stab for the @code{this} pointer implicit argument. The
2747name of the @code{this} pointer is always @code{this}. Type 19, the
2748@code{this} pointer is defined as a pointer to type 20, @code{baseA},
2749but a stab defining @code{baseA} has not yet been emitted. Since the
2750compiler knows it will be emitted shortly, here it just outputs a cross
2751reference to the undefined symbol, by prefixing the symbol name with
2752@samp{xs}.
2753
2754@example
2755.stabs "name:sym_desc(register param)type_def(19)=
2756 type_desc(ptr to)type_ref(baseA)=
2757 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2758
2759.stabs "this:P19=*20=xsbaseA:",64,0,0,8
2760@end example
2761
2762The stab for the explicit integer argument looks just like a parameter
2763to a C function. The last field of the stab is the offset from the
2764argument pointer, which in most systems is the same as the frame
2765pointer.
2766
2767@example
2768.stabs "name:sym_desc(value parameter)type_ref(int)",
2769 N_PSYM,NIL,NIL,offset_from_arg_ptr
2770
2771.stabs "in:p1",160,0,0,72
2772@end example
2773
2774<< The examples that follow are based on A1.C >>
2775
2776@node Method Type Descriptor
2777@section The @samp{#} Type Descriptor
2778
2779This is used to describe a class method. This is a function which takes
2780an extra argument as its first argument, for the @code{this} pointer.
2781
2782If the @samp{#} is immediately followed by another @samp{#}, the second
2783one will be followed by the return type and a semicolon. The class and
2784argument types are not specified, and must be determined by demangling
2785the name of the method if it is available.
2786
2787Otherwise, the single @samp{#} is followed by the class type, a comma,
2788the return type, a comma, and zero or more parameter types separated by
2789commas. The list of arguments is terminated by a semicolon. In the
2790debugging output generated by gcc, a final argument type of @code{void}
2791indicates a method which does not take a variable number of arguments.
2792If the final argument type of @code{void} does not appear, the method
2793was declared with an ellipsis.
2794
2795Note that although such a type will normally be used to describe fields
2796in structures, unions, or classes, for at least some versions of the
2797compiler it can also be used in other contexts.
2798
2799@node Member Type Descriptor
2800@section The @samp{@@} Type Descriptor
2801
2802The @samp{@@} type descriptor is used for a
2803pointer-to-non-static-member-data type. It is followed
2804by type information for the class (or union), a comma, and type
2805information for the member data.
2806
2807The following C@t{++} source:
2808
2809@smallexample
2810typedef int A::*int_in_a;
2811@end smallexample
2812
2813generates the following stab:
2814
2815@smallexample
2816.stabs "int_in_a:t20=21=@@19,1",128,0,0,0
2817@end smallexample
2818
2819Note that there is a conflict between this and type attributes
2820(@pxref{String Field}); both use type descriptor @samp{@@}.
2821Fortunately, the @samp{@@} type descriptor used in this C@t{++} sense always
2822will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2823never start with those things.
2824
2825@node Protections
2826@section Protections
2827
2828In the simple class definition shown above all member data and
2829functions were publicly accessible. The example that follows
2830contrasts public, protected and privately accessible fields and shows
2831how these protections are encoded in C@t{++} stabs.
2832
2833If the character following the @samp{@var{field-name}:} part of the
2834string is @samp{/}, then the next character is the visibility. @samp{0}
2835means private, @samp{1} means protected, and @samp{2} means public.
2836Debuggers should ignore visibility characters they do not recognize, and
2837assume a reasonable default (such as public) (GDB 4.11 does not, but
2838this should be fixed in the next GDB release). If no visibility is
2839specified the field is public. The visibility @samp{9} means that the
2840field has been optimized out and is public (there is no way to specify
2841an optimized out field with a private or protected visibility).
2842Visibility @samp{9} is not supported by GDB 4.11; this should be fixed
2843in the next GDB release.
2844
2845The following C@t{++} source:
2846
2847@example
2848class vis @{
2849private:
2850 int priv;
2851protected:
2852 char prot;
2853public:
2854 float pub;
2855@};
2856@end example
2857
2858@noindent
2859generates the following stab:
2860
2861@example
2862# @r{128 is N_LSYM}
2863.stabs "vis:T19=s12priv:/01,0,32;prot:/12,32,8;pub:12,64,32;;",128,0,0,0
2864@end example
2865
2866@samp{vis:T19=s12} indicates that type number 19 is a 12 byte structure
2867named @code{vis} The @code{priv} field has public visibility
2868(@samp{/0}), type int (@samp{1}), and offset and size @samp{,0,32;}.
2869The @code{prot} field has protected visibility (@samp{/1}), type char
2870(@samp{2}) and offset and size @samp{,32,8;}. The @code{pub} field has
2871type float (@samp{12}), and offset and size @samp{,64,32;}.
2872
2873Protections for member functions are signified by one digit embedded in
2874the field part of the stab describing the method. The digit is 0 if
2875private, 1 if protected and 2 if public. Consider the C@t{++} class
2876definition below:
2877
2878@example
2879class all_methods @{
2880private:
2881 int priv_meth(int in)@{return in;@};
2882protected:
2883 char protMeth(char in)@{return in;@};
2884public:
2885 float pubMeth(float in)@{return in;@};
2886@};
2887@end example
2888
2889It generates the following stab. The digit in question is to the left
2890of an @samp{A} in each case. Notice also that in this case two symbol
2891descriptors apply to the class name struct tag and struct type.
2892
2893@display
2894.stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2895 sym_desc(struct)struct_bytes(1)
2896 meth_name::type_def(22)=sym_desc(method)returning(int);
2897 :args(int);protection(private)modifier(normal)virtual(no);
2898 meth_name::type_def(23)=sym_desc(method)returning(char);
2899 :args(char);protection(protected)modifier(normal)virtual(no);
2900 meth_name::type_def(24)=sym_desc(method)returning(float);
2901 :args(float);protection(public)modifier(normal)virtual(no);;",
2902 N_LSYM,NIL,NIL,NIL
2903@end display
2904
2905@smallexample
2906.stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2907 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2908@end smallexample
2909
2910@node Method Modifiers
2911@section Method Modifiers (@code{const}, @code{volatile}, @code{const volatile})
2912
2913<< based on a6.C >>
2914
2915In the class example described above all the methods have the normal
2916modifier. This method modifier information is located just after the
2917protection information for the method. This field has four possible
2918character values. Normal methods use @samp{A}, const methods use
2919@samp{B}, volatile methods use @samp{C}, and const volatile methods use
2920@samp{D}. Consider the class definition below:
2921
2922@example
2923class A @{
2924public:
2925 int ConstMeth (int arg) const @{ return arg; @};
2926 char VolatileMeth (char arg) volatile @{ return arg; @};
2927 float ConstVolMeth (float arg) const volatile @{return arg; @};
2928@};
2929@end example
2930
2931This class is described by the following stab:
2932
2933@display
2934.stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2935 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2936 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2937 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2938 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2939 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2940 returning(float);:arg(float);protection(public)modifier(const volatile)
2941 virtual(no);;", @dots{}
2942@end display
2943
2944@example
2945.stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2946 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2947@end example
2948
2949@node Virtual Methods
2950@section Virtual Methods
2951
2952<< The following examples are based on a4.C >>
2953
2954The presence of virtual methods in a class definition adds additional
2955data to the class description. The extra data is appended to the
2956description of the virtual method and to the end of the class
2957description. Consider the class definition below:
2958
2959@example
2960class A @{
2961public:
2962 int Adat;
2963 virtual int A_virt (int arg) @{ return arg; @};
2964@};
2965@end example
2966
2967This results in the stab below describing class A. It defines a new
2968type (20) which is an 8 byte structure. The first field of the class
2969struct is @samp{Adat}, an integer, starting at structure offset 0 and
2970occupying 32 bits.
2971
2972The second field in the class struct is not explicitly defined by the
2973C@t{++} class definition but is implied by the fact that the class
2974contains a virtual method. This field is the vtable pointer. The
2975name of the vtable pointer field starts with @samp{$vf} and continues with a
2976type reference to the class it is part of. In this example the type
2977reference for class A is 20 so the name of its vtable pointer field is
2978@samp{$vf20}, followed by the usual colon.
2979
2980Next there is a type definition for the vtable pointer type (21).
2981This is in turn defined as a pointer to another new type (22).
2982
2983Type 22 is the vtable itself, which is defined as an array, indexed by
2984a range of integers between 0 and 1, and whose elements are of type
298517. Type 17 was the vtable record type defined by the boilerplate C@t{++}
2986type definitions, as shown earlier.
2987
2988The bit offset of the vtable pointer field is 32. The number of bits
2989in the field are not specified when the field is a vtable pointer.
2990
2991Next is the method definition for the virtual member function @code{A_virt}.
2992Its description starts out using the same format as the non-virtual
2993member functions described above, except instead of a dot after the
2994@samp{A} there is an asterisk, indicating that the function is virtual.
2995Since is is virtual some addition information is appended to the end
2996of the method description.
2997
2998The first number represents the vtable index of the method. This is a
299932 bit unsigned number with the high bit set, followed by a
3000semi-colon.
3001
3002The second number is a type reference to the first base class in the
3003inheritance hierarchy defining the virtual member function. In this
3004case the class stab describes a base class so the virtual function is
3005not overriding any other definition of the method. Therefore the
3006reference is to the type number of the class that the stab is
3007describing (20).
3008
3009This is followed by three semi-colons. One marks the end of the
3010current sub-section, one marks the end of the method field, and the
3011third marks the end of the struct definition.
3012
3013For classes containing virtual functions the very last section of the
3014string part of the stab holds a type reference to the first base
3015class. This is preceded by @samp{~%} and followed by a final semi-colon.
3016
3017@display
3018.stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
3019 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
3020 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
3021 sym_desc(array)index_type_ref(range of int from 0 to 1);
3022 elem_type_ref(vtbl elem type),
3023 bit_offset(32);
3024 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
3025 :arg_type(int),protection(public)normal(yes)virtual(yes)
3026 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
3027 N_LSYM,NIL,NIL,NIL
3028@end display
3029
3030@c FIXME: bogus line break.
3031@example
3032.stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
3033 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
3034@end example
3035
3036@node Inheritance
3037@section Inheritance
3038
3039Stabs describing C@t{++} derived classes include additional sections that
3040describe the inheritance hierarchy of the class. A derived class stab
3041also encodes the number of base classes. For each base class it tells
3042if the base class is virtual or not, and if the inheritance is private
3043or public. It also gives the offset into the object of the portion of
3044the object corresponding to each base class.
3045
3046This additional information is embedded in the class stab following the
3047number of bytes in the struct. First the number of base classes
3048appears bracketed by an exclamation point and a comma.
3049
3050Then for each base type there repeats a series: a virtual character, a
3051visibility character, a number, a comma, another number, and a
3052semi-colon.
3053
3054The virtual character is @samp{1} if the base class is virtual and
3055@samp{0} if not. The visibility character is @samp{2} if the derivation
3056is public, @samp{1} if it is protected, and @samp{0} if it is private.
3057Debuggers should ignore virtual or visibility characters they do not
3058recognize, and assume a reasonable default (such as public and
3059non-virtual) (GDB 4.11 does not, but this should be fixed in the next
3060GDB release).
3061
3062The number following the virtual and visibility characters is the offset
3063from the start of the object to the part of the object pertaining to the
3064base class.
3065
3066After the comma, the second number is a type_descriptor for the base
3067type. Finally a semi-colon ends the series, which repeats for each
3068base class.
3069
3070The source below defines three base classes @code{A}, @code{B}, and
3071@code{C} and the derived class @code{D}.
3072
3073
3074@example
3075class A @{
3076public:
3077 int Adat;
3078 virtual int A_virt (int arg) @{ return arg; @};
3079@};
3080
3081class B @{
3082public:
3083 int B_dat;
3084 virtual int B_virt (int arg) @{return arg; @};
3085@};
3086
3087class C @{
3088public:
3089 int Cdat;
3090 virtual int C_virt (int arg) @{return arg; @};
3091@};
3092
3093class D : A, virtual B, public C @{
3094public:
3095 int Ddat;
3096 virtual int A_virt (int arg ) @{ return arg+1; @};
3097 virtual int B_virt (int arg) @{ return arg+2; @};
3098 virtual int C_virt (int arg) @{ return arg+3; @};
3099 virtual int D_virt (int arg) @{ return arg; @};
3100@};
3101@end example
3102
3103Class stabs similar to the ones described earlier are generated for
3104each base class.
3105
3106@c FIXME!!! the linebreaks in the following example probably make the
3107@c examples literally unusable, but I don't know any other way to get
3108@c them on the page.
3109@c One solution would be to put some of the type definitions into
3110@c separate stabs, even if that's not exactly what the compiler actually
3111@c emits.
3112@smallexample
3113.stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
3114 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
3115
3116.stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
3117 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
3118
3119.stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
3120 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
3121@end smallexample
3122
3123In the stab describing derived class @code{D} below, the information about
3124the derivation of this class is encoded as follows.
3125
3126@display
3127.stabs "derived_class_name:symbol_descriptors(struct tag&type)=
3128 type_descriptor(struct)struct_bytes(32)!num_bases(3),
3129 base_virtual(no)inheritance_public(no)base_offset(0),
3130 base_class_type_ref(A);
3131 base_virtual(yes)inheritance_public(no)base_offset(NIL),
3132 base_class_type_ref(B);
3133 base_virtual(no)inheritance_public(yes)base_offset(64),
3134 base_class_type_ref(C); @dots{}
3135@end display
3136
3137@c FIXME! fake linebreaks.
3138@smallexample
3139.stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
3140 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
3141 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
3142 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
3143@end smallexample
3144
3145@node Virtual Base Classes
3146@section Virtual Base Classes
3147
3148A derived class object consists of a concatenation in memory of the data
3149areas defined by each base class, starting with the leftmost and ending
3150with the rightmost in the list of base classes. The exception to this
3151rule is for virtual inheritance. In the example above, class @code{D}
3152inherits virtually from base class @code{B}. This means that an
3153instance of a @code{D} object will not contain its own @code{B} part but
3154merely a pointer to a @code{B} part, known as a virtual base pointer.
3155
3156In a derived class stab, the base offset part of the derivation
3157information, described above, shows how the base class parts are
3158ordered. The base offset for a virtual base class is always given as 0.
3159Notice that the base offset for @code{B} is given as 0 even though
3160@code{B} is not the first base class. The first base class @code{A}
3161starts at offset 0.
3162
3163The field information part of the stab for class @code{D} describes the field
3164which is the pointer to the virtual base class @code{B}. The vbase pointer
3165name is @samp{$vb} followed by a type reference to the virtual base class.
3166Since the type id for @code{B} in this example is 25, the vbase pointer name
3167is @samp{$vb25}.
3168
3169@c FIXME!! fake linebreaks below
3170@smallexample
3171.stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
3172 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
3173 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
3174 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
3175@end smallexample
3176
3177Following the name and a semicolon is a type reference describing the
3178type of the virtual base class pointer, in this case 24. Type 24 was
3179defined earlier as the type of the @code{B} class @code{this} pointer. The
3180@code{this} pointer for a class is a pointer to the class type.
3181
3182@example
3183.stabs "this:P24=*25=xsB:",64,0,0,8
3184@end example
3185
3186Finally the field offset part of the vbase pointer field description
3187shows that the vbase pointer is the first field in the @code{D} object,
3188before any data fields defined by the class. The layout of a @code{D}
3189class object is a follows, @code{Adat} at 0, the vtable pointer for
3190@code{A} at 32, @code{Cdat} at 64, the vtable pointer for C at 96, the
3191virtual base pointer for @code{B} at 128, and @code{Ddat} at 160.
3192
3193
3194@node Static Members
3195@section Static Members
3196
3197The data area for a class is a concatenation of the space used by the
3198data members of the class. If the class has virtual methods, a vtable
3199pointer follows the class data. The field offset part of each field
3200description in the class stab shows this ordering.
3201
3202<< How is this reflected in stabs? See Cygnus bug #677 for some info. >>
3203
3204@node Stab Types
3205@appendix Table of Stab Types
3206
3207The following are all the possible values for the stab type field, for
3208a.out files, in numeric order. This does not apply to XCOFF, but
3209it does apply to stabs in sections (@pxref{Stab Sections}). Stabs in
3210ECOFF use these values but add 0x8f300 to distinguish them from non-stab
3211symbols.
3212
3213The symbolic names are defined in the file @file{include/aout/stabs.def}.
3214
3215@menu
3216* Non-Stab Symbol Types:: Types from 0 to 0x1f
3217* Stab Symbol Types:: Types from 0x20 to 0xff
3218@end menu
3219
3220@node Non-Stab Symbol Types
3221@appendixsec Non-Stab Symbol Types
3222
3223The following types are used by the linker and assembler, not by stab
3224directives. Since this document does not attempt to describe aspects of
3225object file format other than the debugging format, no details are
3226given.
3227
3228@c Try to get most of these to fit on a single line.
3229@iftex
3230@tableindent=1.5in
3231@end iftex
3232
3233@table @code
3234@item 0x0 N_UNDF
3235Undefined symbol
3236
3237@item 0x2 N_ABS
3238File scope absolute symbol
3239
3240@item 0x3 N_ABS | N_EXT
3241External absolute symbol
3242
3243@item 0x4 N_TEXT
3244File scope text symbol
3245
3246@item 0x5 N_TEXT | N_EXT
3247External text symbol
3248
3249@item 0x6 N_DATA
3250File scope data symbol
3251
3252@item 0x7 N_DATA | N_EXT
3253External data symbol
3254
3255@item 0x8 N_BSS
3256File scope BSS symbol
3257
3258@item 0x9 N_BSS | N_EXT
3259External BSS symbol
3260
3261@item 0x0c N_FN_SEQ
3262Same as @code{N_FN}, for Sequent compilers
3263
3264@item 0x0a N_INDR
3265Symbol is indirected to another symbol
3266
3267@item 0x12 N_COMM
3268Common---visible after shared library dynamic link
3269
3270@item 0x14 N_SETA
3271@itemx 0x15 N_SETA | N_EXT
3272Absolute set element
3273
3274@item 0x16 N_SETT
3275@itemx 0x17 N_SETT | N_EXT
3276Text segment set element
3277
3278@item 0x18 N_SETD
3279@itemx 0x19 N_SETD | N_EXT
3280Data segment set element
3281
3282@item 0x1a N_SETB
3283@itemx 0x1b N_SETB | N_EXT
3284BSS segment set element
3285
3286@item 0x1c N_SETV
3287@itemx 0x1d N_SETV | N_EXT
3288Pointer to set vector
3289
3290@item 0x1e N_WARNING
3291Print a warning message during linking
3292
3293@item 0x1f N_FN
3294File name of a @file{.o} file
3295@end table
3296
3297@node Stab Symbol Types
3298@appendixsec Stab Symbol Types
3299
3300The following symbol types indicate that this is a stab. This is the
3301full list of stab numbers, including stab types that are used in
3302languages other than C.
3303
3304@table @code
3305@item 0x20 N_GSYM
3306Global symbol; see @ref{Global Variables}.
3307
3308@item 0x22 N_FNAME
3309Function name (for BSD Fortran); see @ref{Procedures}.
3310
3311@item 0x24 N_FUN
3312Function name (@pxref{Procedures}) or text segment variable
3313(@pxref{Statics}).
3314
3315@item 0x26 N_STSYM
3316Data segment file-scope variable; see @ref{Statics}.
3317
3318@item 0x28 N_LCSYM
3319BSS segment file-scope variable; see @ref{Statics}.
3320
3321@item 0x2a N_MAIN
3322Name of main routine; see @ref{Main Program}.
3323
3324@item 0x2c N_ROSYM
3325Variable in @code{.rodata} section; see @ref{Statics}.
3326
3327@item 0x30 N_PC
3328Global symbol (for Pascal); see @ref{N_PC}.
3329
3330@item 0x32 N_NSYMS
3331Number of symbols (according to Ultrix V4.0); see @ref{N_NSYMS}.
3332
3333@item 0x34 N_NOMAP
3334No DST map; see @ref{N_NOMAP}.
3335
3336@item 0x36 N_MAC_DEFINE
3337Name and body of a @code{#define}d macro; see @ref{Macro define and undefine}.
3338
3339@c FIXME: describe this solaris feature in the body of the text (see
3340@c comments in include/aout/stab.def).
3341@item 0x38 N_OBJ
3342Object file (Solaris2).
3343
3344@item 0x3a N_MAC_UNDEF
3345Name of an @code{#undef}ed macro; see @ref{Macro define and undefine}.
3346
3347@c See include/aout/stab.def for (a little) more info.
3348@item 0x3c N_OPT
3349Debugger options (Solaris2).
3350
3351@item 0x40 N_RSYM
3352Register variable; see @ref{Register Variables}.
3353
3354@item 0x42 N_M2C
3355Modula-2 compilation unit; see @ref{N_M2C}.
3356
3357@item 0x44 N_SLINE
3358Line number in text segment; see @ref{Line Numbers}.
3359
3360@item 0x46 N_DSLINE
3361Line number in data segment; see @ref{Line Numbers}.
3362
3363@item 0x48 N_BSLINE
3364Line number in bss segment; see @ref{Line Numbers}.
3365
3366@item 0x48 N_BROWS
3367Sun source code browser, path to @file{.cb} file; see @ref{N_BROWS}.
3368
3369@item 0x4a N_DEFD
3370GNU Modula2 definition module dependency; see @ref{N_DEFD}.
3371
3372@item 0x4c N_FLINE
3373Function start/body/end line numbers (Solaris2).
3374
3375@item 0x50 N_EHDECL
3376GNU C@t{++} exception variable; see @ref{N_EHDECL}.
3377
3378@item 0x50 N_MOD2
3379Modula2 info "for imc" (according to Ultrix V4.0); see @ref{N_MOD2}.
3380
3381@item 0x54 N_CATCH
3382GNU C@t{++} @code{catch} clause; see @ref{N_CATCH}.
3383
3384@item 0x60 N_SSYM
3385Structure of union element; see @ref{N_SSYM}.
3386
3387@item 0x62 N_ENDM
3388Last stab for module (Solaris2).
3389
3390@item 0x64 N_SO
3391Path and name of source file; see @ref{Source Files}.
3392
3393@item 0x80 N_LSYM
3394Stack variable (@pxref{Stack Variables}) or type (@pxref{Typedefs}).
3395
3396@item 0x82 N_BINCL
3397Beginning of an include file (Sun only); see @ref{Include Files}.
3398
3399@item 0x84 N_SOL
3400Name of include file; see @ref{Include Files}.
3401
3402@item 0xa0 N_PSYM
3403Parameter variable; see @ref{Parameters}.
3404
3405@item 0xa2 N_EINCL
3406End of an include file; see @ref{Include Files}.
3407
3408@item 0xa4 N_ENTRY
3409Alternate entry point; see @ref{Alternate Entry Points}.
3410
3411@item 0xc0 N_LBRAC
3412Beginning of a lexical block; see @ref{Block Structure}.
3413
3414@item 0xc2 N_EXCL
3415Place holder for a deleted include file; see @ref{Include Files}.
3416
3417@item 0xc4 N_SCOPE
3418Modula2 scope information (Sun linker); see @ref{N_SCOPE}.
3419
3420@item 0xe0 N_RBRAC
3421End of a lexical block; see @ref{Block Structure}.
3422
3423@item 0xe2 N_BCOMM
3424Begin named common block; see @ref{Common Blocks}.
3425
3426@item 0xe4 N_ECOMM
3427End named common block; see @ref{Common Blocks}.
3428
3429@item 0xe8 N_ECOML
3430Member of a common block; see @ref{Common Blocks}.
3431
3432@c FIXME: How does this really work? Move it to main body of document.
3433@item 0xea N_WITH
3434Pascal @code{with} statement: type,,0,0,offset (Solaris2).
3435
3436@item 0xf0 N_NBTEXT
3437Gould non-base registers; see @ref{Gould}.
3438
3439@item 0xf2 N_NBDATA
3440Gould non-base registers; see @ref{Gould}.
3441
3442@item 0xf4 N_NBBSS
3443Gould non-base registers; see @ref{Gould}.
3444
3445@item 0xf6 N_NBSTS
3446Gould non-base registers; see @ref{Gould}.
3447
3448@item 0xf8 N_NBLCS
3449Gould non-base registers; see @ref{Gould}.
3450@end table
3451
3452@c Restore the default table indent
3453@iftex
3454@tableindent=.8in
3455@end iftex
3456
3457@node Symbol Descriptors
3458@appendix Table of Symbol Descriptors
3459
3460The symbol descriptor is the character which follows the colon in many
3461stabs, and which tells what kind of stab it is. @xref{String Field},
3462for more information about their use.
3463
3464@c Please keep this alphabetical
3465@table @code
3466@c In TeX, this looks great, digit is in italics. But makeinfo insists
3467@c on putting it in `', not realizing that @var should override @code.
3468@c I don't know of any way to make makeinfo do the right thing. Seems
3469@c like a makeinfo bug to me.
3470@item @var{digit}
3471@itemx (
3472@itemx -
3473Variable on the stack; see @ref{Stack Variables}.
3474
3475@item :
3476C@t{++} nested symbol; see @xref{Nested Symbols}.
3477
3478@item a
3479Parameter passed by reference in register; see @ref{Reference Parameters}.
3480
3481@item b
3482Based variable; see @ref{Based Variables}.
3483
3484@item c
3485Constant; see @ref{Constants}.
3486
3487@item C
3488Conformant array bound (Pascal, maybe other languages); @ref{Conformant
3489Arrays}. Name of a caught exception (GNU C@t{++}). These can be
3490distinguished because the latter uses @code{N_CATCH} and the former uses
3491another symbol type.
3492
3493@item d
3494Floating point register variable; see @ref{Register Variables}.
3495
3496@item D
3497Parameter in floating point register; see @ref{Register Parameters}.
3498
3499@item f
3500File scope function; see @ref{Procedures}.
3501
3502@item F
3503Global function; see @ref{Procedures}.
3504
3505@item G
3506Global variable; see @ref{Global Variables}.
3507
3508@item i
3509@xref{Register Parameters}.
3510
3511@item I
3512Internal (nested) procedure; see @ref{Nested Procedures}.
3513
3514@item J
3515Internal (nested) function; see @ref{Nested Procedures}.
3516
3517@item L
3518Label name (documented by AIX, no further information known).
3519
3520@item m
3521Module; see @ref{Procedures}.
3522
3523@item p
3524Argument list parameter; see @ref{Parameters}.
3525
3526@item pP
3527@xref{Parameters}.
3528
3529@item pF
3530Fortran Function parameter; see @ref{Parameters}.
3531
3532@item P
3533Unfortunately, three separate meanings have been independently invented
3534for this symbol descriptor. At least the GNU and Sun uses can be
3535distinguished by the symbol type. Global Procedure (AIX) (symbol type
3536used unknown); see @ref{Procedures}. Register parameter (GNU) (symbol
3537type @code{N_PSYM}); see @ref{Parameters}. Prototype of function
3538referenced by this file (Sun @code{acc}) (symbol type @code{N_FUN}).
3539
3540@item Q
3541Static Procedure; see @ref{Procedures}.
3542
3543@item R
3544Register parameter; see @ref{Register Parameters}.
3545
3546@item r
3547Register variable; see @ref{Register Variables}.
3548
3549@item S
3550File scope variable; see @ref{Statics}.
3551
3552@item s
3553Local variable (OS9000).
3554
3555@item t
3556Type name; see @ref{Typedefs}.
3557
3558@item T
3559Enumeration, structure, or union tag; see @ref{Typedefs}.
3560
3561@item v
3562Parameter passed by reference; see @ref{Reference Parameters}.
3563
3564@item V
3565Procedure scope static variable; see @ref{Statics}.
3566
3567@item x
3568Conformant array; see @ref{Conformant Arrays}.
3569
3570@item X
3571Function return variable; see @ref{Parameters}.
3572@end table
3573
3574@node Type Descriptors
3575@appendix Table of Type Descriptors
3576
3577The type descriptor is the character which follows the type number and
3578an equals sign. It specifies what kind of type is being defined.
3579@xref{String Field}, for more information about their use.
3580
3581@table @code
3582@item @var{digit}
3583@itemx (
3584Type reference; see @ref{String Field}.
3585
3586@item -
3587Reference to builtin type; see @ref{Negative Type Numbers}.
3588
3589@item #
3590Method (C@t{++}); see @ref{Method Type Descriptor}.
3591
3592@item *
3593Pointer; see @ref{Miscellaneous Types}.
3594
3595@item &
3596Reference (C@t{++}).
3597
3598@item @@
3599Type Attributes (AIX); see @ref{String Field}. Member (class and variable)
3600type (GNU C@t{++}); see @ref{Member Type Descriptor}.
3601
3602@item a
3603Array; see @ref{Arrays}.
3604
3605@item A
3606Open array; see @ref{Arrays}.
3607
3608@item b
3609Pascal space type (AIX); see @ref{Miscellaneous Types}. Builtin integer
3610type (Sun); see @ref{Builtin Type Descriptors}. Const and volatile
3611qualified type (OS9000).
3612
3613@item B
3614Volatile-qualified type; see @ref{Miscellaneous Types}.
3615
3616@item c
3617Complex builtin type (AIX); see @ref{Builtin Type Descriptors}.
3618Const-qualified type (OS9000).
3619
3620@item C
3621COBOL Picture type. See AIX documentation for details.
3622
3623@item d
3624File type; see @ref{Miscellaneous Types}.
3625
3626@item D
3627N-dimensional dynamic array; see @ref{Arrays}.
3628
3629@item e
3630Enumeration type; see @ref{Enumerations}.
3631
3632@item E
3633N-dimensional subarray; see @ref{Arrays}.
3634
3635@item f
3636Function type; see @ref{Function Types}.
3637
3638@item F
3639Pascal function parameter; see @ref{Function Types}
3640
3641@item g
3642Builtin floating point type; see @ref{Builtin Type Descriptors}.
3643
3644@item G
3645COBOL Group. See AIX documentation for details.
3646
3647@item i
3648Imported type (AIX); see @ref{Cross-References}. Volatile-qualified
3649type (OS9000).
3650
3651@item k
3652Const-qualified type; see @ref{Miscellaneous Types}.
3653
3654@item K
3655COBOL File Descriptor. See AIX documentation for details.
3656
3657@item M
3658Multiple instance type; see @ref{Miscellaneous Types}.
3659
3660@item n
3661String type; see @ref{Strings}.
3662
3663@item N
3664Stringptr; see @ref{Strings}.
3665
3666@item o
3667Opaque type; see @ref{Typedefs}.
3668
3669@item p
3670Procedure; see @ref{Function Types}.
3671
3672@item P
3673Packed array; see @ref{Arrays}.
3674
3675@item r
3676Range type; see @ref{Subranges}.
3677
3678@item R
3679Builtin floating type; see @ref{Builtin Type Descriptors} (Sun). Pascal
3680subroutine parameter; see @ref{Function Types} (AIX). Detecting this
3681conflict is possible with careful parsing (hint: a Pascal subroutine
3682parameter type will always contain a comma, and a builtin type
3683descriptor never will).
3684
3685@item s
3686Structure type; see @ref{Structures}.
3687
3688@item S
3689Set type; see @ref{Miscellaneous Types}.
3690
3691@item u
3692Union; see @ref{Unions}.
3693
3694@item v
3695Variant record. This is a Pascal and Modula-2 feature which is like a
3696union within a struct in C. See AIX documentation for details.
3697
3698@item w
3699Wide character; see @ref{Builtin Type Descriptors}.
3700
3701@item x
3702Cross-reference; see @ref{Cross-References}.
3703
3704@item Y
3705Used by IBM's xlC C@t{++} compiler (for structures, I think).
3706
3707@item z
3708gstring; see @ref{Strings}.
3709@end table
3710
3711@node Expanded Reference
3712@appendix Expanded Reference by Stab Type
3713
3714@c FIXME: This appendix should go away; see N_PSYM or N_SO for an example.
3715
3716For a full list of stab types, and cross-references to where they are
3717described, see @ref{Stab Types}. This appendix just covers certain
3718stabs which are not yet described in the main body of this document;
3719eventually the information will all be in one place.
3720
3721Format of an entry:
3722
3723The first line is the symbol type (see @file{include/aout/stab.def}).
3724
3725The second line describes the language constructs the symbol type
3726represents.
3727
3728The third line is the stab format with the significant stab fields
3729named and the rest NIL.
3730
3731Subsequent lines expand upon the meaning and possible values for each
3732significant stab field.
3733
3734Finally, any further information.
3735
3736@menu
3737* N_PC:: Pascal global symbol
3738* N_NSYMS:: Number of symbols
3739* N_NOMAP:: No DST map
3740* N_M2C:: Modula-2 compilation unit
3741* N_BROWS:: Path to .cb file for Sun source code browser
3742* N_DEFD:: GNU Modula2 definition module dependency
3743* N_EHDECL:: GNU C++ exception variable
3744* N_MOD2:: Modula2 information "for imc"
3745* N_CATCH:: GNU C++ "catch" clause
3746* N_SSYM:: Structure or union element
3747* N_SCOPE:: Modula2 scope information (Sun only)
3748* Gould:: non-base register symbols used on Gould systems
3749* N_LENG:: Length of preceding entry
3750@end menu
3751
3752@node N_PC
3753@section N_PC
3754
3755@deffn @code{.stabs} N_PC
3756@findex N_PC
3757Global symbol (for Pascal).
3758
3759@example
3760"name" -> "symbol_name" <<?>>
3761value -> supposedly the line number (stab.def is skeptical)
3762@end example
3763
3764@display
3765@file{stabdump.c} says:
3766
3767global pascal symbol: name,,0,subtype,line
3768<< subtype? >>
3769@end display
3770@end deffn
3771
3772@node N_NSYMS
3773@section N_NSYMS
3774
3775@deffn @code{.stabn} N_NSYMS
3776@findex N_NSYMS
3777Number of symbols (according to Ultrix V4.0).
3778
3779@display
3780 0, files,,funcs,lines (stab.def)
3781@end display
3782@end deffn
3783
3784@node N_NOMAP
3785@section N_NOMAP
3786
3787@deffn @code{.stabs} N_NOMAP
3788@findex N_NOMAP
3789No DST map for symbol (according to Ultrix V4.0). I think this means a
3790variable has been optimized out.
3791
3792@display
3793 name, ,0,type,ignored (stab.def)
3794@end display
3795@end deffn
3796
3797@node N_M2C
3798@section N_M2C
3799
3800@deffn @code{.stabs} N_M2C
3801@findex N_M2C
3802Modula-2 compilation unit.
3803
3804@example
3805"string" -> "unit_name,unit_time_stamp[,code_time_stamp]"
3806desc -> unit_number
3807value -> 0 (main unit)
3808 1 (any other unit)
3809@end example
3810
3811See @cite{Dbx and Dbxtool Interfaces}, 2nd edition, by Sun, 1988, for
3812more information.
3813
3814@end deffn
3815
3816@node N_BROWS
3817@section N_BROWS
3818
3819@deffn @code{.stabs} N_BROWS
3820@findex N_BROWS
3821Sun source code browser, path to @file{.cb} file
3822
3823<<?>>
3824"path to associated @file{.cb} file"
3825
3826Note: N_BROWS has the same value as N_BSLINE.
3827@end deffn
3828
3829@node N_DEFD
3830@section N_DEFD
3831
3832@deffn @code{.stabn} N_DEFD
3833@findex N_DEFD
3834GNU Modula2 definition module dependency.
3835
3836GNU Modula-2 definition module dependency. The value is the
3837modification time of the definition file. The other field is non-zero
3838if it is imported with the GNU M2 keyword @code{%INITIALIZE}. Perhaps
3839@code{N_M2C} can be used if there are enough empty fields?
3840@end deffn
3841
3842@node N_EHDECL
3843@section N_EHDECL
3844
3845@deffn @code{.stabs} N_EHDECL
3846@findex N_EHDECL
3847GNU C@t{++} exception variable <<?>>.
3848
3849"@var{string} is variable name"
3850
3851Note: conflicts with @code{N_MOD2}.
3852@end deffn
3853
3854@node N_MOD2
3855@section N_MOD2
3856
3857@deffn @code{.stab?} N_MOD2
3858@findex N_MOD2
3859Modula2 info "for imc" (according to Ultrix V4.0)
3860
3861Note: conflicts with @code{N_EHDECL} <<?>>
3862@end deffn
3863
3864@node N_CATCH
3865@section N_CATCH
3866
3867@deffn @code{.stabn} N_CATCH
3868@findex N_CATCH
3869GNU C@t{++} @code{catch} clause
3870
3871GNU C@t{++} @code{catch} clause. The value is its address. The desc field
3872is nonzero if this entry is immediately followed by a @code{CAUGHT} stab
3873saying what exception was caught. Multiple @code{CAUGHT} stabs means
3874that multiple exceptions can be caught here. If desc is 0, it means all
3875exceptions are caught here.
3876@end deffn
3877
3878@node N_SSYM
3879@section N_SSYM
3880
3881@deffn @code{.stabn} N_SSYM
3882@findex N_SSYM
3883Structure or union element.
3884
3885The value is the offset in the structure.
3886
3887<<?looking at structs and unions in C I didn't see these>>
3888@end deffn
3889
3890@node N_SCOPE
3891@section N_SCOPE
3892
3893@deffn @code{.stab?} N_SCOPE
3894@findex N_SCOPE
3895Modula2 scope information (Sun linker)
3896<<?>>
3897@end deffn
3898
3899@node Gould
3900@section Non-base registers on Gould systems
3901
3902@deffn @code{.stab?} N_NBTEXT
3903@deffnx @code{.stab?} N_NBDATA
3904@deffnx @code{.stab?} N_NBBSS
3905@deffnx @code{.stab?} N_NBSTS
3906@deffnx @code{.stab?} N_NBLCS
3907@findex N_NBTEXT
3908@findex N_NBDATA
3909@findex N_NBBSS
3910@findex N_NBSTS
3911@findex N_NBLCS
3912These are used on Gould systems for non-base registers syms.
3913
3914However, the following values are not the values used by Gould; they are
3915the values which GNU has been documenting for these values for a long
3916time, without actually checking what Gould uses. I include these values
3917only because perhaps some someone actually did something with the GNU
3918information (I hope not, why GNU knowingly assigned wrong values to
3919these in the header file is a complete mystery to me).
3920