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1.\" Copyright (c) 1993 Paul Kranenburg
2.\" All rights reserved.
3.\"
4.\" Redistribution and use in source and binary forms, with or without
5.\" modification, are permitted provided that the following conditions
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8079d092 29.\" $FreeBSD: src/share/man/man5/link.5,v 1.14.2.10 2004/05/17 11:38:55 brueffer Exp $
44cb301e 30.\" $DragonFly: src/share/man/man5/link.5,v 1.5 2006/05/26 19:39:40 swildner Exp $
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31.\"
32.Dd October 23, 1993
33.Dt LINK 5
34.Os
35.Sh NAME
36.Nm link
37.Nd dynamic loader and link editor interface
38.Sh SYNOPSIS
39.In sys/types.h
40.In nlist.h
41.In link.h
42.Sh DESCRIPTION
43The include file
44cb301e 44.In link.h
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45declares several structures that are present in dynamically linked
46programs and libraries.
47The structures define the interface between several components of the
48link-editor and loader mechanism.
49The layout of a number of these
50structures within the binaries resembles the a.out format in many places
51as it serves such similar functions as symbol definitions (including the
52accompanying string table) and relocation records needed to resolve
53references to external entities.
54It also records a number of data structures
55unique to the dynamic loading and linking process.
56These include references
57to other objects that are required to complete the link-editing process and
58indirection tables to facilitate
59.Em Position Independent Code
60(PIC for short) to improve sharing of code pages among different processes.
61The collection of data structures described here will be referred to as the
62.Em Run-time Relocation Section (RRS)
63and is embedded in the standard text and data segments of the dynamically
64linked program or shared object image as the existing
65.Xr a.out 5
66format offers no room for it elsewhere.
67.Pp
68Several utilities cooperate to ensure that the task of getting a program
69ready to run can complete successfully in a way that optimizes the use
70of system resources.
71The compiler emits PIC code from which shared libraries
72can be built by
73.Xr ld 1 .
74The compiler also includes size information of any initialized data items
75through the .size assembler directive.
76PIC code differs from conventional code
77in that it accesses data variables through an indirection table, the
78Global Offset Table, by convention accessible by the reserved name
79.Dv _GLOBAL_OFFSET_TABLE_ .
80The exact mechanism used for this is machine dependent, usually a machine
81register is reserved for the purpose.
82The rational behind this construct
83is to generate code that is independent of the actual load address.
84Only
85the values contained in the Global Offset Table may need updating at run-time
86depending on the load addresses of the various shared objects in the address
87space.
88.Pp
89Likewise, procedure calls to globally defined functions are redirected through
90the Procedure Linkage Table (PLT) residing in the data segment of the core
91image.
92Again, this is done to avoid run-time modifications to the text segment.
93.Pp
94The linker-editor allocates the Global Offset Table and Procedure Linkage Table
95when combining PIC object files into an image suitable for mapping into the
96process address space.
97It also collects all symbols that may be needed by the
98run-time link-editor and stores these along with the image's text and data bits.
99Another reserved symbol,
100.Em _DYNAMIC
101is used to indicate the presence of the run-time linker structures.
102Whenever
103_DYNAMIC is relocated to 0, there is no need to invoke the run-time
104link-editor.
105If this symbol is non-zero, it points at a data structure from
106which the location of the necessary relocation- and symbol information can
107be derived.
108This is most notably used by the start-up module,
109.Em crt0 .
110The _DYNAMIC structure is conventionally located at the start of the data
111segment of the image to which it pertains.
112.Sh DATA STRUCTURES
113The data structures supporting dynamic linking and run-time relocation
114reside both in the text and data segments of the image they apply to.
115The text segments contain read-only data such as symbols descriptions and
116names, while the data segments contain the tables that need to be modified by
117during the relocation process.
118.Pp
119The _DYNAMIC symbol references a
120.Fa _dynamic
121structure:
122.Bd -literal -offset indent
123struct _dynamic {
124 int d_version;
125 struct so_debug *d_debug;
126 union {
127 struct section_dispatch_table *d_sdt;
128 } d_un;
129 struct ld_entry *d_entry;
130};
131.Ed
132.Bl -tag -width d_version
133.It Fa d_version
134This field provides for different versions of the dynamic linking
135implementation.
136The current version numbers understood by
137.Xr ld 1
138and
139.Xr ld.so 1
140are
141.Em LD_VERSION_SUN (3) ,
142which is used by the
143.Tn SunOS
1444.x releases, and
145.Em LD_VERSION_BSD (8) ,
146which has been in use since
147.Fx 1.1 .
148.It Fa d_un
149Refers to a
150.Em d_version
151dependent data structure.
152.It Fa so_debug
153this field provides debuggers with a hook to access symbol tables of shared
154objects loaded as a result of the actions of the run-time link-editor.
155.El
156.Pp
157The
158.Fa section_dispatch_table
159structure is the main
160.Dq dispatcher
161table, containing offsets into the image's segments where various symbol
162and relocation information is located.
163.Bd -literal -offset indent
164struct section_dispatch_table {
165 struct so_map *sdt_loaded;
166 long sdt_sods;
167 long sdt_filler1;
168 long sdt_got;
169 long sdt_plt;
170 long sdt_rel;
171 long sdt_hash;
172 long sdt_nzlist;
173 long sdt_filler2;
174 long sdt_buckets;
175 long sdt_strings;
176 long sdt_str_sz;
177 long sdt_text_sz;
178 long sdt_plt_sz;
179};
180.Ed
181.Pp
182.Bl -tag -width sdt_filler1
183.It Fa sdt_loaded
184A pointer to the first link map loaded (see below). This field is set by
185.Nm ld.so
186.It Fa sdt_sods
187The start of a (linked) list of shared object descriptors needed by
188.Em this
189object.
190.It Fa sdt_filler1
191Deprecated (used by SunOS to specify library search rules).
192.It Fa sdt_got
193The location of the Global Offset Table within this image.
194.It Fa sdt_plt
195The location of the Procedure Linkage Table within this image.
196.It Fa sdt_rel
197The location of an array of
198.Fa relocation_info
199structures
200(see
201.Xr a.out 5 )
202specifying run-time relocations.
203.It Fa sdt_hash
204The location of the hash table for fast symbol lookup in this object's
205symbol table.
206.It Fa sdt_nzlist
207The location of the symbol table.
208.It Fa sdt_filler2
209Currently unused.
210.It Fa sdt_buckets
211The number of buckets in
212.Fa sdt_hash
213.It Fa sdt_strings
214The location of the symbol string table that goes with
215.Fa sdt_nzlist .
216.It Fa sdt_str_sz
217The size of the string table.
218.It Fa sdt_text_sz
219The size of the object's text segment.
220.It Fa sdt_plt_sz
221The size of the Procedure Linkage Table.
222.El
223.Pp
224A
225.Fa sod
226structure describes a shared object that is needed
227to complete the link edit process of the object containing it.
228A list of such objects
229(chained through
230.Fa sod_next )
231is pointed at
232by the
233.Fa sdt_sods
234in the section_dispatch_table structure.
235.Bd -literal -offset indent
236struct sod {
237 long sod_name;
238 u_int sod_library : 1,
239 sod_reserved : 31;
240 short sod_major;
241 short sod_minor;
242 long sod_next;
243};
244.Ed
245.Pp
246.Bl -tag -width sod_library
247.It Fa sod_name
248The offset in the text segment of a string describing this link object.
249.It Fa sod_library
250If set,
251.Fa sod_name
252specifies a library that is to be searched for by
253.Nm ld.so .
254The path name
255is obtained by searching a set of directories
256(see also
257.Xr ldconfig 8 )
258for a shared object matching
259.Em lib\&<sod_name>\&.so.n.m .
260If not set,
261.Fa sod_name
262should point at a full path name for the desired shared object.
263.It Fa sod_major
264Specifies the major version number of the shared object to load.
265.It Fa sod_minor
266Specifies the preferred minor version number of the shared object to load.
267.El
268.Pp
269The run-time link-editor maintains a list of structures called
270.Em link maps
271to keep track of all shared objects loaded into a process' address space.
272These structures are only used at run-time and do not occur within
273the text or data segment of an executable or shared library.
274.Bd -literal -offset indent
275struct so_map {
276 caddr_t som_addr;
277 char *som_path;
278 struct so_map *som_next;
279 struct sod *som_sod;
280 caddr_t som_sodbase;
281 u_int som_write : 1;
282 struct _dynamic *som_dynamic;
283 caddr_t som_spd;
284};
285.Ed
286.Bl -tag -width som_dynamic
287.It Fa som_addr
288The address at which the shared object associated with this link map has
289been loaded.
290.It Fa som_path
291The full path name of the loaded object.
292.It Fa som_next
293Pointer to the next link map.
294.It Fa som_sod
295The
296.Fa sod
297structure that was responsible for loading this shared object.
298.It Fa som_sodbase
8079d092 299Tossed out in later versions of the run-time linker.
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300.It Fa som_write
301Set if (some portion of) this object's text segment is currently writable.
302.It Fa som_dynamic
303Pointer to this object's
304.Fa _dynamic
305structure.
306.It Fa som_spd
307Hook for attaching private data maintained by the run-time link-editor.
308.El
309.Pp
310Symbol description with size.
311This is simply an
312.Fa nlist
313structure with one field
314.Pq Fa nz_size
315added.
316Used to convey size information on items in the data segment
317of shared objects.
318An array of these lives in the shared object's
319text segment and is addressed by the
320.Fa sdt_nzlist
321field of
322.Fa section_dispatch_table .
323.Bd -literal -offset indent
324struct nzlist {
325 struct nlist nlist;
326 u_long nz_size;
327#define nz_un nlist.n_un
328#define nz_strx nlist.n_un.n_strx
329#define nz_name nlist.n_un.n_name
330#define nz_type nlist.n_type
331#define nz_value nlist.n_value
332#define nz_desc nlist.n_desc
333#define nz_other nlist.n_other
334};
335.Ed
336.Bl -tag -width nz_size
337.It Fa nlist
338(see
339.Xr nlist 3 ) .
340.It Fa nz_size
341The size of the data represented by this symbol.
342.El
343.Pp
344A hash table is included within the text segment of shared object
345to facilitate quick lookup of symbols during run-time link-editing.
346The
347.Fa sdt_hash
348field of the
349.Fa section_dispatch_table
350structure points at an array of
351.Fa rrs_hash
352structures:
353.Bd -literal -offset indent
354struct rrs_hash {
355 int rh_symbolnum; /* symbol number */
356 int rh_next; /* next hash entry */
357};
358.Ed
359.Pp
360.Bl -tag -width rh_symbolnum
361.It Fa rh_symbolnum
362The index of the symbol in the shared object's symbol table (as given by the
363.Fa ld_symbols
364field).
365.It Fa rh_next
366In case of collisions, this field is the offset of the next entry in this
367hash table bucket.
368It is zero for the last bucket element.
369.El
370The
371.Fa rt_symbol
372structure is used to keep track of run-time allocated commons
373and data items copied from shared objects.
374These items are kept on linked list
375and is exported through the
376.Fa dd_cc
377field in the
378.Fa so_debug
379structure (see below) for use by debuggers.
380.Bd -literal -offset indent
381struct rt_symbol {
382 struct nzlist *rt_sp;
383 struct rt_symbol *rt_next;
384 struct rt_symbol *rt_link;
385 caddr_t rt_srcaddr;
386 struct so_map *rt_smp;
387};
388.Ed
389.Pp
390.Bl -tag -width rt_scraddr
391.It Fa rt_sp
392The symbol description.
393.It Fa rt_next
394Virtual address of next rt_symbol.
395.It Fa rt_link
396Next in hash bucket.
8079d092 397Used internally by
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398.Nm ld.so .
399.It Fa rt_srcaddr
400Location of the source of initialized data within a shared object.
401.It Fa rt_smp
402The shared object which is the original source of the data that this
403run-time symbol describes.
404.El
405.Pp
406The
407.Fa so_debug
408structure is used by debuggers to gain knowledge of any shared objects
409that have been loaded in the process's address space as a result of run-time
410link-editing.
411Since the run-time link-editor runs as a part of process
412initialization, a debugger that wishes to access symbols from shared objects
413can only do so after the link-editor has been called from crt0.
414A dynamically linked binary contains a
415.Fa so_debug
416structure which can be located by means of the
417.Fa d_debug
418field in
419.Fa _dynamic .
420.Bd -literal -offset indent
421struct so_debug {
422 int dd_version;
423 int dd_in_debugger;
424 int dd_sym_loaded;
425 char *dd_bpt_addr;
426 int dd_bpt_shadow;
427 struct rt_symbol *dd_cc;
428};
429.Ed
430.Pp
431.Bl -tag -width dd_in_debugger
432.It Fa dd_version
433Version number of this interface.
434.It Fa dd_in_debugger
435Set by the debugger to indicate to the run-time linker that the program is
436run under control of a debugger.
437.It Fa dd_sym_loaded
438Set by the run-time linker whenever it adds symbols by loading shared objects.
439.It Fa dd_bpt_addr
8079d092 440The address where a breakpoint will be set by the run-time linker to
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441divert control to the debugger.
442This address is determined by the start-up
443module,
444.Pa crt0.o ,
445to be some convenient place before the call to _main.
446.It Fa dd_bpt_shadow
447Contains the original instruction that was at
448.Fa dd_bpt_addr .
449The debugger is expected to put this instruction back before continuing the
450program.
451.It Fa dd_cc
452A pointer to the linked list of run-time allocated symbols that the debugger
453may be interested in.
454.El
455.Pp
456The
457.Em ld_entry
458structure defines a set of service routines within
459.Nm ld.so .
460.\" See
461.\" .Xr libdl.a
462.\" for more information.
463.Bd -literal -offset indent
464struct ld_entry {
465 void *(*dlopen)(char *, int);
466 int (*dlclose)(void *);
467 void *(*dlsym)(void *, char *);
468 char *(*dlerror)(void);
469};
470.Ed
471.Pp
472The
473.Fa crt_ldso
474structure defines the interface between the start-up code in crt0 and
475.Nm ld.so .
476.Bd -literal -offset indent
477struct crt_ldso {
478 int crt_ba;
479 int crt_dzfd;
480 int crt_ldfd;
481 struct _dynamic *crt_dp;
482 char **crt_ep;
483 caddr_t crt_bp;
484 char *crt_prog;
485 char *crt_ldso;
486 struct ld_entry *crt_ldentry;
487};
488#define CRT_VERSION_SUN 1
489#define CRT_VERSION_BSD_2 2
490#define CRT_VERSION_BSD_3 3
491#define CRT_VERSION_BSD_4 4
492.Ed
493.Bl -tag -width crt_dzfd
494.It Fa crt_ba
495The virtual address at which
496.Nm ld.so
497was loaded by crt0.
498.It Fa crt_dzfd
499On SunOS systems, this field contains an open file descriptor to
500.Dq Pa /dev/zero
501used to get demand paged zeroed pages.
502On
9bb2a92d 503.Dx
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504systems it contains -1.
505.It Fa crt_ldfd
506Contains an open file descriptor that was used by crt0 to load
507.Nm ld.so .
508.It Fa crt_dp
509A pointer to main's
510.Fa _dynamic
511structure.
512.It Fa crt_ep
513A pointer to the environment strings.
514.It Fa crt_bp
515The address at which a breakpoint will be placed by the run-time linker
516if the main program is run by a debugger.
517See
518.Fa so_debug
519.It Fa crt_prog
520The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only).
521.It Fa crt_ldso
522The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only).
523.El
524.Pp
525The
526.Fa hints_header
527and
528.Fa hints_bucket
529structures define the layout of the library hints, normally found in
530.Dq Pa /var/run/ld.so.hints ,
531which is used by
532.Nm ld.so
533to quickly locate the shared object images in the
534filesystem.
535The organization of the hints file is not unlike that of an
536.Dq a.out
537object file, in that it contains a header determining the offset and size
538of a table of fixed sized hash buckets and a common string pool.
539.Bd -literal -offset indent
540struct hints_header {
541 long hh_magic;
542#define HH_MAGIC 011421044151
543 long hh_version;
544#define LD_HINTS_VERSION_1 1
545 long hh_hashtab;
546 long hh_nbucket;
547 long hh_strtab;
548 long hh_strtab_sz;
549 long hh_ehints;
550};
551.Ed
552.Bl -tag -width hh_strtab_sz
553.It Fa hh_magic
554Hints file magic number.
555.It Fa hh_version
556Interface version number.
557.It Fa hh_hashtab
558Offset of hash table.
559.It Fa hh_strtab
560Offset of string table.
561.It Fa hh_strtab_sz
562Size of strings.
563.It Fa hh_ehints
564Maximum usable offset in hints file.
565.El
566.Pp
567.Bd -literal -offset indent
568/*
569 * Hash table element in hints file.
570 */
571struct hints_bucket {
572 int hi_namex;
573 int hi_pathx;
574 int hi_dewey[MAXDEWEY];
575 int hi_ndewey;
576#define hi_major hi_dewey[0]
577#define hi_minor hi_dewey[1]
578 int hi_next;
579};
580.Ed
581.Bl -tag -width hi_ndewey
582.It Fa hi_namex
583Index of the string identifying the library.
584.It Fa hi_pathx
585Index of the string representing the full path name of the library.
586.It Fa hi_dewey
587The version numbers of the shared library.
588.It Fa hi_ndewey
589The number of valid entries in
590.Fa hi_dewey .
591.It Fa hi_next
592Next bucket in case of hashing collisions.
593.El
594.Sh CAVEATS
595Only the (GNU) C compiler currently supports the creation of shared libraries.
596Other programming languages cannot be used.