1 .\" Copyright (c) 1995 Paul Kranenburg
2 .\" All rights reserved.
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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39 .Nd run-time link-editor
42 .Fn _rtld_functrace "const char *callerso" "const char *calleeso" "const char *calleefun" "void *stack"
46 utility is a self-contained shared object providing run-time
47 support for loading and link-editing shared objects into a process'
49 It is also commonly known as the dynamic linker.
50 It uses the data structures
51 contained within dynamically linked programs to determine which shared
52 libraries are needed and loads them using the
56 After all shared libraries have been successfully loaded,
58 proceeds to resolve external references from both the main program and
60 A mechanism is provided for initialization routines
61 to be called on a per-object basis, giving a shared object an opportunity
62 to perform any extra set-up before execution of the program proper begins.
63 This is useful for C++ libraries that contain static constructors.
65 When resolving dependencies for the loaded objects,
67 may be allowed to translate dynamic token strings in rpath and soname
70 option of the static linker
72 The following strings are recognized now:
73 .Bl -tag -width ".Pa $PLATFORM"
75 Translated to the full path of the loaded object.
77 Translated to the name of the operating system implementation.
79 Translated to the release level of the operating system.
81 Translated to the machine hardware platform.
86 utility itself is loaded by the kernel together with any dynamically-linked
87 program that is to be executed.
88 The kernel transfers control to the
90 After the dynamic linker has finished loading,
91 relocating, and initializing the program and its required shared
92 objects, it transfers control to the entry point of the program.
93 The following search order is used to locate required shared objects:
95 .Bl -enum -offset indent -compact
98 of the referencing object unless that object also contains a
103 of the program unless the referencing object contains a
112 of the referencing object
114 Hints file produced by the
120 directory unless the referencing object was linked using the
121 .Dq Fl z Ar nodefaultlib
128 recognizes a number of environment variables that can be used to modify
129 its behaviour as follows:
130 .Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
131 .It Ev LD_DUMP_REL_POST
134 will print a table containing all relocations after symbol
135 binding and relocation.
136 .It Ev LD_DUMP_REL_PRE
139 will print a table containing all relocations before symbol
140 binding and relocation.
142 A library replacement list in the same format as
144 For convenience, the characters
148 can be used instead of a space and a newline.
149 This variable is parsed after
151 and will override its entries.
152 This variable is unset for set-user-ID and set-group-ID programs.
153 .It Ev LD_LIBMAP_DISABLE
154 If set, disables the use of
158 This variable is unset for set-user-ID and set-group-ID programs.
159 .It Ev LD_ELF_HINTS_PATH
160 This variable will override the default location of
163 This variable is unset for set-user-ID and set-group-ID programs.
164 .It Ev LD_LIBRARY_PATH
165 A colon separated list of directories, overriding the default search path
166 for shared libraries.
167 This variable is unset for set-user-ID and set-group-ID programs.
169 A list of shared libraries, separated by colons and/or white space,
170 to be linked in before any
171 other shared libraries.
172 If the directory is not specified then
173 the directories specified by
175 will be searched first
176 followed by the set of built-in standard directories.
177 This variable is unset for set-user-ID and set-group-ID programs.
179 When set to a nonempty string, causes
181 to relocate all external function calls before starting execution of the
183 Normally, function calls are bound lazily, at the first call
186 increases the start-up time of a program, but it avoids run-time
187 surprises caused by unexpectedly undefined functions.
188 .It Ev LD_TRACE_LOADED_OBJECTS
189 When set to a nonempty string, causes
191 to exit after loading the shared objects and printing a summary which includes
192 the absolute pathnames of all objects, to standard output.
193 .It Ev LD_TRACE_LOADED_OBJECTS_ALL
194 When set to a nonempty string, causes
196 to expand the summary to indicate which objects caused each object to
198 .It Ev LD_TRACE_LOADED_OBJECTS_FMT1
199 .It Ev LD_TRACE_LOADED_OBJECTS_FMT2
200 When set, these variables are interpreted as format strings a la
202 to customize the trace output and are used by
207 to be operated as a filter more conveniently.
208 If the dependency name starts with string
210 .Ev LD_TRACE_LOADED_OBJECTS_FMT1
212 .Ev LD_TRACE_LOADED_OBJECTS_FMT2
214 The following conversions can be used:
217 The main program's name
221 The value of the environment variable
222 .Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
223 Typically used to print both the names of programs and shared libraries
224 being inspected using
229 The full pathname as determined by
231 library search rules.
233 The library's load address.
240 are recognized and have their usual meaning.
244 will log events such as the loading and unloading of shared objects via
250 will process the filtee dependencies of the loaded objects immediately,
251 instead of postponing it until required.
252 Normally, the filtees are opened at the time of the first symbol resolution
253 from the filter object.
256 If a shared object preloaded by the
258 mechanism contains a public symbol
259 .Dq _rtld_functrace ,
261 will transfer control to this function each time
262 it needs to resolve an unbound function symbol.
263 By returning a non-zero value,
265 can advise the linker to keep tracing the specified
266 combination of caller shared object and called function;
267 returning 0 will prevent
269 to be called for this combination again.
271 When implementing a custom
273 function, be aware of the possibility that
275 might be called for functions called on its behalf,
276 or that multiple threads could enter
279 .Sh DIFFERENCES BETWEEN .1 and .2
280 ABI changes have been made to support TLS allocation and initialization
281 and to give threading libraries a chance to complete initialization of the
282 TCB prior to the calling of the _init() functions for the dynamically loaded
285 .Bl -tag -width ".Pa /var/run/ld-elf.so.hints" -compact
286 .It Pa /var/run/ld-elf.so.hints
288 .It Pa /etc/libmap.conf
289 The libmap configuration file.
294 for printing out the functions as they are called, this code can be used:
295 .Bd -literal -offset indent
301 _rtld_functrace(const char *callerso, const char *calleeso,
302 const char *calleefun, void *stack)
304 write(2, "calling ", 8);
305 write(2, calleefun, strlen(calleefun));
311 If put in a file named
314 .Bd -literal -offset indent
315 $ cc -shared -fPIC ft.c -o ft.so