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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14 .\" This product includes software developed by Paul Kranenburg.
15 .\" 3. The name of the author may not be used to endorse or promote products
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38 .Nd run-time link-editor
41 .Fn _rtld_functrace "const char *callerso" "const char *calleeso" "const char *calleefun" "void *stack"
45 utility is a self-contained shared object providing run-time
46 support for loading and link-editing shared objects into a process'
48 It is also commonly known as the dynamic linker.
49 It uses the data structures
50 contained within dynamically linked programs to determine which shared
51 libraries are needed and loads them using the
55 After all shared libraries have been successfully loaded,
57 proceeds to resolve external references from both the main program and
59 A mechanism is provided for initialization routines
60 to be called on a per-object basis, giving a shared object an opportunity
61 to perform any extra set-up before execution of the program proper begins.
62 This is useful for C++ libraries that contain static constructors.
64 When resolving dependencies for the loaded objects,
66 may be allowed to translate dynamic token strings in rpath and soname
69 option of the static linker
71 The following strings are recognized now:
72 .Bl -tag -width ".Pa $PLATFORM"
74 Translated to the full path of the loaded object.
76 Translated to the name of the operating system implementation.
78 Translated to the release level of the operating system.
80 Translated to the machine hardware platform.
85 utility itself is loaded by the kernel together with any dynamically-linked
86 program that is to be executed.
87 The kernel transfers control to the
89 After the dynamic linker has finished loading,
90 relocating, and initializing the program and its required shared
91 objects, it transfers control to the entry point of the program.
92 The following search order is used to locate required shared objects:
94 .Bl -enum -offset indent -compact
97 of the referencing object unless that object also contains a
102 of the program unless the referencing object contains a
111 of the referencing object
113 Hints file produced by the
121 directories, unless the referencing object was linked using the
122 .Dq Fl z Ar nodefaultlib
129 recognizes a number of environment variables that can be used to modify
132 .Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
133 .It Ev LD_DUMP_REL_POST
136 will print a table containing all relocations after symbol
137 binding and relocation.
138 .It Ev LD_DUMP_REL_PRE
141 will print a table containing all relocations before symbol
142 binding and relocation.
144 A library replacement list in the same format as
146 For convenience, the characters
150 can be used instead of a space and a newline.
151 This variable is parsed after
153 and will override its entries.
154 This variable is unset for set-user-ID and set-group-ID programs.
155 .It Ev LD_LIBMAP_DISABLE
156 If set, disables the use of
160 This variable is unset for set-user-ID and set-group-ID programs.
161 .It Ev LD_ELF_HINTS_PATH
162 This variable will override the default location of
165 This variable is unset for set-user-ID and set-group-ID programs.
166 .It Ev LD_LIBRARY_PATH
167 A colon separated list of directories, overriding the default search path
168 for shared libraries.
169 This variable is unset for set-user-ID and set-group-ID programs.
171 A list of shared libraries, separated by colons and/or white space,
172 to be linked in before any
173 other shared libraries.
174 If the directory is not specified then
175 the directories specified by
177 will be searched first
178 followed by the set of built-in standard directories.
179 This variable is unset for set-user-ID and set-group-ID programs.
180 .Ev LD_LIBRARY_PATH_FDS
181 A colon separated list of file descriptor numbers for library directories.
182 This is intended for future use within capsicum
183 sandboxes, when global namespaces such as the filesystem are unavailable.
184 It is consulted just after LD_LIBRARY_PATH.
185 This variable is unset for set-user-ID and set-group-ID programs.
187 When set to a nonempty string, causes
189 to relocate all external function calls before starting execution of the
191 Normally, function calls are bound lazily, at the first call
194 increases the start-up time of a program, but it avoids run-time
195 surprises caused by unexpectedly undefined functions.
196 .It Ev LD_TRACE_LOADED_OBJECTS
197 When set to a nonempty string, causes
199 to exit after loading the shared objects and printing a summary which includes
200 the absolute pathnames of all objects, to standard output.
201 .It Ev LD_TRACE_LOADED_OBJECTS_ALL
202 When set to a nonempty string, causes
204 to expand the summary to indicate which objects caused each object to
206 .It Ev LD_TRACE_LOADED_OBJECTS_FMT1
207 .It Ev LD_TRACE_LOADED_OBJECTS_FMT2
208 When set, these variables are interpreted as format strings a la
210 to customize the trace output and are used by
215 to be operated as a filter more conveniently.
216 If the dependency name starts with string
218 .Ev LD_TRACE_LOADED_OBJECTS_FMT1
220 .Ev LD_TRACE_LOADED_OBJECTS_FMT2
222 The following conversions can be used:
225 The main program's name
229 The value of the environment variable
230 .Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
231 Typically used to print both the names of programs and shared libraries
232 being inspected using
237 The full pathname as determined by
239 library search rules.
241 The library's load address.
248 are recognized and have their usual meaning.
252 will log events such as the loading and unloading of shared objects via
257 will process the filtee dependencies of the loaded objects immediately,
258 instead of postponing it until required.
259 Normally, the filtees are opened at the time of the first symbol resolution
260 from the filter object.
263 If a shared object preloaded by the
265 mechanism contains a public symbol
266 .Dq _rtld_functrace ,
268 will transfer control to this function each time
269 it needs to resolve an unbound function symbol.
270 By returning a non-zero value,
272 can advise the linker to keep tracing the specified
273 combination of caller shared object and called function;
274 returning 0 will prevent
276 to be called for this combination again.
278 When implementing a custom
280 function, be aware of the possibility that
282 might be called for functions called on its behalf,
283 or that multiple threads could enter
286 .Sh DIFFERENCES BETWEEN .1 and .2
287 ABI changes have been made to support TLS allocation and initialization
288 and to give threading libraries a chance to complete initialization of the
289 TCB prior to the calling of the
291 functions for the dynamically loaded libraries.
293 .Bl -tag -width ".Pa /var/run/ld-elf.so.hints" -compact
294 .It Pa /var/run/ld-elf.so.hints
296 .It Pa /etc/libmap.conf
297 The libmap configuration file.
302 for printing out the functions as they are called, this code can be used:
303 .Bd -literal -offset indent
310 _rtld_functrace(const char *callerso, const char *calleeso,
311 const char *calleefun, void *stack)
313 write(2, "calling ", 8);
314 write(2, calleefun, strlen(calleefun));
320 If put in a file named
323 .Bd -literal -offset indent
324 $ cc -shared -fPIC ft.c -o ft.so