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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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
122 directories, unless the referencing object was linked using the
123 .Dq Fl z Ar nodefaultlib
130 recognizes a number of environment variables that can be used to modify
133 .Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
134 .It Ev LD_DUMP_REL_POST
137 will print a table containing all relocations after symbol
138 binding and relocation.
139 .It Ev LD_DUMP_REL_PRE
142 will print a table containing all relocations before symbol
143 binding and relocation.
145 A library replacement list in the same format as
147 For convenience, the characters
151 can be used instead of a space and a newline.
152 This variable is parsed after
154 and will override its entries.
155 This variable is unset for set-user-ID and set-group-ID programs.
156 .It Ev LD_LIBMAP_DISABLE
157 If set, disables the use of
161 This variable is unset for set-user-ID and set-group-ID programs.
162 .It Ev LD_ELF_HINTS_PATH
163 This variable will override the default location of
166 This variable is unset for set-user-ID and set-group-ID programs.
167 .It Ev LD_LIBRARY_PATH
168 A colon separated list of directories, overriding the default search path
169 for shared libraries.
170 This variable is unset for set-user-ID and set-group-ID programs.
172 A list of shared libraries, separated by colons and/or white space,
173 to be linked in before any
174 other shared libraries.
175 If the directory is not specified then
176 the directories specified by
178 will be searched first
179 followed by the set of built-in standard directories.
180 This variable is unset for set-user-ID and set-group-ID programs.
181 .Ev LD_LIBRARY_PATH_FDS
182 A colon separated list of file descriptor numbers for library directories.
183 This is intended for future use within capsicum
184 sandboxes, when global namespaces such as the filesystem are unavailable.
185 It is consulted just after LD_LIBRARY_PATH.
186 This variable is unset for set-user-ID and set-group-ID programs.
188 When set to a nonempty string, causes
190 to relocate all external function calls before starting execution of the
192 Normally, function calls are bound lazily, at the first call
195 increases the start-up time of a program, but it avoids run-time
196 surprises caused by unexpectedly undefined functions.
197 .It Ev LD_TRACE_LOADED_OBJECTS
198 When set to a nonempty string, causes
200 to exit after loading the shared objects and printing a summary which includes
201 the absolute pathnames of all objects, to standard output.
202 .It Ev LD_TRACE_LOADED_OBJECTS_ALL
203 When set to a nonempty string, causes
205 to expand the summary to indicate which objects caused each object to
207 .It Ev LD_TRACE_LOADED_OBJECTS_FMT1
208 .It Ev LD_TRACE_LOADED_OBJECTS_FMT2
209 When set, these variables are interpreted as format strings a la
211 to customize the trace output and are used by
216 to be operated as a filter more conveniently.
217 If the dependency name starts with string
219 .Ev LD_TRACE_LOADED_OBJECTS_FMT1
221 .Ev LD_TRACE_LOADED_OBJECTS_FMT2
223 The following conversions can be used:
226 The main program's name
230 The value of the environment variable
231 .Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
232 Typically used to print both the names of programs and shared libraries
233 being inspected using
238 The full pathname as determined by
240 library search rules.
242 The library's load address.
249 are recognized and have their usual meaning.
253 will log events such as the loading and unloading of shared objects via
258 will process the filtee dependencies of the loaded objects immediately,
259 instead of postponing it until required.
260 Normally, the filtees are opened at the time of the first symbol resolution
261 from the filter object.
264 If a shared object preloaded by the
266 mechanism contains a public symbol
267 .Dq _rtld_functrace ,
269 will transfer control to this function each time
270 it needs to resolve an unbound function symbol.
271 By returning a non-zero value,
273 can advise the linker to keep tracing the specified
274 combination of caller shared object and called function;
275 returning 0 will prevent
277 to be called for this combination again.
279 When implementing a custom
281 function, be aware of the possibility that
283 might be called for functions called on its behalf,
284 or that multiple threads could enter
287 .Sh DIFFERENCES BETWEEN .1 and .2
288 ABI changes have been made to support TLS allocation and initialization
289 and to give threading libraries a chance to complete initialization of the
290 TCB prior to the calling of the _init() functions for the dynamically loaded
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