1 .\" Copyright (c) 1995 Paul Kranenburg
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38 .Nd run-time link-editor
42 utility is a self-contained shared object providing run-time
43 support for loading and link-editing shared objects into a process'
45 It is also commonly known as the dynamic linker.
46 It uses the data structures
47 contained within dynamically linked programs to determine which shared
48 libraries are needed and loads them using the
52 After all shared libraries have been successfully loaded,
54 proceeds to resolve external references from both the main program and
56 A mechanism is provided for initialization routines
57 to be called on a per-object basis, giving a shared object an opportunity
58 to perform any extra set-up before execution of the program proper begins.
59 This is useful for C++ libraries that contain static constructors.
61 When resolving dependencies for the loaded objects,
63 translates dynamic token strings in rpath and soname.
66 option of the static linker was set when linking the binary,
67 the token expansion is performed at the object load time, see
69 The following strings are recognized now:
70 .Bl -tag -width ".Pa $PLATFORM"
72 Translated to the full path of the loaded object.
74 Translated to the name of the operating system implementation.
76 Translated to the release level of the operating system.
78 Translated to the machine hardware platform.
83 utility itself is loaded by the kernel together with any dynamically-linked
84 program that is to be executed.
85 The kernel transfers control to the
87 After the dynamic linker has finished loading,
88 relocating, and initializing the program and its required shared
89 objects, it transfers control to the entry point of the program.
90 The following search order is used to locate required shared objects:
92 .Bl -enum -offset indent -compact
95 of the referencing object unless that object also contains a
100 of the program unless the referencing object contains a
109 of the referencing object
111 Hints file produced by the
119 directories, unless the referencing object was linked using the
120 .Dq Fl z Ar nodefaultlib
127 recognizes a number of environment variables that can be used to modify
129 On 64-bit architectures, the linker for 32-bit objects recognizes
130 all the environment variables listed below, but is being prefixed with
133 .Ev LD_32_TRACE_LOADED_OBJECTS .
134 .Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
135 .It Ev LD_DUMP_REL_POST
138 will print a table containing all relocations after symbol
139 binding and relocation.
140 .It Ev LD_DUMP_REL_PRE
143 will print a table containing all relocations before symbol
144 binding and relocation.
146 A library replacement list in the same format as
148 For convenience, the characters
152 can be used instead of a space and a newline.
153 This variable is parsed after
155 and will override its entries.
156 This variable is unset for set-user-ID and set-group-ID programs.
157 .It Ev LD_LIBMAP_DISABLE
158 If set, disables the use of
162 This variable is unset for set-user-ID and set-group-ID programs.
163 .It Ev LD_ELF_HINTS_PATH
164 This variable will override the default location of
167 This variable is unset for set-user-ID and set-group-ID programs.
168 .It Ev LD_LIBRARY_PATH
169 A colon separated list of directories, overriding the default search path
170 for shared libraries.
171 This variable is unset for set-user-ID and set-group-ID programs.
172 .It Ev LD_LIBRARY_PATH_RPATH
173 If the variable is specified and has a value starting with
174 any of \'y\', \'Y\' or \'1\' symbols, the path specified by
176 variable is allowed to override the path from
178 for binaries which does not contain
181 For such binaries, when the variable
182 .Ev LD_LIBRARY_PATH_RPATH
184 .Dq Fl z Ar nodefaultlib
185 link-time option is ignored as well.
187 A list of shared libraries, separated by colons and/or white space,
188 to be linked in before any
189 other shared libraries.
190 If the directory is not specified then
191 the directories specified by
193 will be searched first
194 followed by the set of built-in standard directories.
195 This variable is unset for set-user-ID and set-group-ID programs.
196 .It Ev LD_LIBRARY_PATH_FDS
197 A colon separated list of file descriptor numbers for library directories.
198 This is intended for use within
200 sandboxes, when global namespaces such as the filesystem are unavailable.
201 It is consulted just after LD_LIBRARY_PATH.
202 This variable is unset for set-user-ID and set-group-ID programs.
204 When set to a nonempty string, prevents modifications of the PLT slots when
206 As result, each call of the PLT-resolved function is resolved.
207 In combination with debug output, this provides complete account of
208 all bind actions at runtime.
209 This variable is unset for set-user-ID and set-group-ID programs.
211 When set to a nonempty string, causes
213 to relocate all external function calls before starting execution of the
215 Normally, function calls are bound lazily, at the first call
218 increases the start-up time of a program, but it avoids run-time
219 surprises caused by unexpectedly undefined functions.
220 .It Ev LD_TRACE_LOADED_OBJECTS
221 When set to a nonempty string, causes
223 to exit after loading the shared objects and printing a summary which includes
224 the absolute pathnames of all objects, to standard output.
225 .It Ev LD_TRACE_LOADED_OBJECTS_ALL
226 When set to a nonempty string, causes
228 to expand the summary to indicate which objects caused each object to
230 .It Ev LD_TRACE_LOADED_OBJECTS_FMT1
231 .It Ev LD_TRACE_LOADED_OBJECTS_FMT2
232 When set, these variables are interpreted as format strings a la
234 to customize the trace output and are used by
239 to be operated as a filter more conveniently.
240 If the dependency name starts with string
242 .Ev LD_TRACE_LOADED_OBJECTS_FMT1
244 .Ev LD_TRACE_LOADED_OBJECTS_FMT2
246 The following conversions can be used:
249 The main program's name
253 The value of the environment variable
254 .Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
255 Typically used to print both the names of programs and shared libraries
256 being inspected using
261 The full pathname as determined by
263 library search rules.
265 The library's load address.
272 are recognized and have their usual meaning.
276 will log events such as the loading and unloading of shared objects via
281 will process the filtee dependencies of the loaded objects immediately,
282 instead of postponing it until required.
283 Normally, the filtees are opened at the time of the first symbol resolution
284 from the filter object.
286 .Sh DIRECT EXECUTION MODE
288 is typically used implicitly, loaded by the kernel as requested by the
290 program header of the executed binary.
292 also supports a direct execution mode for the dynamic linker.
293 In this mode, the user explicitly executes
295 and provides the path of the program to be linked and executed as
297 This mode allows use of a non-standard dynamic linker for a program
298 activation without changing the binary or without changing
299 the installed dynamic linker.
300 Execution options may be specified.
302 The syntax of the direct invocation is
303 .Bd -ragged -offset indent
304 .Pa /libexec/ld-elf.so.1
310 .Op Ar image arguments
314 .Bl -tag -width indent
321 If this option is specified,
323 is only used to provide the
325 value to the program.
329 references the binary to be activated by
331 It must already be opened in the process when executing
333 If this option is specified,
335 is only used to provide the
337 value to the program.
341 argument specifies a name which does not contain a slash
345 uses the search path provided by the environment variable
347 to find the binary to execute.
349 Display information about this run-time linker binary, then exit.
354 The argument following
356 is interpreted as the path of the binary to execute.
359 In the direct execution mode,
361 emulates verification of the binary execute permission for the
363 This is done to avoid breaking user expectations in naively restricted
364 execution environments.
365 The verification only uses Unix
369 and is naturally prone to race conditions.
370 Environments which rely on such restrictions are weak
371 and breakable on their own.
375 might provide some features or changes in runtime behavior that cannot be
376 easily detected at runtime by checking of the normal exported symbols.
377 Note that it is almost always wrong to verify
378 .Dv __FreeBSD_version
379 in userspace to detect features, either at compile or at run time,
380 because either kernel, or libc, or environment variables could not
384 To solve the problem,
386 exports some feature indicators in the
388 private symbols namespace
389 .Dv FBSDprivate_1.0 .
390 Symbols start with the
393 Current list of defined symbols and corresponding features is:
394 .Bl -tag -width indent
395 .It Dv _rtld_version__FreeBSD_version
396 Symbol exports the value of the
397 .Dv __FreeBSD_version
398 definition as it was provided during the
401 The symbol is always present since the
403 facility was introduced.
404 .It Dv _rtld_version_laddr_offset
409 structure contains the load offset of the shared object.
412 contained the base address of the library.
416 Also it indicates the presence of
418 member of the structure.
421 .Bl -tag -width ".Pa /var/run/ld-elf32.so.hints" -compact
422 .It Pa /var/run/ld-elf.so.hints
424 .It Pa /var/run/ld-elf32.so.hints
425 Hints file for 32-bit binaries on 64-bit system.
426 .It Pa /etc/libmap.conf
427 The libmap configuration file.
428 .It Pa /etc/libmap32.conf
429 The libmap configuration file for 32-bit binaries on 64-bit system.