1 .\" %%%LICENSE_START(PUBLIC_DOMAIN)
2 .\" This is in the public domain
5 .TH LD.SO 8 2015-08-08 "GNU" "Linux Programmer's Manual"
7 ld.so, ld-linux.so* \- dynamic linker/loader
9 The dynamic linker can be run either indirectly by running some
10 dynamically linked program or shared object
11 (in which case no command-line options
12 to the dynamic linker can be passed and, in the ELF case, the dynamic linker
13 which is stored in the
15 section of the program is executed) or directly by running:
18 [OPTIONS] [PROGRAM [ARGUMENTS]]
24 find and load the shared objects (shared libraries) needed by a program,
25 prepare the program to run, and then run it.
27 Linux binaries require dynamic linking (linking at run time)
36 handles a.out binaries, a format used long ago;
38 handles ELF (\fI/lib/ld-linux.so.1\fP for libc5, \fI/lib/ld-linux.so.2\fP
39 for glibc2), which everybody has been using for years now.
40 Otherwise, both have the same behavior, and use the same
41 support files and programs
47 When resolving shared object dependencies,
48 the dynamic linker first inspects each dependency
49 string to see if it contains a slash (this can occur if
50 a shared object pathname containing slashes was specified at link time).
51 If a slash is found, then the dependency string is interpreted as
52 a (relative or absolute) pathname,
53 and the shared object is loaded using that pathname.
55 If a shared object dependency does not contain a slash,
56 then it is searched for in the following order:
58 (ELF only) Using the directories specified in the
59 DT_RPATH dynamic section attribute
60 of the binary if present and DT_RUNPATH attribute does not exist.
61 Use of DT_RPATH is deprecated.
63 Using the environment variable
65 (unless the executable is being run in secure-execution mode; see below).
66 in which case it is ignored.
68 (ELF only) Using the directories specified in the
69 DT_RUNPATH dynamic section attribute
70 of the binary if present.
73 .IR /etc/ld.so.cache ,
74 which contains a compiled list of candidate shared objects previously found
75 in the augmented library path.
76 If, however, the binary was linked with the
78 linker option, shared objects in the default paths are skipped.
79 Shared objects installed in hardware capability directories (see below)
80 are preferred to other shared objects.
86 (On some 64-bit architectures, the default paths for 64-bit shared objects are
90 If the binary was linked with the
92 linker option, this step is skipped.
93 .SS Rpath token expansion
96 understands certain strings in an rpath specification (DT_RPATH or DT_RUNPATH); those strings are substituted as follows
98 .IR $ORIGIN " (or equivalently " ${ORIGIN} )
100 the directory containing the program or shared object.
101 Thus, an application located in
103 could be compiled with
105 gcc \-Wl,\-rpath,\(aq$ORIGIN/../lib\(aq
107 so that it finds an associated shared object in
111 is located in the directory hierarchy.
112 This facilitates the creation of "turn-key" applications that
113 do not need to be installed into special directories,
114 but can instead be unpacked into any directory
115 and still find their own shared objects.
117 .IR $LIB " (or equivalently " ${LIB} )
122 depending on the architecture
123 (e.g., on x86-64, it expands to
126 on x86-32, it expands to
129 .IR $PLATFORM " (or equivalently " ${PLATFORM} )
130 This expands to a string corresponding to the processor type
131 of the host system (e.g., "x86_64").
132 On some architectures, the Linux kernel doesn't provide a platform
133 string to the dynamic linker.
134 The value of this string is taken from the
136 value in the auxiliary vector (see
138 .\" To get an idea of the places that $PLATFORM would match,
139 .\" look at the output of the following:
142 .\" LD_LIBRARY_PATH=/tmp/d strace -e open /bin/date 2>&1 | grep /tmp/d
144 .\" ld.so lets names be abbreviated, so $O will work for $ORIGIN;
149 List all dependencies and how they are resolved.
152 Verify that program is dynamically linked and this dynamic linker can handle
157 .IR /etc/ld.so.cache .
159 .BI \-\-library\-path " path"
164 environment variable setting (see below).
166 .BI \-\-inhibit\-rpath " list"
167 Ignore RPATH and RUNPATH information in object names in
169 This option is ignored when running in secure-execution mode (see below).
171 .BI \-\-audit " list"
176 Various environment variables influence the operation of the dynamic linker.
178 .SS Secure-execution mode
179 For security reasons,
180 the effects of some environment variables are voided or modified if
181 the dynamic linker determines that the binary should be
182 run in secure-execution mode.
183 This determination is made by checking whether the
185 entry in the auxiliary vector (see
188 This entry may have a nonzero value for various reasons, including:
190 The process's real and effective user IDs differ,
191 or the real and effective group IDs differ.
192 This typically occurs as a result of executing
193 a set-user-ID or set-group-ID program.
195 A process with a non-root user ID executed a binary that
196 conferred permitted or effective capabilities.
198 A nonzero value may have been set by a Linux Security Module.
200 .SS Environment variables
201 Among the more important environment variables are the following:
203 .BR LD_ASSUME_KERNEL " (glibc since 2.2.3)"
204 Each shared object can inform the dynamic linker of the minimum kernel ABI
205 version that it requires.
206 (This requirement is encoded in an ELF note section that is viewable via
209 .BR NT_GNU_ABI_TAG .)
211 the dynamic linker determines the ABI version of the running kernel and
212 will reject loading shared objects that specify minimum ABI versions
213 that exceed that ABI version.
217 cause the dynamic linker to assume that it is running on a system with
218 a different kernel ABI version.
219 For example, the following command line causes the
220 dynamic linker to assume it is running on Linux 2.2.5 when loading
221 the shared objects required by
226 $ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
230 On systems that provide multiple versions of a shared object
231 (in different directories in the search path) that have
232 different minimum kernel ABI version requirements,
234 can be used to select the version of the object that is used
235 (dependent on the directory search order).
236 Historically, the most common use of the
238 feature was to manually select the older
239 LinuxThreads POSIX threads implementation on systems that provided both
240 LinuxThreads and NPTL
241 (which latter was typically the default on such systems);
246 (libc5; glibc since 2.1.1)
247 If set to a nonempty string,
248 causes the dynamic linker to resolve all symbols
249 at program startup instead of deferring function call resolution to the point
250 when they are first referenced.
251 This is useful when using a debugger.
254 A list of directories in which to search for
255 ELF libraries at execution-time.
256 The items in the list are separated by either colons or semicolons.
259 environment variable.
260 This variable is ignored in secure-execution mode.
263 A list of additional, user-specified, ELF shared
264 objects to be loaded before all others.
265 The items of the list can be separated by spaces or colons.
266 This can be used to selectively override functions in other shared objects.
267 The objects are searched for using the rules given under DESCRIPTION.
268 In secure-execution mode,
269 preload pathnames containing slashes are ignored,
270 and shared objects in the standard search directories are loaded
271 only if the set-user-ID mode bit is enabled on the shared object file.
273 .B LD_TRACE_LOADED_OBJECTS
275 If set (to any value), causes the program to list its dynamic
276 dependencies, as if run by
278 instead of running normally.
280 Then there are lots of more or less obscure variables,
281 many obsolete or only for internal use.
283 .B LD_AOUT_LIBRARY_PATH
287 for a.out binaries only.
288 Old versions of ld\-linux.so.1 also supported
289 .BR LD_ELF_LIBRARY_PATH .
295 for a.out binaries only.
296 Old versions of ld\-linux.so.1 also supported
301 A colon-separated list of user-specified, ELF shared objects
302 to be loaded before all others in a separate linker namespace
303 (i.e., one that does not intrude upon the normal symbol bindings that
304 would occur in the process).
305 These objects can be used to audit the operation of the dynamic linker.
307 is ignored in secure-execution mode.
309 The dynamic linker will notify the audit
310 shared objects at so-called auditing checkpoints\(emfor example,
311 loading a new shared object, resolving a symbol,
312 or calling a symbol from another shared object\(emby
313 calling an appropriate function within the audit shared object.
316 The auditing interface is largely compatible with that provided on Solaris,
318 .IR "Linker and Libraries Guide" ,
320 .IR "Runtime Linker Auditing Interface" .
324 Do not update the GOT (global offset table) and PLT (procedure linkage table)
325 after resolving a symbol.
329 Output verbose debugging information about the dynamic linker.
332 prints all debugging information it has, if set to
334 prints a help message about which categories can be specified in this
335 environment variable.
338 is ignored in secure-execution mode, unless the file
340 exists (the content of the file is irrelevant).
346 output should be written.
347 The default is standard error.
349 is ignored in secure-execution mode.
353 Allow weak symbols to be overridden (reverting to old glibc behavior).
354 .\" See weak handling
355 .\" https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
356 .\" To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
357 .\" Subject: weak handling
358 .\" From: Ulrich Drepper <drepper at redhat dot com>
359 .\" Date: 07 Jun 2000 20:08:12 -0700
360 .\" Reply-To: drepper at cygnus dot com (Ulrich Drepper)
363 is ignored in secure-execution mode.
367 Mask for hardware capabilities.
371 Don't ignore the directory in the names of a.out libraries to be loaded.
372 Use of this option is strongly discouraged.
376 Suppress warnings about a.out libraries with incompatible minor
381 Path where the binary is found (for non-set-user-ID programs).
384 is ignored in secure-execution mode.
385 .\" Only used if $ORIGIN can't be determined by normal means
386 .\" (from the origin path saved at load time, or from /proc/self/exe)?
389 (glibc from 2.4 to 2.22)
390 Set to 0 to disable pointer guarding.
391 Any other value enables pointer guarding, which is also the default.
392 Pointer guarding is a security mechanism whereby some pointers to code
393 stored in writable program memory (return addresses saved by
395 or function pointers used by various glibc internals) are mangled
396 semi-randomly to make it more difficult for an attacker to hijack
397 the pointers for use in the event of a buffer overrun or
398 stack-smashing attack.
400 .\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
402 can no longer be used to disable pointer guarding,
403 which is now always enabled.
407 The name of a (single) shared object to be profiled,
408 specified either as a pathname or a soname.
409 Profiling output is appended to the file whose name is:
410 "\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
416 output should be written.
417 If this variable is not defined, or is defined as an empty string,
421 is ignored in secure-execution mode; instead
427 Show auxiliary array passed up from the kernel.
430 is ignored in secure-execution mode.
432 .B LD_TRACE_PRELINKING
434 Trace prelinking of the object whose name is assigned to
435 this environment variable.
438 to get a list of the objects that might be traced.)
439 If the object name is not recognized,
440 .\" (This is what seems to happen, from experimenting)
441 then all prelinking activity is traced.
444 .\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
445 .\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
447 By default (i.e., if this variable is not defined),
448 executables and prelinked
449 shared objects will honor base addresses of their dependent shared objects
450 and (nonprelinked) position-independent executables (PIEs)
451 and other shared objects will not honor them.
454 is defined with the value 1, both executables and PIEs
455 will honor the base addresses.
458 is defined with the value 0,
459 neither executables nor PIEs will honor the base addresses.
460 This variable is ignored in secure-execution mode.
464 If set to a nonempty string,
465 output symbol versioning information about the
467 .B LD_TRACE_LOADED_OBJECTS
468 environment variable has been set.
471 (ELF only)(glibc since 2.1.3)
472 If set to a nonempty string, warn about unresolved symbols.
474 .B LD_PREFER_MAP_32BIT_EXEC
475 (x86-64 only)(glibc since 2.23)
476 According to the Intel Silvermont software optimization guide, for 64-bit
477 applications, branch prediction performance can be negatively impacted
478 when the target of a branch is more than 4GB away from the branch.
479 If this environment variable is set (to any value),
481 will first try to map executable pages using the
484 flag, and fall back to mapping without that flag if that attempt fails.
485 NB: MAP_32BIT will map to the low 2GB (not 2GB) of the address space.
488 reduces the address range available for address space layout
489 randomization (ASLR),
490 .B LD_PREFER_MAP_32BIT_EXEC
491 is always disabled in secure-execution mode.
498 when none is present.
503 a.out dynamic linker/loader
505 .IR /lib/ld\-linux.so. { 1 , 2 }
506 ELF dynamic linker/loader
509 File containing a compiled list of directories in which to search for
510 shared objects and an ordered list of candidate shared objects.
512 .I /etc/ld.so.preload
513 File containing a whitespace-separated list of ELF shared objects to
514 be loaded before the program.
522 functionality is available for executables compiled using libc version
524 ELF functionality is available since Linux 1.1.52 and libc5.
525 .SS Hardware capabilities
526 Some shared objects are compiled using hardware-specific instructions which do
527 not exist on every CPU.
528 Such objects should be installed in directories whose names define the
529 required hardware capabilities, such as
531 The dynamic linker checks these directories against the hardware of the
532 machine and selects the most suitable version of a given shared object.
533 Hardware capability directories can be cascaded to combine CPU features.
534 The list of supported hardware capability names depends on the CPU.
535 The following names are currently recognized:
538 ev4, ev5, ev56, ev6, ev67
541 loongson2e, loongson2f, octeon, octeon2
544 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
545 fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
546 ppc64, smt, spe, ucache, vsx
549 flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
552 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
553 z900, z990, z9-109, z10, zarch
556 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
557 mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
565 .BR capabilities (7),
570 .\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
571 .\" Torvalds, Lars Wirzenius and Mitch D'Souza
572 .\" ld-linux.so: Roland McGrath, Ulrich Drepper and others.
574 .\" In the above, (libc5) stands for David Engel's ld.so/ld-linux.so.