1 .\" %%%LICENSE_START(PUBLIC_DOMAIN)
2 .\" This is in the public domain
5 .TH LD.SO 8 2018-04-30 "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:
17 .I /lib/ld\-linux.so.*
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 (\fI/lib/ld\-linux.so.1\fP for libc5, \fI/lib/ld\-linux.so.2\fP for glibc2)
40 which everybody has been using for years now.
41 Otherwise, both have the same behavior, and use the same
42 support files and programs as
48 When resolving shared object dependencies,
49 the dynamic linker first inspects each dependency
50 string to see if it contains a slash (this can occur if
51 a shared object pathname containing slashes was specified at link time).
52 If a slash is found, then the dependency string is interpreted as
53 a (relative or absolute) pathname,
54 and the shared object is loaded using that pathname.
56 If a shared object dependency does not contain a slash,
57 then it is searched for in the following order:
59 Using the directories specified in the
60 DT_RPATH dynamic section attribute
61 of the binary if present and DT_RUNPATH attribute does not exist.
62 Use of DT_RPATH is deprecated.
64 Using the environment variable
66 unless the executable is being run in secure-execution mode (see below),
67 in which case this variable is ignored.
69 Using the directories specified in the
70 DT_RUNPATH dynamic section attribute
71 of the binary if present.
72 Such directories are searched only to
73 find those objects required by DT_NEEDED (direct dependencies) entries
74 and do not apply to those objects' children,
75 which must themselves have their own DT_RUNPATH entries.
76 This is unlike DT_RPATH, which is applied
77 to searches for all children in the dependency tree.
80 .IR /etc/ld.so.cache ,
81 which contains a compiled list of candidate shared objects previously found
82 in the augmented library path.
83 If, however, the binary was linked with the
85 linker option, shared objects in the default paths are skipped.
86 Shared objects installed in hardware capability directories (see below)
87 are preferred to other shared objects.
93 (On some 64-bit architectures, the default paths for 64-bit shared objects are
97 If the binary was linked with the
99 linker option, this step is skipped.
100 .SS Rpath token expansion
103 understands certain token strings in an rpath specification
104 (DT_RPATH or DT_RUNPATH).
105 Those strings are substituted as follows:
107 .IR $ORIGIN " (or equivalently " ${ORIGIN} )
109 the directory containing the program or shared object.
110 Thus, an application located in
112 could be compiled with
114 gcc \-Wl,\-rpath,\(aq$ORIGIN/../lib\(aq
116 so that it finds an associated shared object in
120 is located in the directory hierarchy.
121 This facilitates the creation of "turn-key" applications that
122 do not need to be installed into special directories,
123 but can instead be unpacked into any directory
124 and still find their own shared objects.
126 .IR $LIB " (or equivalently " ${LIB} )
131 depending on the architecture
132 (e.g., on x86-64, it expands to
135 on x86-32, it expands to
138 .IR $PLATFORM " (or equivalently " ${PLATFORM} )
139 This expands to a string corresponding to the processor type
140 of the host system (e.g., "x86_64").
141 On some architectures, the Linux kernel doesn't provide a platform
142 string to the dynamic linker.
143 The value of this string is taken from the
145 value in the auxiliary vector (see
147 .\" To get an idea of the places that $PLATFORM would match,
148 .\" look at the output of the following:
151 .\" LD_LIBRARY_PATH=/tmp/d strace -e open /bin/date 2>&1 | grep /tmp/d
153 .\" ld.so lets names be abbreviated, so $O will work for $ORIGIN;
158 List all dependencies and how they are resolved.
161 Verify that program is dynamically linked and this dynamic linker can handle
164 .B \-\-inhibit\-cache
166 .IR /etc/ld.so.cache .
168 .BI \-\-library\-path " path"
173 environment variable setting (see below).
179 are interpreted as for the
181 environment variable.
183 .BI \-\-inhibit\-rpath " list"
184 Ignore RPATH and RUNPATH information in object names in
186 This option is ignored when running in secure-execution mode (see below).
189 are delimited by colons or spaces.
191 .BI \-\-audit " list"
197 are delimited by colons.
199 Various environment variables influence the operation of the dynamic linker.
201 .SS Secure-execution mode
202 For security reasons,
203 the effects of some environment variables are voided or modified if
204 the dynamic linker determines that the binary should be
205 run in secure-execution mode.
206 (For details, see the discussion of individual environment variables below.)
207 A binary is executed in secure-execution mode if the
209 entry in the auxiliary vector (see
212 This entry may have a nonzero value for various reasons, including:
214 The process's real and effective user IDs differ,
215 or the real and effective group IDs differ.
216 This typically occurs as a result of executing
217 a set-user-ID or set-group-ID program.
219 A process with a non-root user ID executed a binary that
220 conferred capabilities to the process.
222 A nonzero value may have been set by a Linux Security Module.
224 .SS Environment variables
225 Among the more important environment variables are the following:
227 .BR LD_ASSUME_KERNEL " (since glibc 2.2.3)"
228 Each shared object can inform the dynamic linker of the minimum kernel ABI
229 version that it requires.
230 (This requirement is encoded in an ELF note section that is viewable via
233 .BR NT_GNU_ABI_TAG .)
235 the dynamic linker determines the ABI version of the running kernel and
236 will reject loading shared objects that specify minimum ABI versions
237 that exceed that ABI version.
241 cause the dynamic linker to assume that it is running on a system with
242 a different kernel ABI version.
243 For example, the following command line causes the
244 dynamic linker to assume it is running on Linux 2.2.5 when loading
245 the shared objects required by
250 $ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
254 On systems that provide multiple versions of a shared object
255 (in different directories in the search path) that have
256 different minimum kernel ABI version requirements,
258 can be used to select the version of the object that is used
259 (dependent on the directory search order).
261 Historically, the most common use of the
263 feature was to manually select the older
264 LinuxThreads POSIX threads implementation on systems that provided both
265 LinuxThreads and NPTL
266 (which latter was typically the default on such systems);
270 .BR LD_BIND_NOW " (since glibc 2.1.1)"
271 If set to a nonempty string,
272 causes the dynamic linker to resolve all symbols
273 at program startup instead of deferring function call resolution to the point
274 when they are first referenced.
275 This is useful when using a debugger.
278 A list of directories in which to search for
279 ELF libraries at execution time.
280 The items in the list are separated by either colons or semicolons,
281 and there is no support for escaping either separator.
283 This variable is ignored in secure-execution mode.
285 Within the pathnames specified in
286 .BR LD_LIBRARY_PATH ,
287 the dynamic linker expands the tokens
292 (or the versions using curly braces around the names)
293 as described above in
294 .IR "Rpath token expansion" .
296 the following would cause a library to be searched for in either the
300 subdirectory below the directory containing the program to be executed:
304 $ \fBLD_LIBRARY_PATH='$ORIGIN/$LIB' prog\fP
308 (Note the use of single quotes, which prevent expansion of
315 A list of additional, user-specified, ELF shared
316 objects to be loaded before all others.
317 This feature can be used to selectively override functions
318 in other shared objects.
320 The items of the list can be separated by spaces or colons,
321 and there is no support for escaping either separator.
322 The objects are searched for using the rules given under DESCRIPTION.
324 In secure-execution mode,
325 preload pathnames containing slashes are ignored.
326 Furthermore, shared objects are preloaded only
327 from the standard search directories and only
328 if they have set-user-ID mode bit enabled (which is not typical).
330 Within the names specified in the
332 list, the dynamic linker understands the tokens
337 (or the versions using curly braces around the names)
338 as described above in
339 .IR "Rpath token expansion" .
340 (See also the discussion of quoting under the description of
341 .BR LD_LIBRARY_PATH .)
342 .\" Tested with the following:
344 .\" LD_PRELOAD='$LIB/libmod.so' LD_LIBRARY_PATH=. ./prog
346 .\" which will preload the libmod.so in 'lib' or 'lib64', using it
347 .\" in preference to the version in '.'.
349 .BR LD_TRACE_LOADED_OBJECTS
350 If set (to any value), causes the program to list its dynamic
351 dependencies, as if run by
353 instead of running normally.
355 Then there are lots of more or less obscure variables,
356 many obsolete or only for internal use.
358 .BR LD_AUDIT " (since glibc 2.4)"
359 A colon-separated list of user-specified, ELF shared objects
360 to be loaded before all others in a separate linker namespace
361 (i.e., one that does not intrude upon the normal symbol bindings that
362 would occur in the process)
363 and there is no support for escaping the separator.
364 These objects can be used to audit the operation of the dynamic linker.
367 is ignored in secure-execution mode.
369 The dynamic linker will notify the audit
370 shared objects at so-called auditing checkpoints\(emfor example,
371 loading a new shared object, resolving a symbol,
372 or calling a symbol from another shared object\(emby
373 calling an appropriate function within the audit shared object.
376 The auditing interface is largely compatible with that provided on Solaris,
378 .IR "Linker and Libraries Guide" ,
380 .IR "Runtime Linker Auditing Interface" .
382 Within the names specified in the
384 list, the dynamic linker understands the tokens
389 (or the versions using curly braces around the names)
390 as described above in
391 .IR "Rpath token expansion" .
392 (See also the discussion of quoting under the description of
393 .BR LD_LIBRARY_PATH .)
396 .\" commit 8e9f92e9d5d7737afdacf79b76d98c4c42980508
397 in secure-execution mode,
398 names in the audit list that contain slashes are ignored,
399 and only shared objects in the standard search directories that
400 have the set-user-ID mode bit enabled are loaded.
402 .BR LD_BIND_NOT " (since glibc 2.1.95)"
403 If this environment variable is set to a nonempty string,
404 do not update the GOT (global offset table) and PLT (procedure linkage table)
405 after resolving a function symbol.
406 By combining the use of this variable with
412 one can observe all run-time function bindings.
414 .BR LD_DEBUG " (since glibc 2.1)"
415 Output verbose debugging information about operation of the dynamic linker.
416 The content of this variable is one of more of the following categories,
417 separated by colons, commas, or (if the value is quoted) spaces:
423 in the value of this variable does not run the specified program,
424 and displays a help message about which categories can be specified in this
425 environment variable.
428 Print all debugging information (except
435 Display information about which definition each symbol is bound to.
438 Display progress for input file.
441 Display library search paths.
444 Display relocation processing.
447 Display scope information.
450 Display relocation statistics.
453 Display search paths for each symbol look-up.
456 Determine unused DSOs.
459 Display version dependencies.
464 is ignored in secure-execution mode, unless the file
466 exists (the content of the file is irrelevant).
468 .BR LD_DEBUG_OUTPUT " (since glibc 2.1)"
471 output is written to standard error.
474 is defined, then output is written to the pathname specified by its value,
475 with the suffix "." (dot) followed by the process ID appended to the pathname.
478 is ignored in secure-execution mode.
480 .BR LD_DYNAMIC_WEAK " (since glibc 2.1.91)"
481 By default, when searching shared libraries to resolve a symbol reference,
482 the dynamic linker will resolve to the first definition it finds.
484 Old glibc versions (before 2.2), provided a different behavior:
485 if the linker found a symbol that was weak,
486 it would remember that symbol and
487 keep searching in the remaining shared libraries.
488 If it subsequently found a strong definition of the same symbol,
489 then it would instead use that definition.
490 (If no further symbol was found,
491 then the dynamic linker would use the weak symbol that it initially found.)
493 The old glibc behavior was nonstandard.
494 (Standard practice is that the distinction between
495 weak and strong symbols should have effect only at static link time.)
497 .\" More precisely 2.1.92
498 .\" See weak handling
499 .\" https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
500 .\" To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
501 .\" Subject: weak handling
502 .\" From: Ulrich Drepper <drepper at redhat dot com>
503 .\" Date: 07 Jun 2000 20:08:12 -0700
504 .\" Reply-To: drepper at cygnus dot com (Ulrich Drepper)
505 the dynamic linker was modified to provide the current behavior
506 (which was the behavior that was provided by most other implementations
511 environment variable (with any value) provides
512 the old (nonstandard) glibc behavior,
513 whereby a weak symbol in one shared library may be overridden by
514 a strong symbol subsequently discovered in another shared library.
515 (Note that even when this variable is set,
516 a strong symbol in a shared library will not override
517 a weak definition of the same symbol in the main program.)
521 is ignored in secure-execution mode.
523 .BR LD_HWCAP_MASK " (since glibc 2.1)"
524 Mask for hardware capabilities.
526 .BR LD_ORIGIN_PATH " (since glibc 2.1)"
527 Path where the binary is found.
528 .\" Used only if $ORIGIN can't be determined by normal means
529 .\" (from the origin path saved at load time, or from /proc/self/exe)?
533 is ignored in secure-execution mode.
535 .BR LD_POINTER_GUARD " (glibc from 2.4 to 2.22)"
536 Set to 0 to disable pointer guarding.
537 Any other value enables pointer guarding, which is also the default.
538 Pointer guarding is a security mechanism whereby some pointers to code
539 stored in writable program memory (return addresses saved by
541 or function pointers used by various glibc internals) are mangled
542 semi-randomly to make it more difficult for an attacker to hijack
543 the pointers for use in the event of a buffer overrun or
544 stack-smashing attack.
546 .\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
548 can no longer be used to disable pointer guarding,
549 which is now always enabled.
551 .BR LD_PROFILE " (since glibc 2.1)"
552 The name of a (single) shared object to be profiled,
553 specified either as a pathname or a soname.
554 Profiling output is appended to the file whose name is:
555 "\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
559 is ignored in secure-execution mode.
561 .BR LD_PROFILE_OUTPUT " (since glibc 2.1)"
564 output should be written.
565 If this variable is not defined, or is defined as an empty string,
570 is ignored in secure-execution mode; instead
573 (This detail is relevant only before glibc 2.2.5,
574 since in later glibc versions,
576 is also ignored in secure-execution mode.)
578 .BR LD_SHOW_AUXV " (since glibc 2.1)"
579 If this environment variable is defined (with any value),
580 show the auxiliary array passed up from the kernel (see also
585 is ignored in secure-execution mode.
587 .BR LD_TRACE_PRELINKING " (since glibc 2.4)"
588 If this environment variable is defined,
589 trace prelinking of the object whose name is assigned to
590 this environment variable.
593 to get a list of the objects that might be traced.)
594 If the object name is not recognized,
595 .\" (This is what seems to happen, from experimenting)
596 then all prelinking activity is traced.
598 .BR LD_USE_LOAD_BIAS " (since glibc 2.3.3)"
599 .\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
600 .\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
602 By default (i.e., if this variable is not defined),
603 executables and prelinked
604 shared objects will honor base addresses of their dependent shared objects
605 and (nonprelinked) position-independent executables (PIEs)
606 and other shared objects will not honor them.
609 is defined with the value 1, both executables and PIEs
610 will honor the base addresses.
613 is defined with the value 0,
614 neither executables nor PIEs will honor the base addresses.
616 Since glibc 2.3.3, this variable is ignored in secure-execution mode.
618 .BR LD_VERBOSE " (since glibc 2.1)"
619 If set to a nonempty string,
620 output symbol versioning information about the
622 .B LD_TRACE_LOADED_OBJECTS
623 environment variable has been set.
625 .BR LD_WARN " (since glibc 2.1.3)
626 If set to a nonempty string, warn about unresolved symbols.
628 .BR LD_PREFER_MAP_32BIT_EXEC " (x86-64 only; since glibc 2.23)"
629 According to the Intel Silvermont software optimization guide, for 64-bit
630 applications, branch prediction performance can be negatively impacted
631 when the target of a branch is more than 4\ GB away from the branch.
632 If this environment variable is set (to any value),
634 will first try to map executable pages using the
637 flag, and fall back to mapping without that flag if that attempt fails.
638 NB: MAP_32BIT will map to the low 2\ GB (not 4\ GB) of the address space.
642 reduces the address range available for address space layout
643 randomization (ASLR),
644 .B LD_PREFER_MAP_32BIT_EXEC
645 is always disabled in secure-execution mode.
650 a.out dynamic linker/loader
652 .IR /lib/ld\-linux.so. { 1 , 2 }
653 ELF dynamic linker/loader
656 File containing a compiled list of directories in which to search for
657 shared objects and an ordered list of candidate shared objects.
661 .I /etc/ld.so.preload
662 File containing a whitespace-separated list of ELF shared objects to
663 be loaded before the program.
664 See the discussion of
670 .I /etc/ld.so.preload
671 are employed, the libraries specified by
674 .I /etc/ld.so.preload
675 has a system-wide effect,
676 causing the specified libraries to be preloaded for
677 all programs that are executed on the system.
678 (This is usually undesirable,
679 and is typically employed only as an emergency remedy, for example,
680 as a temporary workaround to a library misconfiguration issue.)
686 .SS Hardware capabilities
687 Some shared objects are compiled using hardware-specific instructions which do
688 not exist on every CPU.
689 Such objects should be installed in directories whose names define the
690 required hardware capabilities, such as
692 The dynamic linker checks these directories against the hardware of the
693 machine and selects the most suitable version of a given shared object.
694 Hardware capability directories can be cascaded to combine CPU features.
695 The list of supported hardware capability names depends on the CPU.
696 The following names are currently recognized:
699 ev4, ev5, ev56, ev6, ev67
702 loongson2e, loongson2f, octeon, octeon2
705 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
706 fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
707 ppc64, smt, spe, ucache, vsx
710 flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
713 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
714 z900, z990, z9-109, z10, zarch
717 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
718 mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
727 .BR capabilities (7),
732 .\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
733 .\" Torvalds, Lars Wirzenius and Mitch D'Souza
734 .\" ld\-linux.so: Roland McGrath, Ulrich Drepper and others.
736 .\" In the above, (libc5) stands for David Engel's ld.so/ld\-linux.so.