1 .\" %%%LICENSE_START(PUBLIC_DOMAIN)
2 .\" This is in the public domain
5 .TH LD.SO 8 2017-05-03 "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
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 it 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).
188 .BI \-\-audit " list"
193 Various environment variables influence the operation of the dynamic linker.
195 .SS Secure-execution mode
196 For security reasons,
197 the effects of some environment variables are voided or modified if
198 the dynamic linker determines that the binary should be
199 run in secure-execution mode.
200 (For details, see the discussion of individual environment variables below.)
201 A binary is executed in secure-execution mode if the
203 entry in the auxiliary vector (see
206 This entry may have a nonzero value for various reasons, including:
208 The process's real and effective user IDs differ,
209 or the real and effective group IDs differ.
210 This typically occurs as a result of executing
211 a set-user-ID or set-group-ID program.
213 A process with a non-root user ID executed a binary that
214 conferred permitted or effective capabilities.
216 A nonzero value may have been set by a Linux Security Module.
218 .SS Environment variables
219 Among the more important environment variables are the following:
221 .BR LD_ASSUME_KERNEL " (since glibc 2.2.3)"
222 Each shared object can inform the dynamic linker of the minimum kernel ABI
223 version that it requires.
224 (This requirement is encoded in an ELF note section that is viewable via
227 .BR NT_GNU_ABI_TAG .)
229 the dynamic linker determines the ABI version of the running kernel and
230 will reject loading shared objects that specify minimum ABI versions
231 that exceed that ABI version.
235 cause the dynamic linker to assume that it is running on a system with
236 a different kernel ABI version.
237 For example, the following command line causes the
238 dynamic linker to assume it is running on Linux 2.2.5 when loading
239 the shared objects required by
244 $ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
248 On systems that provide multiple versions of a shared object
249 (in different directories in the search path) that have
250 different minimum kernel ABI version requirements,
252 can be used to select the version of the object that is used
253 (dependent on the directory search order).
255 Historically, the most common use of the
257 feature was to manually select the older
258 LinuxThreads POSIX threads implementation on systems that provided both
259 LinuxThreads and NPTL
260 (which latter was typically the default on such systems);
264 .BR LD_BIND_NOW " (since glibc 2.1.1)"
265 If set to a nonempty string,
266 causes the dynamic linker to resolve all symbols
267 at program startup instead of deferring function call resolution to the point
268 when they are first referenced.
269 This is useful when using a debugger.
272 A list of directories in which to search for
273 ELF libraries at execution time.
274 The items in the list are separated by either colons or semicolons.
277 environment variable.
279 This variable is ignored in secure-execution mode.
281 Within the pathnames specified in
282 .BR LD_LIBRARY_PATH ,
283 the dynamic linker expands the tokens
288 (or the versions using curly braces around the names)
289 as described above in
290 .IR "Rpath token expansion" .
292 the following would cause a library to be searched for in either the
296 subdirectory below the directory containing the program to be executed:
300 $ \fBLD_LIBRARY_PATH='$ORIGIN/$LIB' prog\fP
304 (Note the use of single quotes, which prevent expansion of
311 A list of additional, user-specified, ELF shared
312 objects to be loaded before all others.
313 The items of the list can be separated by spaces or colons.
314 This can be used to selectively override functions in other shared objects.
315 The objects are searched for using the rules given under DESCRIPTION.
317 In secure-execution mode,
318 preload pathnames containing slashes are ignored.
319 Furthermore, shared objects are preloaded only
320 from the standard search directories and only
321 if they have set-user-ID mode bit enabled (which is not typical).
323 Within the names specified in the
325 list, the dynamic linker understands the tokens
330 (or the versions using curly braces around the names)
331 as described above in
332 .IR "Rpath token expansion" .
333 (See also the discussion of quoting under the description of
334 .BR LD_LIBRARY_PATH .)
335 .\" Tested with the following:
337 .\" LD_PRELOAD='$LIB/libmod.so' LD_LIBRARY_PATH=. ./prog
339 .\" which will preload the libmod.so in 'lib' or 'lib64', using it
340 .\" in preference to the version in '.'.
342 .BR LD_TRACE_LOADED_OBJECTS
343 If set (to any value), causes the program to list its dynamic
344 dependencies, as if run by
346 instead of running normally.
348 Then there are lots of more or less obscure variables,
349 many obsolete or only for internal use.
351 .BR LD_AUDIT " (since glibc 2.4)"
352 A colon-separated list of user-specified, ELF shared objects
353 to be loaded before all others in a separate linker namespace
354 (i.e., one that does not intrude upon the normal symbol bindings that
355 would occur in the process).
356 These objects can be used to audit the operation of the dynamic linker.
359 is ignored in secure-execution mode.
361 The dynamic linker will notify the audit
362 shared objects at so-called auditing checkpoints\(emfor example,
363 loading a new shared object, resolving a symbol,
364 or calling a symbol from another shared object\(emby
365 calling an appropriate function within the audit shared object.
368 The auditing interface is largely compatible with that provided on Solaris,
370 .IR "Linker and Libraries Guide" ,
372 .IR "Runtime Linker Auditing Interface" .
374 Within the names specified in the
376 list, the dynamic linker understands the tokens
381 (or the versions using curly braces around the names)
382 as described above in
383 .IR "Rpath token expansion" .
384 (See also the discussion of quoting under the description of
385 .BR LD_LIBRARY_PATH .)
388 .\" commit 8e9f92e9d5d7737afdacf79b76d98c4c42980508
389 in secure-execution mode,
390 names in the audit list that contain slashes are ignored,
391 and only shared objects in the standard search directories that
392 have the set-user-ID mode bit enabled are loaded.
394 .BR LD_BIND_NOT " (since glibc 2.1.95)"
395 If this environment variable is set to a nonempty string,
396 do not update the GOT (global offset table) and PLT (procedure linkage table)
397 after resolving a function symbol.
398 By combining the use of this variable with
404 one can observe all run-time function bindings.
406 .BR LD_DEBUG " (since glibc 2.1)"
407 Output verbose debugging information about operation of the dynamic linker.
408 The content of this variable is one of more of the following categories,
409 separated by colons, commas, or (if the value is quoted) spaces:
415 in the value of this variable does not run the specified program,
416 and displays a help message about which categories can be specified in this
417 environment variable.
420 Print all debugging information (except
427 Display information about which definition each symbol is bound to.
430 Display progress for input file.
433 Display library search paths.
436 Display relocation processing.
439 Display scope information.
442 Display relocation statistics.
445 Display search paths for each symbol look-up.
448 Determine unused DSOs.
451 Display version dependencies.
456 is ignored in secure-execution mode, unless the file
458 exists (the content of the file is irrelevant).
460 .BR LD_DEBUG_OUTPUT " (since glibc 2.1)"
463 output is written to standard error.
466 is defined, then output is written to the pathname specified by its value,
467 with the suffix "." (dot) followed by the process ID appended to the pathname.
470 is ignored in secure-execution mode.
472 .BR LD_DYNAMIC_WEAK " (since glibc 2.1.91)"
473 By default, when searching shared libraries to resolve a symbol reference,
474 the dynamic linker will resolve to the first definition it finds.
476 Old glibc versions (before 2.2), provided a different behavior:
477 if the linker found a symbol that was weak,
478 it would remember that symbol and
479 keep searching in the remaining shared libraries.
480 If it subsequently found a strong definition of the same symbol,
481 then it would instead use that definition.
482 (If no further symbol was found,
483 then the dynamic linker would use the weak symbol that it initially found.)
485 The old glibc behavior was nonstandard.
486 (Standard practice is that the distinction between
487 weak and strong symbols should have effect only at static link time.)
489 .\" More precisely 2.1.92
490 .\" See weak handling
491 .\" https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
492 .\" To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
493 .\" Subject: weak handling
494 .\" From: Ulrich Drepper <drepper at redhat dot com>
495 .\" Date: 07 Jun 2000 20:08:12 -0700
496 .\" Reply-To: drepper at cygnus dot com (Ulrich Drepper)
497 the dynamic linker was modified to provide the current behavior
498 (which was the behavior that was provided by most other implementations
503 environment variable (with any value) provides
504 the old (nonstandard) glibc behavior,
505 whereby a weak symbol in one shared library may be overridden by
506 a strong symbol subsequently discovered in another shared library.
507 (Note that even when this variable is set,
508 a strong symbol in a shared library will not override
509 a weak definition of the same symbol in the main program.)
513 is ignored in secure-execution mode.
515 .BR LD_HWCAP_MASK " (since glibc 2.1)"
516 Mask for hardware capabilities.
518 .BR LD_ORIGIN_PATH " (since glibc 2.1)"
519 Path where the binary is found.
520 .\" Used only if $ORIGIN can't be determined by normal means
521 .\" (from the origin path saved at load time, or from /proc/self/exe)?
525 is ignored in secure-execution mode.
527 .BR LD_POINTER_GUARD " (glibc from 2.4 to 2.22)"
528 Set to 0 to disable pointer guarding.
529 Any other value enables pointer guarding, which is also the default.
530 Pointer guarding is a security mechanism whereby some pointers to code
531 stored in writable program memory (return addresses saved by
533 or function pointers used by various glibc internals) are mangled
534 semi-randomly to make it more difficult for an attacker to hijack
535 the pointers for use in the event of a buffer overrun or
536 stack-smashing attack.
538 .\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
540 can no longer be used to disable pointer guarding,
541 which is now always enabled.
543 .BR LD_PROFILE " (since glibc 2.1)"
544 The name of a (single) shared object to be profiled,
545 specified either as a pathname or a soname.
546 Profiling output is appended to the file whose name is:
547 "\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
551 is ignored in secure-execution mode.
553 .BR LD_PROFILE_OUTPUT " (since glibc 2.1)"
556 output should be written.
557 If this variable is not defined, or is defined as an empty string,
562 is ignored in secure-execution mode; instead
565 (This detail is relevant only before glibc 2.2.5,
566 since in later glibc versions,
568 is also ignored in secure-execution mode.)
570 .BR LD_SHOW_AUXV " (since glibc 2.1)"
571 If this environment variable is defined (with any value),
572 show the auxiliary array passed up from the kernel (see also
577 is ignored in secure-execution mode.
579 .BR LD_TRACE_PRELINKING " (since glibc 2.4)"
580 If this environment variable is defined,
581 trace prelinking of the object whose name is assigned to
582 this environment variable.
585 to get a list of the objects that might be traced.)
586 If the object name is not recognized,
587 .\" (This is what seems to happen, from experimenting)
588 then all prelinking activity is traced.
590 .BR LD_USE_LOAD_BIAS " (since glibc 2.3.3)"
591 .\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
592 .\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
594 By default (i.e., if this variable is not defined),
595 executables and prelinked
596 shared objects will honor base addresses of their dependent shared objects
597 and (nonprelinked) position-independent executables (PIEs)
598 and other shared objects will not honor them.
601 is defined with the value 1, both executables and PIEs
602 will honor the base addresses.
605 is defined with the value 0,
606 neither executables nor PIEs will honor the base addresses.
608 Since glibc 2.3.3, this variable is ignored in secure-execution mode.
610 .BR LD_VERBOSE " (since glibc 2.1)"
611 If set to a nonempty string,
612 output symbol versioning information about the
614 .B LD_TRACE_LOADED_OBJECTS
615 environment variable has been set.
617 .BR LD_WARN " (since glibc 2.1.3)
618 If set to a nonempty string, warn about unresolved symbols.
620 .BR LD_PREFER_MAP_32BIT_EXEC " (x86-64 only; since glibc 2.23)"
621 According to the Intel Silvermont software optimization guide, for 64-bit
622 applications, branch prediction performance can be negatively impacted
623 when the target of a branch is more than 4\ GB away from the branch.
624 If this environment variable is set (to any value),
626 will first try to map executable pages using the
629 flag, and fall back to mapping without that flag if that attempt fails.
630 NB: MAP_32BIT will map to the low 2\ GB (not 4\ GB) of the address space.
634 reduces the address range available for address space layout
635 randomization (ASLR),
636 .B LD_PREFER_MAP_32BIT_EXEC
637 is always disabled in secure-execution mode.
642 a.out dynamic linker/loader
644 .IR /lib/ld\-linux.so. { 1 , 2 }
645 ELF dynamic linker/loader
648 File containing a compiled list of directories in which to search for
649 shared objects and an ordered list of candidate shared objects.
653 .I /etc/ld.so.preload
654 File containing a whitespace-separated list of ELF shared objects to
655 be loaded before the program.
656 See the discussion of
662 .I /etc/ld.so.preload
663 are employed, the libraries specified by
666 .I /etc/ld.so.preload
667 has a system-wide effect,
668 causing the specified libraries to be preloaded for
669 all programs that are executed on the system.
670 (This is usually undesirable,
671 and is typically employed only as an emergency remedy, for example,
672 as a temporary workaround to a library misconfiguration issue.)
678 .SS Hardware capabilities
679 Some shared objects are compiled using hardware-specific instructions which do
680 not exist on every CPU.
681 Such objects should be installed in directories whose names define the
682 required hardware capabilities, such as
684 The dynamic linker checks these directories against the hardware of the
685 machine and selects the most suitable version of a given shared object.
686 Hardware capability directories can be cascaded to combine CPU features.
687 The list of supported hardware capability names depends on the CPU.
688 The following names are currently recognized:
691 ev4, ev5, ev56, ev6, ev67
694 loongson2e, loongson2f, octeon, octeon2
697 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
698 fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
699 ppc64, smt, spe, ucache, vsx
702 flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
705 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
706 z900, z990, z9-109, z10, zarch
709 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
710 mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
719 .BR capabilities (7),
724 .\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
725 .\" Torvalds, Lars Wirzenius and Mitch D'Souza
726 .\" ld\-linux.so: Roland McGrath, Ulrich Drepper and others.
728 .\" In the above, (libc5) stands for David Engel's ld.so/ld\-linux.so.