1 .\" %%%LICENSE_START(PUBLIC_DOMAIN)
2 .\" This is in the public domain
5 .TH LD.SO 8 2017-03-13 "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 (\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.
74 .IR /etc/ld.so.cache ,
75 which contains a compiled list of candidate shared objects previously found
76 in the augmented library path.
77 If, however, the binary was linked with the
79 linker option, shared objects in the default paths are skipped.
80 Shared objects installed in hardware capability directories (see below)
81 are preferred to other shared objects.
87 (On some 64-bit architectures, the default paths for 64-bit shared objects are
91 If the binary was linked with the
93 linker option, this step is skipped.
94 .SS Rpath token expansion
97 understands certain strings in an rpath specification (DT_RPATH or DT_RUNPATH); those strings are substituted as follows
99 .IR $ORIGIN " (or equivalently " ${ORIGIN} )
101 the directory containing the program or shared object.
102 Thus, an application located in
104 could be compiled with
106 gcc \-Wl,\-rpath,\(aq$ORIGIN/../lib\(aq
108 so that it finds an associated shared object in
112 is located in the directory hierarchy.
113 This facilitates the creation of "turn-key" applications that
114 do not need to be installed into special directories,
115 but can instead be unpacked into any directory
116 and still find their own shared objects.
118 .IR $LIB " (or equivalently " ${LIB} )
123 depending on the architecture
124 (e.g., on x86-64, it expands to
127 on x86-32, it expands to
130 .IR $PLATFORM " (or equivalently " ${PLATFORM} )
131 This expands to a string corresponding to the processor type
132 of the host system (e.g., "x86_64").
133 On some architectures, the Linux kernel doesn't provide a platform
134 string to the dynamic linker.
135 The value of this string is taken from the
137 value in the auxiliary vector (see
139 .\" To get an idea of the places that $PLATFORM would match,
140 .\" look at the output of the following:
143 .\" LD_LIBRARY_PATH=/tmp/d strace -e open /bin/date 2>&1 | grep /tmp/d
145 .\" ld.so lets names be abbreviated, so $O will work for $ORIGIN;
150 List all dependencies and how they are resolved.
153 Verify that program is dynamically linked and this dynamic linker can handle
158 .IR /etc/ld.so.cache .
160 .BI \-\-library\-path " path"
165 environment variable setting (see below).
171 are interpreted as for the
173 environment variable.
175 .BI \-\-inhibit\-rpath " list"
176 Ignore RPATH and RUNPATH information in object names in
178 This option is ignored when running in secure-execution mode (see below).
180 .BI \-\-audit " list"
185 Various environment variables influence the operation of the dynamic linker.
187 .SS Secure-execution mode
188 For security reasons,
189 the effects of some environment variables are voided or modified if
190 the dynamic linker determines that the binary should be
191 run in secure-execution mode.
192 (For details, see the discussion of individual environment variables below.)
193 A binary is executed in secure-execution mode if the
195 entry in the auxiliary vector (see
198 This entry may have a nonzero value for various reasons, including:
200 The process's real and effective user IDs differ,
201 or the real and effective group IDs differ.
202 This typically occurs as a result of executing
203 a set-user-ID or set-group-ID program.
205 A process with a non-root user ID executed a binary that
206 conferred permitted or effective capabilities.
208 A nonzero value may have been set by a Linux Security Module.
210 .SS Environment variables
211 Among the more important environment variables are the following:
213 .BR LD_ASSUME_KERNEL " (since glibc 2.2.3)"
214 Each shared object can inform the dynamic linker of the minimum kernel ABI
215 version that it requires.
216 (This requirement is encoded in an ELF note section that is viewable via
219 .BR NT_GNU_ABI_TAG .)
221 the dynamic linker determines the ABI version of the running kernel and
222 will reject loading shared objects that specify minimum ABI versions
223 that exceed that ABI version.
227 cause the dynamic linker to assume that it is running on a system with
228 a different kernel ABI version.
229 For example, the following command line causes the
230 dynamic linker to assume it is running on Linux 2.2.5 when loading
231 the shared objects required by
236 $ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
240 On systems that provide multiple versions of a shared object
241 (in different directories in the search path) that have
242 different minimum kernel ABI version requirements,
244 can be used to select the version of the object that is used
245 (dependent on the directory search order).
246 Historically, the most common use of the
248 feature was to manually select the older
249 LinuxThreads POSIX threads implementation on systems that provided both
250 LinuxThreads and NPTL
251 (which latter was typically the default on such systems);
255 .BR LD_BIND_NOW " (since glibc 2.1.1)"
256 If set to a nonempty string,
257 causes the dynamic linker to resolve all symbols
258 at program startup instead of deferring function call resolution to the point
259 when they are first referenced.
260 This is useful when using a debugger.
263 A list of directories in which to search for
264 ELF libraries at execution-time.
265 The items in the list are separated by either colons or semicolons.
268 environment variable.
269 This variable is ignored in secure-execution mode.
271 Within the pathnames specified in
272 .BR LD_LIBRARY_PATH ,
273 the dynamic linker expands the strings
278 (or the versions using curly braces around the names)
279 as described above in
280 .IR "Rpath token expansion" .
282 the following would cause a library to be searched for in either the
286 subdirectory below the directory containing the program to be executed:
288 $ \fBLD_LIBRARY_PATH='$ORIGIN/$LIB' prog\fP
290 (Note the use of single quotes, which prevent expansion of
297 A list of additional, user-specified, ELF shared
298 objects to be loaded before all others.
299 The items of the list can be separated by spaces or colons.
300 This can be used to selectively override functions in other shared objects.
301 The objects are searched for using the rules given under DESCRIPTION.
303 In secure-execution mode,
304 preload pathnames containing slashes are ignored,
305 and only shared objects in the standard search directories that
306 have the set-user-ID mode bit enabled are loaded.
308 Within the names specified in the
310 list, the dynamic linker understands the strings
315 (or the versions using curly braces around the names)
316 as described above in
317 .IR "Rpath token expansion" .
318 .\" Tested with the following:
320 .\" LD_PRELOAD='$LIB/libmod.so' LD_LIBRARY_PATH=. ./prog
322 .\" which will reload the libmod.so in 'lib' or 'lib64', using it
323 .\" in preference to the version in '.'.
325 .BR LD_TRACE_LOADED_OBJECTS
326 If set (to any value), causes the program to list its dynamic
327 dependencies, as if run by
329 instead of running normally.
331 Then there are lots of more or less obscure variables,
332 many obsolete or only for internal use.
334 .BR LD_AUDIT " (since glibc 2.4)"
335 A colon-separated list of user-specified, ELF shared objects
336 to be loaded before all others in a separate linker namespace
337 (i.e., one that does not intrude upon the normal symbol bindings that
338 would occur in the process).
339 These objects can be used to audit the operation of the dynamic linker.
341 is ignored in secure-execution mode.
343 The dynamic linker will notify the audit
344 shared objects at so-called auditing checkpoints\(emfor example,
345 loading a new shared object, resolving a symbol,
346 or calling a symbol from another shared object\(emby
347 calling an appropriate function within the audit shared object.
350 The auditing interface is largely compatible with that provided on Solaris,
352 .IR "Linker and Libraries Guide" ,
354 .IR "Runtime Linker Auditing Interface" .
356 Within the names specified in the
358 list, the dynamic linker understands the strings
363 (or the versions using curly braces around the names)
364 as described above in
365 .IR "Rpath token expansion" .
368 .\" commit 8e9f92e9d5d7737afdacf79b76d98c4c42980508
369 in secure-execution mode,
370 names in the audit list that contain slashes are ignored,
371 and only shared objects in the standard search directories that
372 have the set-user-ID mode bit enabled are loaded.
374 .BR LD_BIND_NOT " (since glibc 2.1.95)"
375 If this environment variable is set to a nonempty string,
376 do not update the GOT (global offset table) and PLT (procedure linkage table)
377 after resolving a function symbol.
378 By combining the use of this variable with
384 one can observe all run-time function bindings.
386 .BR LD_DEBUG " (since glibc 2.1)"
387 Output verbose debugging information about the dynamic linker.
390 print all debugging information,
391 Setting this variable to
393 does not run the specified program,
394 and displays a help message about which categories can be specified in this
395 environment variable.
400 Display information about which definition each symbol is bound to.
403 Display progress for input file.
406 Display library search paths.
409 Display relocation processing.
412 Display scope information.
415 Display relocation statistics.
418 Display search paths for each symbol look-up.
421 Determine unused DSOs.
424 Display version dependencies.
429 can specify multiple categories, separated by colons, commas,
430 or (if the value is quoted) spaces.
434 is ignored in secure-execution mode, unless the file
436 exists (the content of the file is irrelevant).
438 .BR LD_DEBUG_OUTPUT " (since glibc 2.1)"
441 output should be written.
442 The default is standard error.
444 is ignored in secure-execution mode.
446 .BR LD_DYNAMIC_WEAK " (since glibc 2.1.91)"
447 If this environment variable is defined (with any value),
448 allow weak symbols to be overridden (reverting to old glibc behavior).
449 .\" See weak handling
450 .\" https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
451 .\" To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
452 .\" Subject: weak handling
453 .\" From: Ulrich Drepper <drepper at redhat dot com>
454 .\" Date: 07 Jun 2000 20:08:12 -0700
455 .\" Reply-To: drepper at cygnus dot com (Ulrich Drepper)
458 is ignored in secure-execution mode.
460 .BR LD_HWCAP_MASK " (since glibc 2.1)"
461 Mask for hardware capabilities.
463 .BR LD_ORIGIN_PATH " (since glibc 2.1)"
464 Path where the binary is found.
467 is ignored in secure-execution mode.
468 .\" Used only if $ORIGIN can't be determined by normal means
469 .\" (from the origin path saved at load time, or from /proc/self/exe)?
471 .BR LD_POINTER_GUARD " (glibc from 2.4 to 2.22)"
472 Set to 0 to disable pointer guarding.
473 Any other value enables pointer guarding, which is also the default.
474 Pointer guarding is a security mechanism whereby some pointers to code
475 stored in writable program memory (return addresses saved by
477 or function pointers used by various glibc internals) are mangled
478 semi-randomly to make it more difficult for an attacker to hijack
479 the pointers for use in the event of a buffer overrun or
480 stack-smashing attack.
482 .\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
484 can no longer be used to disable pointer guarding,
485 which is now always enabled.
487 .BR LD_PROFILE " (since glibc 2.1)"
488 The name of a (single) shared object to be profiled,
489 specified either as a pathname or a soname.
490 Profiling output is appended to the file whose name is:
491 "\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
493 .BR LD_PROFILE_OUTPUT " (since glibc 2.1)"
496 output should be written.
497 If this variable is not defined, or is defined as an empty string,
501 is ignored in secure-execution mode; instead
505 .BR LD_SHOW_AUXV " (since glibc 2.1)"
506 If this environment variable is defined (with any value),
507 show the auxiliary array passed up from the kernel (see also
511 is ignored in secure-execution mode.
513 .BR LD_TRACE_PRELINKING " (since glibc 2.4)"
514 If this environment variable is defined,
515 trace prelinking of the object whose name is assigned to
516 this environment variable.
519 to get a list of the objects that might be traced.)
520 If the object name is not recognized,
521 .\" (This is what seems to happen, from experimenting)
522 then all prelinking activity is traced.
524 .BR LD_USE_LOAD_BIAS " (since glibc 2.3.3)"
525 .\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
526 .\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
528 By default (i.e., if this variable is not defined),
529 executables and prelinked
530 shared objects will honor base addresses of their dependent shared objects
531 and (nonprelinked) position-independent executables (PIEs)
532 and other shared objects will not honor them.
535 is defined with the value 1, both executables and PIEs
536 will honor the base addresses.
539 is defined with the value 0,
540 neither executables nor PIEs will honor the base addresses.
541 This variable is ignored in secure-execution mode.
543 .BR LD_VERBOSE " (since glibc 2.1)"
544 If set to a nonempty string,
545 output symbol versioning information about the
547 .B LD_TRACE_LOADED_OBJECTS
548 environment variable has been set.
550 .BR LD_WARN " (since glibc 2.1.3)
551 If set to a nonempty string, warn about unresolved symbols.
553 .BR LD_PREFER_MAP_32BIT_EXEC " (x86-64 only; since glibc 2.23)"
554 According to the Intel Silvermont software optimization guide, for 64-bit
555 applications, branch prediction performance can be negatively impacted
556 when the target of a branch is more than 4GB away from the branch.
557 If this environment variable is set (to any value),
559 will first try to map executable pages using the
562 flag, and fall back to mapping without that flag if that attempt fails.
563 NB: MAP_32BIT will map to the low 2GB (not 4GB) of the address space.
566 reduces the address range available for address space layout
567 randomization (ASLR),
568 .B LD_PREFER_MAP_32BIT_EXEC
569 is always disabled in secure-execution mode.
574 a.out dynamic linker/loader
576 .IR /lib/ld\-linux.so. { 1 , 2 }
577 ELF dynamic linker/loader
580 File containing a compiled list of directories in which to search for
581 shared objects and an ordered list of candidate shared objects.
583 .I /etc/ld.so.preload
584 File containing a whitespace-separated list of ELF shared objects to
585 be loaded before the program.
591 .SS Hardware capabilities
592 Some shared objects are compiled using hardware-specific instructions which do
593 not exist on every CPU.
594 Such objects should be installed in directories whose names define the
595 required hardware capabilities, such as
597 The dynamic linker checks these directories against the hardware of the
598 machine and selects the most suitable version of a given shared object.
599 Hardware capability directories can be cascaded to combine CPU features.
600 The list of supported hardware capability names depends on the CPU.
601 The following names are currently recognized:
604 ev4, ev5, ev56, ev6, ev67
607 loongson2e, loongson2f, octeon, octeon2
610 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
611 fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
612 ppc64, smt, spe, ucache, vsx
615 flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
618 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
619 z900, z990, z9-109, z10, zarch
622 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
623 mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
631 .BR capabilities (7),
636 .\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
637 .\" Torvalds, Lars Wirzenius and Mitch D'Souza
638 .\" ld-linux.so: Roland McGrath, Ulrich Drepper and others.
640 .\" In the above, (libc5) stands for David Engel's ld.so/ld-linux.so.