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 token strings in an rpath specification
98 (DT_RPATH or DT_RUNPATH).
99 Those strings are substituted as follows:
101 .IR $ORIGIN " (or equivalently " ${ORIGIN} )
103 the directory containing the program or shared object.
104 Thus, an application located in
106 could be compiled with
108 gcc \-Wl,\-rpath,\(aq$ORIGIN/../lib\(aq
110 so that it finds an associated shared object in
114 is located in the directory hierarchy.
115 This facilitates the creation of "turn-key" applications that
116 do not need to be installed into special directories,
117 but can instead be unpacked into any directory
118 and still find their own shared objects.
120 .IR $LIB " (or equivalently " ${LIB} )
125 depending on the architecture
126 (e.g., on x86-64, it expands to
129 on x86-32, it expands to
132 .IR $PLATFORM " (or equivalently " ${PLATFORM} )
133 This expands to a string corresponding to the processor type
134 of the host system (e.g., "x86_64").
135 On some architectures, the Linux kernel doesn't provide a platform
136 string to the dynamic linker.
137 The value of this string is taken from the
139 value in the auxiliary vector (see
141 .\" To get an idea of the places that $PLATFORM would match,
142 .\" look at the output of the following:
145 .\" LD_LIBRARY_PATH=/tmp/d strace -e open /bin/date 2>&1 | grep /tmp/d
147 .\" ld.so lets names be abbreviated, so $O will work for $ORIGIN;
152 List all dependencies and how they are resolved.
155 Verify that program is dynamically linked and this dynamic linker can handle
160 .IR /etc/ld.so.cache .
162 .BI \-\-library\-path " path"
167 environment variable setting (see below).
173 are interpreted as for the
175 environment variable.
177 .BI \-\-inhibit\-rpath " list"
178 Ignore RPATH and RUNPATH information in object names in
180 This option is ignored when running in secure-execution mode (see below).
182 .BI \-\-audit " list"
187 Various environment variables influence the operation of the dynamic linker.
189 .SS Secure-execution mode
190 For security reasons,
191 the effects of some environment variables are voided or modified if
192 the dynamic linker determines that the binary should be
193 run in secure-execution mode.
194 (For details, see the discussion of individual environment variables below.)
195 A binary is executed in secure-execution mode if the
197 entry in the auxiliary vector (see
200 This entry may have a nonzero value for various reasons, including:
202 The process's real and effective user IDs differ,
203 or the real and effective group IDs differ.
204 This typically occurs as a result of executing
205 a set-user-ID or set-group-ID program.
207 A process with a non-root user ID executed a binary that
208 conferred permitted or effective capabilities.
210 A nonzero value may have been set by a Linux Security Module.
212 .SS Environment variables
213 Among the more important environment variables are the following:
215 .BR LD_ASSUME_KERNEL " (since glibc 2.2.3)"
216 Each shared object can inform the dynamic linker of the minimum kernel ABI
217 version that it requires.
218 (This requirement is encoded in an ELF note section that is viewable via
221 .BR NT_GNU_ABI_TAG .)
223 the dynamic linker determines the ABI version of the running kernel and
224 will reject loading shared objects that specify minimum ABI versions
225 that exceed that ABI version.
229 cause the dynamic linker to assume that it is running on a system with
230 a different kernel ABI version.
231 For example, the following command line causes the
232 dynamic linker to assume it is running on Linux 2.2.5 when loading
233 the shared objects required by
238 $ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
242 On systems that provide multiple versions of a shared object
243 (in different directories in the search path) that have
244 different minimum kernel ABI version requirements,
246 can be used to select the version of the object that is used
247 (dependent on the directory search order).
248 Historically, the most common use of the
250 feature was to manually select the older
251 LinuxThreads POSIX threads implementation on systems that provided both
252 LinuxThreads and NPTL
253 (which latter was typically the default on such systems);
257 .BR LD_BIND_NOW " (since glibc 2.1.1)"
258 If set to a nonempty string,
259 causes the dynamic linker to resolve all symbols
260 at program startup instead of deferring function call resolution to the point
261 when they are first referenced.
262 This is useful when using a debugger.
265 A list of directories in which to search for
266 ELF libraries at execution-time.
267 The items in the list are separated by either colons or semicolons.
270 environment variable.
271 This variable is ignored in secure-execution mode.
273 Within the pathnames specified in
274 .BR LD_LIBRARY_PATH ,
275 the dynamic linker expands the tokens
280 (or the versions using curly braces around the names)
281 as described above in
282 .IR "Rpath token expansion" .
284 the following would cause a library to be searched for in either the
288 subdirectory below the directory containing the program to be executed:
290 $ \fBLD_LIBRARY_PATH='$ORIGIN/$LIB' prog\fP
292 (Note the use of single quotes, which prevent expansion of
299 A list of additional, user-specified, ELF shared
300 objects to be loaded before all others.
301 The items of the list can be separated by spaces or colons.
302 This can be used to selectively override functions in other shared objects.
303 The objects are searched for using the rules given under DESCRIPTION.
305 In secure-execution mode,
306 preload pathnames containing slashes are ignored,
307 and only shared objects in the standard search directories that
308 have the set-user-ID mode bit enabled are loaded.
310 Within the names specified in the
312 list, the dynamic linker understands the tokens
317 (or the versions using curly braces around the names)
318 as described above in
319 .IR "Rpath token expansion" .
320 (See also the discussion of quoting under the description of
321 .BR LD_LIBRARY_PATH .)
322 .\" Tested with the following:
324 .\" LD_PRELOAD='$LIB/libmod.so' LD_LIBRARY_PATH=. ./prog
326 .\" which will preload the libmod.so in 'lib' or 'lib64', using it
327 .\" in preference to the version in '.'.
329 .BR LD_TRACE_LOADED_OBJECTS
330 If set (to any value), causes the program to list its dynamic
331 dependencies, as if run by
333 instead of running normally.
335 Then there are lots of more or less obscure variables,
336 many obsolete or only for internal use.
338 .BR LD_AUDIT " (since glibc 2.4)"
339 A colon-separated list of user-specified, ELF shared objects
340 to be loaded before all others in a separate linker namespace
341 (i.e., one that does not intrude upon the normal symbol bindings that
342 would occur in the process).
343 These objects can be used to audit the operation of the dynamic linker.
345 is ignored in secure-execution mode.
347 The dynamic linker will notify the audit
348 shared objects at so-called auditing checkpoints\(emfor example,
349 loading a new shared object, resolving a symbol,
350 or calling a symbol from another shared object\(emby
351 calling an appropriate function within the audit shared object.
354 The auditing interface is largely compatible with that provided on Solaris,
356 .IR "Linker and Libraries Guide" ,
358 .IR "Runtime Linker Auditing Interface" .
360 Within the names specified in the
362 list, the dynamic linker understands the tokens
367 (or the versions using curly braces around the names)
368 as described above in
369 .IR "Rpath token expansion" .
370 (See also the discussion of quoting under the description of
371 .BR LD_LIBRARY_PATH .)
374 .\" commit 8e9f92e9d5d7737afdacf79b76d98c4c42980508
375 in secure-execution mode,
376 names in the audit list that contain slashes are ignored,
377 and only shared objects in the standard search directories that
378 have the set-user-ID mode bit enabled are loaded.
380 .BR LD_BIND_NOT " (since glibc 2.1.95)"
381 If this environment variable is set to a nonempty string,
382 do not update the GOT (global offset table) and PLT (procedure linkage table)
383 after resolving a function symbol.
384 By combining the use of this variable with
390 one can observe all run-time function bindings.
392 .BR LD_DEBUG " (since glibc 2.1)"
393 Output verbose debugging information about the dynamic linker.
396 print all debugging information,
397 Setting this variable to
399 does not run the specified program,
400 and displays a help message about which categories can be specified in this
401 environment variable.
406 Display information about which definition each symbol is bound to.
409 Display progress for input file.
412 Display library search paths.
415 Display relocation processing.
418 Display scope information.
421 Display relocation statistics.
424 Display search paths for each symbol look-up.
427 Determine unused DSOs.
430 Display version dependencies.
435 can specify multiple categories, separated by colons, commas,
436 or (if the value is quoted) spaces.
440 is ignored in secure-execution mode, unless the file
442 exists (the content of the file is irrelevant).
444 .BR LD_DEBUG_OUTPUT " (since glibc 2.1)"
447 output should be written.
448 The default is standard error.
450 is ignored in secure-execution mode.
452 .BR LD_DYNAMIC_WEAK " (since glibc 2.1.91)"
453 If this environment variable is defined (with any value),
454 allow weak symbols to be overridden (reverting to old glibc behavior).
455 .\" See weak handling
456 .\" https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
457 .\" To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
458 .\" Subject: weak handling
459 .\" From: Ulrich Drepper <drepper at redhat dot com>
460 .\" Date: 07 Jun 2000 20:08:12 -0700
461 .\" Reply-To: drepper at cygnus dot com (Ulrich Drepper)
464 is ignored in secure-execution mode.
466 .BR LD_HWCAP_MASK " (since glibc 2.1)"
467 Mask for hardware capabilities.
469 .BR LD_ORIGIN_PATH " (since glibc 2.1)"
470 Path where the binary is found.
473 is ignored in secure-execution mode.
474 .\" Used only if $ORIGIN can't be determined by normal means
475 .\" (from the origin path saved at load time, or from /proc/self/exe)?
477 .BR LD_POINTER_GUARD " (glibc from 2.4 to 2.22)"
478 Set to 0 to disable pointer guarding.
479 Any other value enables pointer guarding, which is also the default.
480 Pointer guarding is a security mechanism whereby some pointers to code
481 stored in writable program memory (return addresses saved by
483 or function pointers used by various glibc internals) are mangled
484 semi-randomly to make it more difficult for an attacker to hijack
485 the pointers for use in the event of a buffer overrun or
486 stack-smashing attack.
488 .\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
490 can no longer be used to disable pointer guarding,
491 which is now always enabled.
493 .BR LD_PROFILE " (since glibc 2.1)"
494 The name of a (single) shared object to be profiled,
495 specified either as a pathname or a soname.
496 Profiling output is appended to the file whose name is:
497 "\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
499 .BR LD_PROFILE_OUTPUT " (since glibc 2.1)"
502 output should be written.
503 If this variable is not defined, or is defined as an empty string,
507 is ignored in secure-execution mode; instead
511 .BR LD_SHOW_AUXV " (since glibc 2.1)"
512 If this environment variable is defined (with any value),
513 show the auxiliary array passed up from the kernel (see also
517 is ignored in secure-execution mode.
519 .BR LD_TRACE_PRELINKING " (since glibc 2.4)"
520 If this environment variable is defined,
521 trace prelinking of the object whose name is assigned to
522 this environment variable.
525 to get a list of the objects that might be traced.)
526 If the object name is not recognized,
527 .\" (This is what seems to happen, from experimenting)
528 then all prelinking activity is traced.
530 .BR LD_USE_LOAD_BIAS " (since glibc 2.3.3)"
531 .\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
532 .\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
534 By default (i.e., if this variable is not defined),
535 executables and prelinked
536 shared objects will honor base addresses of their dependent shared objects
537 and (nonprelinked) position-independent executables (PIEs)
538 and other shared objects will not honor them.
541 is defined with the value 1, both executables and PIEs
542 will honor the base addresses.
545 is defined with the value 0,
546 neither executables nor PIEs will honor the base addresses.
547 This variable is ignored in secure-execution mode.
549 .BR LD_VERBOSE " (since glibc 2.1)"
550 If set to a nonempty string,
551 output symbol versioning information about the
553 .B LD_TRACE_LOADED_OBJECTS
554 environment variable has been set.
556 .BR LD_WARN " (since glibc 2.1.3)
557 If set to a nonempty string, warn about unresolved symbols.
559 .BR LD_PREFER_MAP_32BIT_EXEC " (x86-64 only; since glibc 2.23)"
560 According to the Intel Silvermont software optimization guide, for 64-bit
561 applications, branch prediction performance can be negatively impacted
562 when the target of a branch is more than 4GB away from the branch.
563 If this environment variable is set (to any value),
565 will first try to map executable pages using the
568 flag, and fall back to mapping without that flag if that attempt fails.
569 NB: MAP_32BIT will map to the low 2GB (not 4GB) of the address space.
572 reduces the address range available for address space layout
573 randomization (ASLR),
574 .B LD_PREFER_MAP_32BIT_EXEC
575 is always disabled in secure-execution mode.
580 a.out dynamic linker/loader
582 .IR /lib/ld\-linux.so. { 1 , 2 }
583 ELF dynamic linker/loader
586 File containing a compiled list of directories in which to search for
587 shared objects and an ordered list of candidate shared objects.
589 .I /etc/ld.so.preload
590 File containing a whitespace-separated list of ELF shared objects to
591 be loaded before the program.
597 .SS Hardware capabilities
598 Some shared objects are compiled using hardware-specific instructions which do
599 not exist on every CPU.
600 Such objects should be installed in directories whose names define the
601 required hardware capabilities, such as
603 The dynamic linker checks these directories against the hardware of the
604 machine and selects the most suitable version of a given shared object.
605 Hardware capability directories can be cascaded to combine CPU features.
606 The list of supported hardware capability names depends on the CPU.
607 The following names are currently recognized:
610 ev4, ev5, ev56, ev6, ev67
613 loongson2e, loongson2f, octeon, octeon2
616 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
617 fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
618 ppc64, smt, spe, ucache, vsx
621 flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
624 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
625 z900, z990, z9-109, z10, zarch
628 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
629 mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
637 .BR capabilities (7),
642 .\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
643 .\" Torvalds, Lars Wirzenius and Mitch D'Souza
644 .\" ld-linux.so: Roland McGrath, Ulrich Drepper and others.
646 .\" In the above, (libc5) stands for David Engel's ld.so/ld-linux.so.