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:
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.
272 This variable is ignored in secure-execution mode.
274 Within the pathnames specified in
275 .BR LD_LIBRARY_PATH ,
276 the dynamic linker expands the tokens
281 (or the versions using curly braces around the names)
282 as described above in
283 .IR "Rpath token expansion" .
285 the following would cause a library to be searched for in either the
289 subdirectory below the directory containing the program to be executed:
291 $ \fBLD_LIBRARY_PATH='$ORIGIN/$LIB' prog\fP
293 (Note the use of single quotes, which prevent expansion of
300 A list of additional, user-specified, ELF shared
301 objects to be loaded before all others.
302 The items of the list can be separated by spaces or colons.
303 This can be used to selectively override functions in other shared objects.
304 The objects are searched for using the rules given under DESCRIPTION.
306 In secure-execution mode,
307 preload pathnames containing slashes are ignored.
308 Furthermore, shared objects are preloaded only
309 from the standard search directories and and only
310 if they have set-user-ID mode bit enabled (which is not typical).
312 Within the names specified in the
314 list, the dynamic linker understands the tokens
319 (or the versions using curly braces around the names)
320 as described above in
321 .IR "Rpath token expansion" .
322 (See also the discussion of quoting under the description of
323 .BR LD_LIBRARY_PATH .)
324 .\" Tested with the following:
326 .\" LD_PRELOAD='$LIB/libmod.so' LD_LIBRARY_PATH=. ./prog
328 .\" which will preload the libmod.so in 'lib' or 'lib64', using it
329 .\" in preference to the version in '.'.
331 .BR LD_TRACE_LOADED_OBJECTS
332 If set (to any value), causes the program to list its dynamic
333 dependencies, as if run by
335 instead of running normally.
337 Then there are lots of more or less obscure variables,
338 many obsolete or only for internal use.
340 .BR LD_AUDIT " (since glibc 2.4)"
341 A colon-separated list of user-specified, ELF shared objects
342 to be loaded before all others in a separate linker namespace
343 (i.e., one that does not intrude upon the normal symbol bindings that
344 would occur in the process).
345 These objects can be used to audit the operation of the dynamic linker.
348 is ignored in secure-execution mode.
350 The dynamic linker will notify the audit
351 shared objects at so-called auditing checkpoints\(emfor example,
352 loading a new shared object, resolving a symbol,
353 or calling a symbol from another shared object\(emby
354 calling an appropriate function within the audit shared object.
357 The auditing interface is largely compatible with that provided on Solaris,
359 .IR "Linker and Libraries Guide" ,
361 .IR "Runtime Linker Auditing Interface" .
363 Within the names specified in the
365 list, the dynamic linker understands the tokens
370 (or the versions using curly braces around the names)
371 as described above in
372 .IR "Rpath token expansion" .
373 (See also the discussion of quoting under the description of
374 .BR LD_LIBRARY_PATH .)
377 .\" commit 8e9f92e9d5d7737afdacf79b76d98c4c42980508
378 in secure-execution mode,
379 names in the audit list that contain slashes are ignored,
380 and only shared objects in the standard search directories that
381 have the set-user-ID mode bit enabled are loaded.
383 .BR LD_BIND_NOT " (since glibc 2.1.95)"
384 If this environment variable is set to a nonempty string,
385 do not update the GOT (global offset table) and PLT (procedure linkage table)
386 after resolving a function symbol.
387 By combining the use of this variable with
393 one can observe all run-time function bindings.
395 .BR LD_DEBUG " (since glibc 2.1)"
396 Output verbose debugging information about the dynamic linker.
399 print all debugging information,
400 Setting this variable to
402 does not run the specified program,
403 and displays a help message about which categories can be specified in this
404 environment variable.
409 Display information about which definition each symbol is bound to.
412 Display progress for input file.
415 Display library search paths.
418 Display relocation processing.
421 Display scope information.
424 Display relocation statistics.
427 Display search paths for each symbol look-up.
430 Determine unused DSOs.
433 Display version dependencies.
438 can specify multiple categories, separated by colons, commas,
439 or (if the value is quoted) spaces.
443 is ignored in secure-execution mode, unless the file
445 exists (the content of the file is irrelevant).
447 .BR LD_DEBUG_OUTPUT " (since glibc 2.1)"
450 output should be written.
451 The default is standard error.
454 is ignored in secure-execution mode.
456 .BR LD_DYNAMIC_WEAK " (since glibc 2.1.91)"
457 If this environment variable is defined (with any value),
458 allow weak symbols to be overridden (reverting to old glibc behavior).
459 .\" See weak handling
460 .\" https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
461 .\" To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
462 .\" Subject: weak handling
463 .\" From: Ulrich Drepper <drepper at redhat dot com>
464 .\" Date: 07 Jun 2000 20:08:12 -0700
465 .\" Reply-To: drepper at cygnus dot com (Ulrich Drepper)
469 is ignored in secure-execution mode.
471 .BR LD_HWCAP_MASK " (since glibc 2.1)"
472 Mask for hardware capabilities.
474 .BR LD_ORIGIN_PATH " (since glibc 2.1)"
475 Path where the binary is found.
476 .\" Used only if $ORIGIN can't be determined by normal means
477 .\" (from the origin path saved at load time, or from /proc/self/exe)?
481 is ignored in secure-execution mode.
483 .BR LD_POINTER_GUARD " (glibc from 2.4 to 2.22)"
484 Set to 0 to disable pointer guarding.
485 Any other value enables pointer guarding, which is also the default.
486 Pointer guarding is a security mechanism whereby some pointers to code
487 stored in writable program memory (return addresses saved by
489 or function pointers used by various glibc internals) are mangled
490 semi-randomly to make it more difficult for an attacker to hijack
491 the pointers for use in the event of a buffer overrun or
492 stack-smashing attack.
494 .\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
496 can no longer be used to disable pointer guarding,
497 which is now always enabled.
499 .BR LD_PROFILE " (since glibc 2.1)"
500 The name of a (single) shared object to be profiled,
501 specified either as a pathname or a soname.
502 Profiling output is appended to the file whose name is:
503 "\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
505 .BR LD_PROFILE_OUTPUT " (since glibc 2.1)"
508 output should be written.
509 If this variable is not defined, or is defined as an empty string,
514 is ignored in secure-execution mode; instead
518 .BR LD_SHOW_AUXV " (since glibc 2.1)"
519 If this environment variable is defined (with any value),
520 show the auxiliary array passed up from the kernel (see also
525 is ignored in secure-execution mode.
527 .BR LD_TRACE_PRELINKING " (since glibc 2.4)"
528 If this environment variable is defined,
529 trace prelinking of the object whose name is assigned to
530 this environment variable.
533 to get a list of the objects that might be traced.)
534 If the object name is not recognized,
535 .\" (This is what seems to happen, from experimenting)
536 then all prelinking activity is traced.
538 .BR LD_USE_LOAD_BIAS " (since glibc 2.3.3)"
539 .\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
540 .\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
542 By default (i.e., if this variable is not defined),
543 executables and prelinked
544 shared objects will honor base addresses of their dependent shared objects
545 and (nonprelinked) position-independent executables (PIEs)
546 and other shared objects will not honor them.
549 is defined with the value 1, both executables and PIEs
550 will honor the base addresses.
553 is defined with the value 0,
554 neither executables nor PIEs will honor the base addresses.
556 This variable is ignored in secure-execution mode.
558 .BR LD_VERBOSE " (since glibc 2.1)"
559 If set to a nonempty string,
560 output symbol versioning information about the
562 .B LD_TRACE_LOADED_OBJECTS
563 environment variable has been set.
565 .BR LD_WARN " (since glibc 2.1.3)
566 If set to a nonempty string, warn about unresolved symbols.
568 .BR LD_PREFER_MAP_32BIT_EXEC " (x86-64 only; since glibc 2.23)"
569 According to the Intel Silvermont software optimization guide, for 64-bit
570 applications, branch prediction performance can be negatively impacted
571 when the target of a branch is more than 4GB away from the branch.
572 If this environment variable is set (to any value),
574 will first try to map executable pages using the
577 flag, and fall back to mapping without that flag if that attempt fails.
578 NB: MAP_32BIT will map to the low 2GB (not 4GB) of the address space.
582 reduces the address range available for address space layout
583 randomization (ASLR),
584 .B LD_PREFER_MAP_32BIT_EXEC
585 is always disabled in secure-execution mode.
590 a.out dynamic linker/loader
592 .IR /lib/ld\-linux.so. { 1 , 2 }
593 ELF dynamic linker/loader
596 File containing a compiled list of directories in which to search for
597 shared objects and an ordered list of candidate shared objects.
601 .I /etc/ld.so.preload
602 File containing a whitespace-separated list of ELF shared objects to
603 be loaded before the program.
604 See the discussion of
610 .I /etc/ld.so.preload
611 are employed, the libraries specified by
614 .I /etc/ld.so.preload
615 has a system-wide effect,
616 causing the specified libraries to be preloaded for
617 all programs that are executed on the system.
618 (This is usually undesirable,
619 and is typically employed only as an emergency remedy, for example,
620 as a temporary workaround to a library misconfiguration issue.)
626 .SS Hardware capabilities
627 Some shared objects are compiled using hardware-specific instructions which do
628 not exist on every CPU.
629 Such objects should be installed in directories whose names define the
630 required hardware capabilities, such as
632 The dynamic linker checks these directories against the hardware of the
633 machine and selects the most suitable version of a given shared object.
634 Hardware capability directories can be cascaded to combine CPU features.
635 The list of supported hardware capability names depends on the CPU.
636 The following names are currently recognized:
639 ev4, ev5, ev56, ev6, ev67
642 loongson2e, loongson2f, octeon, octeon2
645 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
646 fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
647 ppc64, smt, spe, ucache, vsx
650 flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
653 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
654 z900, z990, z9-109, z10, zarch
657 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
658 mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
667 .BR capabilities (7),
672 .\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
673 .\" Torvalds, Lars Wirzenius and Mitch D'Souza
674 .\" ld-linux.so: Roland McGrath, Ulrich Drepper and others.
676 .\" In the above, (libc5) stands for David Engel's ld.so/ld-linux.so.