1LD.SO(8) Linux Programmer's Manual LD.SO(8)
2
6 ld.so, ld-linux.so - dynamic linker/loader
7
9 The dynamic linker can be run either indirectly by running some dynami‐
10 cally linked program or shared object (in which case no command-line
11 options to the dynamic linker can be passed and, in the ELF case, the
12 dynamic linker which is stored in the .interp section of the program is
13 executed) or directly by running:
14 /lib/ld-linux.so.* [OPTIONS] [PROGRAM [ARGUMENTS]]
16
18 The programs ld.so and ld-linux.so* find and load the shared objects
19 (shared libraries) needed by a program, prepare the program to run, and
20 then run it.
21 Linux binaries require dynamic linking (linking at run time) unless the
23 -static option was given to ld(1) during compilation.
24 The program ld.so handles a.out binaries, a binary format used long
26 ago. The program ld-linux.so* (/lib/ld-linux.so.1 for libc5,
27 /lib/ld-linux.so.2 for glibc2) handles binaries that are in the more
28 modern ELF format. Both programs have the same behavior, and use the
29 same support files and programs (ldd(1), ldconfig(8), and
30 /etc/ld.so.conf).
31 When resolving shared object dependencies, the dynamic linker first in‐
33 spects each dependency string to see if it contains a slash (this can
34 occur if a shared object pathname containing slashes was specified at
35 link time). If a slash is found, then the dependency string is inter‐
36 preted as a (relative or absolute) pathname, and the shared object is
37 loaded using that pathname.
38 If a shared object dependency does not contain a slash, then it is
40 searched for in the following order:
41 o Using the directories specified in the DT_RPATH dynamic section at‐
43 tribute of the binary if present and DT_RUNPATH attribute does not
44 exist. Use of DT_RPATH is deprecated.
45 o Using the environment variable LD_LIBRARY_PATH, unless the exe‐
47 cutable is being run in secure-execution mode (see below), in which
48 case this variable is ignored.
49 o Using the directories specified in the DT_RUNPATH dynamic section
51 attribute of the binary if present. Such directories are searched
52 only to find those objects required by DT_NEEDED (direct dependen‐
53 cies) entries and do not apply to those objects' children, which
54 must themselves have their own DT_RUNPATH entries. This is unlike
55 DT_RPATH, which is applied to searches for all children in the de‐
56 pendency tree.
57 o From the cache file /etc/ld.so.cache, which contains a compiled list
59 of candidate shared objects previously found in the augmented li‐
60 brary path. If, however, the binary was linked with the -z nodeflib
61 linker option, shared objects in the default paths are skipped.
62 Shared objects installed in hardware capability directories (see be‐
63 low) are preferred to other shared objects.
64 o In the default path /lib, and then /usr/lib. (On some 64-bit archi‐
66 tectures, the default paths for 64-bit shared objects are /lib64,
67 and then /usr/lib64.) If the binary was linked with the -z nodeflib
68 linker option, this step is skipped.
69 Dynamic string tokens
71 In several places, the dynamic linker expands dynamic string tokens:
72 o In the environment variables LD_LIBRARY_PATH, LD_PRELOAD, and LD_AU‐
74 DIT,
75 o inside the values of the dynamic section tags DT_NEEDED, DT_RPATH,
77 DT_RUNPATH, DT_AUDIT, and DT_DEPAUDIT of ELF binaries,
78 o in the arguments to the ld.so command line options --audit, --li‐
80 brary-path, and --preload (see below), and
81 o in the filename arguments to the dlopen(3) and dlmopen(3) functions.
83 The substituted tokens are as follows:
85 $ORIGIN (or equivalently ${ORIGIN})
87 This expands to the directory containing the program or shared
88 object. Thus, an application located in somedir/app could be
89 compiled with
90 gcc -Wl,-rpath,'$ORIGIN/../lib'
92 so that it finds an associated shared object in somedir/lib no
94 matter where somedir is located in the directory hierarchy.
95 This facilitates the creation of "turn-key" applications that do
96 not need to be installed into special directories, but can in‐
97 stead be unpacked into any directory and still find their own
98 shared objects.
99 $LIB (or equivalently ${LIB})
101 This expands to lib or lib64 depending on the architecture
102 (e.g., on x86-64, it expands to lib64 and on x86-32, it expands
103 to lib).
104 $PLATFORM (or equivalently ${PLATFORM})
106 This expands to a string corresponding to the processor type of
107 the host system (e.g., "x86_64"). On some architectures, the
108 Linux kernel doesn't provide a platform string to the dynamic
109 linker. The value of this string is taken from the AT_PLATFORM
110 value in the auxiliary vector (see getauxval(3)).
111 Note that the dynamic string tokens have to be quoted properly when set
113 from a shell, to prevent their expansion as shell or environment vari‐
114 ables.
115
117 --argv0 string (since glibc 2.33)
118 Set argv[0] to the value string before running the program.
119 --audit list
121 Use objects named in list as auditors. The objects in list are
122 delimited by colons.
123 --inhibit-cache
125 Do not use /etc/ld.so.cache.
126 --library-path path
128 Use path instead of LD_LIBRARY_PATH environment variable setting
129 (see below). The names ORIGIN, LIB, and PLATFORM are inter‐
130 preted as for the LD_LIBRARY_PATH environment variable.
131 --inhibit-rpath list
133 Ignore RPATH and RUNPATH information in object names in list.
134 This option is ignored when running in secure-execution mode
135 (see below). The objects in list are delimited by colons or
136 spaces.
137 --list List all dependencies and how they are resolved.
139 --preload list (since glibc 2.30)
141 Preload the objects specified in list. The objects in list are
142 delimited by colons or spaces. The objects are preloaded as ex‐
143 plained in the description of the LD_PRELOAD environment vari‐
144 able below.
145 By contrast with LD_PRELOAD, the --preload option provides a way
147 to perform preloading for a single executable without affecting
148 preloading performed in any child process that executes a new
149 program.
150 --verify
152 Verify that program is dynamically linked and this dynamic
153 linker can handle it.
154
156 Various environment variables influence the operation of the dynamic
157 linker.
158 Secure-execution mode
160 For security reasons, if the dynamic linker determines that a binary
161 should be run in secure-execution mode, the effects of some environment
162 variables are voided or modified, and furthermore those environment
163 variables are stripped from the environment, so that the program does
164 not even see the definitions. Some of these environment variables af‐
165 fect the operation of the dynamic linker itself, and are described be‐
166 low. Other environment variables treated in this way include:
167 GCONV_PATH, GETCONF_DIR, HOSTALIASES, LOCALDOMAIN, LOCPATH, MAL‐
168 LOC_TRACE, NIS_PATH, NLSPATH, RESOLV_HOST_CONF, RES_OPTIONS, TMPDIR,
169 and TZDIR.
170 A binary is executed in secure-execution mode if the AT_SECURE entry in
172 the auxiliary vector (see getauxval(3)) has a nonzero value. This en‐
173 try may have a nonzero value for various reasons, including:
174 * The process's real and effective user IDs differ, or the real and
176 effective group IDs differ. This typically occurs as a result of
177 executing a set-user-ID or set-group-ID program.
178 * A process with a non-root user ID executed a binary that conferred
180 capabilities to the process.
181 * A nonzero value may have been set by a Linux Security Module.
183 Environment variables
185 Among the more important environment variables are the following:
186 LD_ASSUME_KERNEL (since glibc 2.2.3)
188 Each shared object can inform the dynamic linker of the minimum
189 kernel ABI version that it requires. (This requirement is en‐
190 coded in an ELF note section that is viewable via readelf -n as
191 a section labeled NT_GNU_ABI_TAG.) At run time, the dynamic
192 linker determines the ABI version of the running kernel and will
193 reject loading shared objects that specify minimum ABI versions
194 that exceed that ABI version.
195 LD_ASSUME_KERNEL can be used to cause the dynamic linker to as‐
197 sume that it is running on a system with a different kernel ABI
198 version. For example, the following command line causes the dy‐
199 namic linker to assume it is running on Linux 2.2.5 when loading
200 the shared objects required by myprog:
201 $ LD_ASSUME_KERNEL=2.2.5 ./myprog
203 On systems that provide multiple versions of a shared object (in
205 different directories in the search path) that have different
206 minimum kernel ABI version requirements, LD_ASSUME_KERNEL can be
207 used to select the version of the object that is used (dependent
208 on the directory search order).
209 Historically, the most common use of the LD_ASSUME_KERNEL fea‐
211 ture was to manually select the older LinuxThreads POSIX threads
212 implementation on systems that provided both LinuxThreads and
213 NPTL (which latter was typically the default on such systems);
214 see pthreads(7).
215 LD_BIND_NOW (since glibc 2.1.1)
217 If set to a nonempty string, causes the dynamic linker to re‐
218 solve all symbols at program startup instead of deferring func‐
219 tion call resolution to the point when they are first refer‐
220 enced. This is useful when using a debugger.
221 LD_LIBRARY_PATH
223 A list of directories in which to search for ELF libraries at
224 execution time. The items in the list are separated by either
225 colons or semicolons, and there is no support for escaping ei‐
226 ther separator. A zero-length directory name indicates the cur‐
227 rent working directory.
228 This variable is ignored in secure-execution mode.
230 Within the pathnames specified in LD_LIBRARY_PATH, the dynamic
232 linker expands the tokens $ORIGIN, $LIB, and $PLATFORM (or the
233 versions using curly braces around the names) as described above
234 in Dynamic string tokens. Thus, for example, the following
235 would cause a library to be searched for in either the lib or
236 lib64 subdirectory below the directory containing the program to
237 be executed:
238 $ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
240 (Note the use of single quotes, which prevent expansion of $ORI‐
242 GIN and $LIB as shell variables!)
243 LD_PRELOAD
245 A list of additional, user-specified, ELF shared objects to be
246 loaded before all others. This feature can be used to selec‐
247 tively override functions in other shared objects.
248 The items of the list can be separated by spaces or colons, and
250 there is no support for escaping either separator. The objects
251 are searched for using the rules given under DESCRIPTION. Ob‐
252 jects are searched for and added to the link map in the left-to-
253 right order specified in the list.
254 In secure-execution mode, preload pathnames containing slashes
256 are ignored. Furthermore, shared objects are preloaded only
257 from the standard search directories and only if they have set-
258 user-ID mode bit enabled (which is not typical).
259 Within the names specified in the LD_PRELOAD list, the dynamic
261 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
262 the versions using curly braces around the names) as described
263 above in Dynamic string tokens. (See also the discussion of
264 quoting under the description of LD_LIBRARY_PATH.)
265 There are various methods of specifying libraries to be pre‐
267 loaded, and these are handled in the following order:
268 (1) The LD_PRELOAD environment variable.
270 (2) The --preload command-line option when invoking the dynamic
272 linker directly.
273 (3) The /etc/ld.so.preload file (described below).
275 LD_TRACE_LOADED_OBJECTS
277 If set (to any value), causes the program to list its dynamic
278 dependencies, as if run by ldd(1), instead of running normally.
279 Then there are lots of more or less obscure variables, many obsolete or
281 only for internal use.
282 LD_AUDIT (since glibc 2.4)
284 A list of user-specified, ELF shared objects to be loaded before
285 all others in a separate linker namespace (i.e., one that does
286 not intrude upon the normal symbol bindings that would occur in
287 the process) These objects can be used to audit the operation of
288 the dynamic linker. The items in the list are colon-separated,
289 and there is no support for escaping the separator.
290 LD_AUDIT is ignored in secure-execution mode.
292 The dynamic linker will notify the audit shared objects at so-
294 called auditing checkpoints—for example, loading a new shared
295 object, resolving a symbol, or calling a symbol from another
296 shared object—by calling an appropriate function within the au‐
297 dit shared object. For details, see rtld-audit(7). The audit‐
298 ing interface is largely compatible with that provided on So‐
299 laris, as described in its Linker and Libraries Guide, in the
300 chapter Runtime Linker Auditing Interface.
301 Within the names specified in the LD_AUDIT list, the dynamic
303 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
304 the versions using curly braces around the names) as described
305 above in Dynamic string tokens. (See also the discussion of
306 quoting under the description of LD_LIBRARY_PATH.)
307 Since glibc 2.13, in secure-execution mode, names in the audit
309 list that contain slashes are ignored, and only shared objects
310 in the standard search directories that have the set-user-ID
311 mode bit enabled are loaded.
312 LD_BIND_NOT (since glibc 2.1.95)
314 If this environment variable is set to a nonempty string, do not
315 update the GOT (global offset table) and PLT (procedure linkage
316 table) after resolving a function symbol. By combining the use
317 of this variable with LD_DEBUG (with the categories bindings and
318 symbols), one can observe all run-time function bindings.
319 LD_DEBUG (since glibc 2.1)
321 Output verbose debugging information about operation of the dy‐
322 namic linker. The content of this variable is one of more of
323 the following categories, separated by colons, commas, or (if
324 the value is quoted) spaces:
325 help Specifying help in the value of this variable does
327 not run the specified program, and displays a help
328 message about which categories can be specified in
329 this environment variable.
330 all Print all debugging information (except statistics
332 and unused; see below).
333 bindings Display information about which definition each sym‐
335 bol is bound to.
336 files Display progress for input file.
338 libs Display library search paths.
340 reloc Display relocation processing.
342 scopes Display scope information.
344 statistics Display relocation statistics.
346 symbols Display search paths for each symbol look-up.
348 unused Determine unused DSOs.
350 versions Display version dependencies.
352 Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution mode,
354 unless the file /etc/suid-debug exists (the content of the file
355 is irrelevant).
356 LD_DEBUG_OUTPUT (since glibc 2.1)
358 By default, LD_DEBUG output is written to standard error. If
359 LD_DEBUG_OUTPUT is defined, then output is written to the path‐
360 name specified by its value, with the suffix "." (dot) followed
361 by the process ID appended to the pathname.
362 LD_DEBUG_OUTPUT is ignored in secure-execution mode.
364 LD_DYNAMIC_WEAK (since glibc 2.1.91)
366 By default, when searching shared libraries to resolve a symbol
367 reference, the dynamic linker will resolve to the first defini‐
368 tion it finds.
369 Old glibc versions (before 2.2), provided a different behavior:
371 if the linker found a symbol that was weak, it would remember
372 that symbol and keep searching in the remaining shared li‐
373 braries. If it subsequently found a strong definition of the
374 same symbol, then it would instead use that definition. (If no
375 further symbol was found, then the dynamic linker would use the
376 weak symbol that it initially found.)
377 The old glibc behavior was nonstandard. (Standard practice is
379 that the distinction between weak and strong symbols should have
380 effect only at static link time.) In glibc 2.2, the dynamic
381 linker was modified to provide the current behavior (which was
382 the behavior that was provided by most other implementations at
383 that time).
384 Defining the LD_DYNAMIC_WEAK environment variable (with any
386 value) provides the old (nonstandard) glibc behavior, whereby a
387 weak symbol in one shared library may be overridden by a strong
388 symbol subsequently discovered in another shared library. (Note
389 that even when this variable is set, a strong symbol in a shared
390 library will not override a weak definition of the same symbol
391 in the main program.)
392 Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored in secure-execu‐
394 tion mode.
395 LD_HWCAP_MASK (since glibc 2.1)
397 Mask for hardware capabilities.
398 LD_ORIGIN_PATH (since glibc 2.1)
400 Path where the binary is found.
401 Since glibc 2.4, LD_ORIGIN_PATH is ignored in secure-execution
403 mode.
404 LD_POINTER_GUARD (glibc from 2.4 to 2.22)
406 Set to 0 to disable pointer guarding. Any other value enables
407 pointer guarding, which is also the default. Pointer guarding
408 is a security mechanism whereby some pointers to code stored in
409 writable program memory (return addresses saved by setjmp(3) or
410 function pointers used by various glibc internals) are mangled
411 semi-randomly to make it more difficult for an attacker to hi‐
412 jack the pointers for use in the event of a buffer overrun or
413 stack-smashing attack. Since glibc 2.23, LD_POINTER_GUARD can
414 no longer be used to disable pointer guarding, which is now al‐
415 ways enabled.
416 LD_PROFILE (since glibc 2.1)
418 The name of a (single) shared object to be profiled, specified
419 either as a pathname or a soname. Profiling output is appended
420 to the file whose name is: "$LD_PROFILE_OUTPUT/$LD_PROFILE.pro‐
421 file".
422 Since glibc 2.2.5, LD_PROFILE is ignored in secure-execution
424 mode.
425 LD_PROFILE_OUTPUT (since glibc 2.1)
427 Directory where LD_PROFILE output should be written. If this
428 variable is not defined, or is defined as an empty string, then
429 the default is /var/tmp.
430 LD_PROFILE_OUTPUT is ignored in secure-execution mode; instead
432 /var/profile is always used. (This detail is relevant only be‐
433 fore glibc 2.2.5, since in later glibc versions, LD_PROFILE is
434 also ignored in secure-execution mode.)
435 LD_SHOW_AUXV (since glibc 2.1)
437 If this environment variable is defined (with any value), show
438 the auxiliary array passed up from the kernel (see also getaux‐
439 val(3)).
440 Since glibc 2.3.4, LD_SHOW_AUXV is ignored in secure-execution
442 mode.
443 LD_TRACE_PRELINKING (since glibc 2.4)
445 If this environment variable is defined, trace prelinking of the
446 object whose name is assigned to this environment variable.
447 (Use ldd(1) to get a list of the objects that might be traced.)
448 If the object name is not recognized, then all prelinking activ‐
449 ity is traced.
450 LD_USE_LOAD_BIAS (since glibc 2.3.3)
452 By default (i.e., if this variable is not defined), executables
453 and prelinked shared objects will honor base addresses of their
454 dependent shared objects and (nonprelinked) position-independent
455 executables (PIEs) and other shared objects will not honor them.
456 If LD_USE_LOAD_BIAS is defined with the value 1, both executa‐
457 bles and PIEs will honor the base addresses. If
458 LD_USE_LOAD_BIAS is defined with the value 0, neither executa‐
459 bles nor PIEs will honor the base addresses.
460 Since glibc 2.3.3, this variable is ignored in secure-execution
462 mode.
463 LD_VERBOSE (since glibc 2.1)
465 If set to a nonempty string, output symbol versioning informa‐
466 tion about the program if the LD_TRACE_LOADED_OBJECTS environ‐
467 ment variable has been set.
468 LD_WARN (since glibc 2.1.3)
470 If set to a nonempty string, warn about unresolved symbols.
471 LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
473 According to the Intel Silvermont software optimization guide,
474 for 64-bit applications, branch prediction performance can be
475 negatively impacted when the target of a branch is more than
476 4 GB away from the branch. If this environment variable is set
477 (to any value), the dynamic linker will first try to map exe‐
478 cutable pages using the mmap(2) MAP_32BIT flag, and fall back to
479 mapping without that flag if that attempt fails. NB: MAP_32BIT
480 will map to the low 2 GB (not 4 GB) of the address space.
481 Because MAP_32BIT reduces the address range available for ad‐
483 dress space layout randomization (ASLR), LD_PRE‐
484 FER_MAP_32BIT_EXEC is always disabled in secure-execution mode.
485
487 /lib/ld.so
488 a.out dynamic linker/loader
489 /lib/ld-linux.so.{1,2}
491 ELF dynamic linker/loader
492 /etc/ld.so.cache
494 File containing a compiled list of directories in which to
495 search for shared objects and an ordered list of candidate
496 shared objects. See ldconfig(8).
497 /etc/ld.so.preload
499 File containing a whitespace-separated list of ELF shared ob‐
500 jects to be loaded before the program. See the discussion of
501 LD_PRELOAD above. If both LD_PRELOAD and /etc/ld.so.preload are
502 employed, the libraries specified by LD_PRELOAD are preloaded
503 first. /etc/ld.so.preload has a system-wide effect, causing the
504 specified libraries to be preloaded for all programs that are
505 executed on the system. (This is usually undesirable, and is
506 typically employed only as an emergency remedy, for example, as
507 a temporary workaround to a library misconfiguration issue.)
508 lib*.so*
510 shared objects
511
513 Hardware capabilities
514 Some shared objects are compiled using hardware-specific instructions
515 which do not exist on every CPU. Such objects should be installed in
516 directories whose names define the required hardware capabilities, such
517 as /usr/lib/sse2/. The dynamic linker checks these directories against
518 the hardware of the machine and selects the most suitable version of a
519 given shared object. Hardware capability directories can be cascaded
520 to combine CPU features. The list of supported hardware capability
521 names depends on the CPU. The following names are currently recog‐
522 nized:
523 Alpha ev4, ev5, ev56, ev6, ev67
525 MIPS loongson2e, loongson2f, octeon, octeon2
527 PowerPC
529 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efp‐
530 double, efpsingle, fpu, ic_snoop, mmu, notb, pa6t, power4,
531 power5, power5+, power6x, ppc32, ppc601, ppc64, smt, spe,
532 ucache, vsx
533 SPARC flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
535 s390 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa,
537 stfle, z900, z990, z9-109, z10, zarch
538 x86 (32-bit only)
540 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586,
541 i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse,
542 sse2, tm
543
545 ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5), ca‐
546 pabilities(7), rtld-audit(7), ldconfig(8), sln(8)
547
549 This page is part of release 5.12 of the Linux man-pages project. A
550 description of the project, information about reporting bugs, and the
551 latest version of this page, can be found at
552 https://www.kernel.org/doc/man-pages/.
553GNU 2021-03-22 LD.SO(8)