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 --audit list
118 Use objects named in list as auditors. The objects in list are
119 delimited by colons.
120 --inhibit-cache
122 Do not use /etc/ld.so.cache.
123 --library-path path
125 Use path instead of LD_LIBRARY_PATH environment variable setting
126 (see below). The names ORIGIN, LIB, and PLATFORM are inter‐
127 preted as for the LD_LIBRARY_PATH environment variable.
128 --inhibit-rpath list
130 Ignore RPATH and RUNPATH information in object names in list.
131 This option is ignored when running in secure-execution mode
132 (see below). The objects in list are delimited by colons or
133 spaces.
134 --list List all dependencies and how they are resolved.
136 --preload list (since glibc 2.30)
138 Preload the objects specified in list. The objects in list are
139 delimited by colons or spaces. The objects are preloaded as ex‐
140 plained in the description of the LD_PRELOAD environment vari‐
141 able below.
142 By contrast with LD_PRELOAD, the --preload option provides a way
144 to perform preloading for a single executable without affecting
145 preloading performed in any child process that executes a new
146 program.
147 --verify
149 Verify that program is dynamically linked and this dynamic
150 linker can handle it.
151
153 Various environment variables influence the operation of the dynamic
154 linker.
155 Secure-execution mode
157 For security reasons, if the dynamic linker determines that a binary
158 should be run in secure-execution mode, the effects of some environment
159 variables are voided or modified, and furthermore those environment
160 variables are stripped from the environment, so that the program does
161 not even see the definitions. Some of these environment variables af‐
162 fect the operation of the dynamic linker itself, and are described be‐
163 low. Other environment variables treated in this way include:
164 GCONV_PATH, GETCONF_DIR, HOSTALIASES, LOCALDOMAIN, LOCPATH, MAL‐
165 LOC_TRACE, NIS_PATH, NLSPATH, RESOLV_HOST_CONF, RES_OPTIONS, TMPDIR,
166 and TZDIR.
167 A binary is executed in secure-execution mode if the AT_SECURE entry in
169 the auxiliary vector (see getauxval(3)) has a nonzero value. This en‐
170 try may have a nonzero value for various reasons, including:
171 * The process's real and effective user IDs differ, or the real and
173 effective group IDs differ. This typically occurs as a result of
174 executing a set-user-ID or set-group-ID program.
175 * A process with a non-root user ID executed a binary that conferred
177 capabilities to the process.
178 * A nonzero value may have been set by a Linux Security Module.
180 Environment variables
182 Among the more important environment variables are the following:
183 LD_ASSUME_KERNEL (since glibc 2.2.3)
185 Each shared object can inform the dynamic linker of the minimum
186 kernel ABI version that it requires. (This requirement is en‐
187 coded in an ELF note section that is viewable via readelf -n as
188 a section labeled NT_GNU_ABI_TAG.) At run time, the dynamic
189 linker determines the ABI version of the running kernel and will
190 reject loading shared objects that specify minimum ABI versions
191 that exceed that ABI version.
192 LD_ASSUME_KERNEL can be used to cause the dynamic linker to as‐
194 sume that it is running on a system with a different kernel ABI
195 version. For example, the following command line causes the dy‐
196 namic linker to assume it is running on Linux 2.2.5 when loading
197 the shared objects required by myprog:
198 $ LD_ASSUME_KERNEL=2.2.5 ./myprog
200 On systems that provide multiple versions of a shared object (in
202 different directories in the search path) that have different
203 minimum kernel ABI version requirements, LD_ASSUME_KERNEL can be
204 used to select the version of the object that is used (dependent
205 on the directory search order).
206 Historically, the most common use of the LD_ASSUME_KERNEL fea‐
208 ture was to manually select the older LinuxThreads POSIX threads
209 implementation on systems that provided both LinuxThreads and
210 NPTL (which latter was typically the default on such systems);
211 see pthreads(7).
212 LD_BIND_NOW (since glibc 2.1.1)
214 If set to a nonempty string, causes the dynamic linker to re‐
215 solve all symbols at program startup instead of deferring func‐
216 tion call resolution to the point when they are first refer‐
217 enced. This is useful when using a debugger.
218 LD_LIBRARY_PATH
220 A list of directories in which to search for ELF libraries at
221 execution time. The items in the list are separated by either
222 colons or semicolons, and there is no support for escaping ei‐
223 ther separator. A zero-length directory name indicates the cur‐
224 rent working directory.
225 This variable is ignored in secure-execution mode.
227 Within the pathnames specified in LD_LIBRARY_PATH, the dynamic
229 linker expands the tokens $ORIGIN, $LIB, and $PLATFORM (or the
230 versions using curly braces around the names) as described above
231 in Dynamic string tokens. Thus, for example, the following
232 would cause a library to be searched for in either the lib or
233 lib64 subdirectory below the directory containing the program to
234 be executed:
235 $ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
237 (Note the use of single quotes, which prevent expansion of $ORI‐
239 GIN and $LIB as shell variables!)
240 LD_PRELOAD
242 A list of additional, user-specified, ELF shared objects to be
243 loaded before all others. This feature can be used to selec‐
244 tively override functions in other shared objects.
245 The items of the list can be separated by spaces or colons, and
247 there is no support for escaping either separator. The objects
248 are searched for using the rules given under DESCRIPTION. Ob‐
249 jects are searched for and added to the link map in the left-to-
250 right order specified in the list.
251 In secure-execution mode, preload pathnames containing slashes
253 are ignored. Furthermore, shared objects are preloaded only
254 from the standard search directories and only if they have set-
255 user-ID mode bit enabled (which is not typical).
256 Within the names specified in the LD_PRELOAD list, the dynamic
258 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
259 the versions using curly braces around the names) as described
260 above in Dynamic string tokens. (See also the discussion of
261 quoting under the description of LD_LIBRARY_PATH.)
262 There are various methods of specifying libraries to be pre‐
264 loaded, and these are handled in the following order:
265 (1) The LD_PRELOAD environment variable.
267 (2) The --preload command-line option when invoking the dynamic
269 linker directly.
270 (3) The /etc/ld.so.preload file (described below).
272 LD_TRACE_LOADED_OBJECTS
274 If set (to any value), causes the program to list its dynamic
275 dependencies, as if run by ldd(1), instead of running normally.
276 Then there are lots of more or less obscure variables, many obsolete or
278 only for internal use.
279 LD_AUDIT (since glibc 2.4)
281 A list of user-specified, ELF shared objects to be loaded before
282 all others in a separate linker namespace (i.e., one that does
283 not intrude upon the normal symbol bindings that would occur in
284 the process) These objects can be used to audit the operation of
285 the dynamic linker. The items in the list are colon-separated,
286 and there is no support for escaping the separator.
287 LD_AUDIT is ignored in secure-execution mode.
289 The dynamic linker will notify the audit shared objects at so-
291 called auditing checkpoints—for example, loading a new shared
292 object, resolving a symbol, or calling a symbol from another
293 shared object—by calling an appropriate function within the au‐
294 dit shared object. For details, see rtld-audit(7). The audit‐
295 ing interface is largely compatible with that provided on So‐
296 laris, as described in its Linker and Libraries Guide, in the
297 chapter Runtime Linker Auditing Interface.
298 Within the names specified in the LD_AUDIT list, the dynamic
300 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
301 the versions using curly braces around the names) as described
302 above in Dynamic string tokens. (See also the discussion of
303 quoting under the description of LD_LIBRARY_PATH.)
304 Since glibc 2.13, in secure-execution mode, names in the audit
306 list that contain slashes are ignored, and only shared objects
307 in the standard search directories that have the set-user-ID
308 mode bit enabled are loaded.
309 LD_BIND_NOT (since glibc 2.1.95)
311 If this environment variable is set to a nonempty string, do not
312 update the GOT (global offset table) and PLT (procedure linkage
313 table) after resolving a function symbol. By combining the use
314 of this variable with LD_DEBUG (with the categories bindings and
315 symbols), one can observe all run-time function bindings.
316 LD_DEBUG (since glibc 2.1)
318 Output verbose debugging information about operation of the dy‐
319 namic linker. The content of this variable is one of more of
320 the following categories, separated by colons, commas, or (if
321 the value is quoted) spaces:
322 help Specifying help in the value of this variable does
324 not run the specified program, and displays a help
325 message about which categories can be specified in
326 this environment variable.
327 all Print all debugging information (except statistics
329 and unused; see below).
330 bindings Display information about which definition each sym‐
332 bol is bound to.
333 files Display progress for input file.
335 libs Display library search paths.
337 reloc Display relocation processing.
339 scopes Display scope information.
341 statistics Display relocation statistics.
343 symbols Display search paths for each symbol look-up.
345 unused Determine unused DSOs.
347 versions Display version dependencies.
349 Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution mode,
351 unless the file /etc/suid-debug exists (the content of the file
352 is irrelevant).
353 LD_DEBUG_OUTPUT (since glibc 2.1)
355 By default, LD_DEBUG output is written to standard error. If
356 LD_DEBUG_OUTPUT is defined, then output is written to the path‐
357 name specified by its value, with the suffix "." (dot) followed
358 by the process ID appended to the pathname.
359 LD_DEBUG_OUTPUT is ignored in secure-execution mode.
361 LD_DYNAMIC_WEAK (since glibc 2.1.91)
363 By default, when searching shared libraries to resolve a symbol
364 reference, the dynamic linker will resolve to the first defini‐
365 tion it finds.
366 Old glibc versions (before 2.2), provided a different behavior:
368 if the linker found a symbol that was weak, it would remember
369 that symbol and keep searching in the remaining shared li‐
370 braries. If it subsequently found a strong definition of the
371 same symbol, then it would instead use that definition. (If no
372 further symbol was found, then the dynamic linker would use the
373 weak symbol that it initially found.)
374 The old glibc behavior was nonstandard. (Standard practice is
376 that the distinction between weak and strong symbols should have
377 effect only at static link time.) In glibc 2.2, the dynamic
378 linker was modified to provide the current behavior (which was
379 the behavior that was provided by most other implementations at
380 that time).
381 Defining the LD_DYNAMIC_WEAK environment variable (with any
383 value) provides the old (nonstandard) glibc behavior, whereby a
384 weak symbol in one shared library may be overridden by a strong
385 symbol subsequently discovered in another shared library. (Note
386 that even when this variable is set, a strong symbol in a shared
387 library will not override a weak definition of the same symbol
388 in the main program.)
389 Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored in secure-execu‐
391 tion mode.
392 LD_HWCAP_MASK (since glibc 2.1)
394 Mask for hardware capabilities.
395 LD_ORIGIN_PATH (since glibc 2.1)
397 Path where the binary is found.
398 Since glibc 2.4, LD_ORIGIN_PATH is ignored in secure-execution
400 mode.
401 LD_POINTER_GUARD (glibc from 2.4 to 2.22)
403 Set to 0 to disable pointer guarding. Any other value enables
404 pointer guarding, which is also the default. Pointer guarding
405 is a security mechanism whereby some pointers to code stored in
406 writable program memory (return addresses saved by setjmp(3) or
407 function pointers used by various glibc internals) are mangled
408 semi-randomly to make it more difficult for an attacker to hi‐
409 jack the pointers for use in the event of a buffer overrun or
410 stack-smashing attack. Since glibc 2.23, LD_POINTER_GUARD can
411 no longer be used to disable pointer guarding, which is now al‐
412 ways enabled.
413 LD_PROFILE (since glibc 2.1)
415 The name of a (single) shared object to be profiled, specified
416 either as a pathname or a soname. Profiling output is appended
417 to the file whose name is: "$LD_PROFILE_OUTPUT/$LD_PROFILE.pro‐
418 file".
419 Since glibc 2.2.5, LD_PROFILE is ignored in secure-execution
421 mode.
422 LD_PROFILE_OUTPUT (since glibc 2.1)
424 Directory where LD_PROFILE output should be written. If this
425 variable is not defined, or is defined as an empty string, then
426 the default is /var/tmp.
427 LD_PROFILE_OUTPUT is ignored in secure-execution mode; instead
429 /var/profile is always used. (This detail is relevant only be‐
430 fore glibc 2.2.5, since in later glibc versions, LD_PROFILE is
431 also ignored in secure-execution mode.)
432 LD_SHOW_AUXV (since glibc 2.1)
434 If this environment variable is defined (with any value), show
435 the auxiliary array passed up from the kernel (see also getaux‐
436 val(3)).
437 Since glibc 2.3.4, LD_SHOW_AUXV is ignored in secure-execution
439 mode.
440 LD_TRACE_PRELINKING (since glibc 2.4)
442 If this environment variable is defined, trace prelinking of the
443 object whose name is assigned to this environment variable.
444 (Use ldd(1) to get a list of the objects that might be traced.)
445 If the object name is not recognized, then all prelinking activ‐
446 ity is traced.
447 LD_USE_LOAD_BIAS (since glibc 2.3.3)
449 By default (i.e., if this variable is not defined), executables
450 and prelinked shared objects will honor base addresses of their
451 dependent shared objects and (nonprelinked) position-independent
452 executables (PIEs) and other shared objects will not honor them.
453 If LD_USE_LOAD_BIAS is defined with the value 1, both executa‐
454 bles and PIEs will honor the base addresses. If
455 LD_USE_LOAD_BIAS is defined with the value 0, neither executa‐
456 bles nor PIEs will honor the base addresses.
457 Since glibc 2.3.3, this variable is ignored in secure-execution
459 mode.
460 LD_VERBOSE (since glibc 2.1)
462 If set to a nonempty string, output symbol versioning informa‐
463 tion about the program if the LD_TRACE_LOADED_OBJECTS environ‐
464 ment variable has been set.
465 LD_WARN (since glibc 2.1.3)
467 If set to a nonempty string, warn about unresolved symbols.
468 LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
470 According to the Intel Silvermont software optimization guide,
471 for 64-bit applications, branch prediction performance can be
472 negatively impacted when the target of a branch is more than
473 4 GB away from the branch. If this environment variable is set
474 (to any value), the dynamic linker will first try to map exe‐
475 cutable pages using the mmap(2) MAP_32BIT flag, and fall back to
476 mapping without that flag if that attempt fails. NB: MAP_32BIT
477 will map to the low 2 GB (not 4 GB) of the address space.
478 Because MAP_32BIT reduces the address range available for ad‐
480 dress space layout randomization (ASLR), LD_PRE‐
481 FER_MAP_32BIT_EXEC is always disabled in secure-execution mode.
482
484 /lib/ld.so
485 a.out dynamic linker/loader
486 /lib/ld-linux.so.{1,2}
488 ELF dynamic linker/loader
489 /etc/ld.so.cache
491 File containing a compiled list of directories in which to
492 search for shared objects and an ordered list of candidate
493 shared objects. See ldconfig(8).
494 /etc/ld.so.preload
496 File containing a whitespace-separated list of ELF shared ob‐
497 jects to be loaded before the program. See the discussion of
498 LD_PRELOAD above. If both LD_PRELOAD and /etc/ld.so.preload are
499 employed, the libraries specified by LD_PRELOAD are preloaded
500 first. /etc/ld.so.preload has a system-wide effect, causing the
501 specified libraries to be preloaded for all programs that are
502 executed on the system. (This is usually undesirable, and is
503 typically employed only as an emergency remedy, for example, as
504 a temporary workaround to a library misconfiguration issue.)
505 lib*.so*
507 shared objects
508
510 Hardware capabilities
511 Some shared objects are compiled using hardware-specific instructions
512 which do not exist on every CPU. Such objects should be installed in
513 directories whose names define the required hardware capabilities, such
514 as /usr/lib/sse2/. The dynamic linker checks these directories against
515 the hardware of the machine and selects the most suitable version of a
516 given shared object. Hardware capability directories can be cascaded
517 to combine CPU features. The list of supported hardware capability
518 names depends on the CPU. The following names are currently recog‐
519 nized:
520 Alpha ev4, ev5, ev56, ev6, ev67
522 MIPS loongson2e, loongson2f, octeon, octeon2
524 PowerPC
526 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efp‐
527 double, efpsingle, fpu, ic_snoop, mmu, notb, pa6t, power4,
528 power5, power5+, power6x, ppc32, ppc601, ppc64, smt, spe,
529 ucache, vsx
530 SPARC flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
532 s390 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa,
534 stfle, z900, z990, z9-109, z10, zarch
535 x86 (32-bit only)
537 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586,
538 i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse,
539 sse2, tm
540
542 ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5), ca‐
543 pabilities(7), rtld-audit(7), ldconfig(8), sln(8)
544
546 This page is part of release 5.10 of the Linux man-pages project. A
547 description of the project, information about reporting bugs, and the
548 latest version of this page, can be found at
549 https://www.kernel.org/doc/man-pages/.
550GNU 2020-08-13 LD.SO(8)