1ld.so(8) System Manager'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 (1) Using the directories specified in the DT_RPATH dynamic section
43 attribute of the binary if present and DT_RUNPATH attribute does
44 not exist. Use of DT_RPATH is deprecated.
45 (2) Using the environment variable LD_LIBRARY_PATH, unless the exe‐
47 cutable is being run in secure-execution mode (see below), in
48 which case this variable is ignored.
49 (3) 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 (4) From the cache file /etc/ld.so.cache, which contains a compiled
59 list of candidate shared objects previously found in the augmented
60 library path. If, however, the binary was linked with the -z
61 nodeflib linker option, shared objects in the default paths are
62 skipped. Shared objects installed in hardware capability directo‐
63 ries (see below) are preferred to other shared objects.
64 (5) In the default path /lib, and then /usr/lib. (On some 64-bit ar‐
66 chitectures, the default paths for 64-bit shared objects are
67 /lib64, and then /usr/lib64.) If the binary was linked with the
68 -z nodeflib linker option, this step is skipped.
69 Dynamic string tokens
71 In several places, the dynamic linker expands dynamic string tokens:
72 • In the environment variables LD_LIBRARY_PATH, LD_PRELOAD, and LD_AU‐
74 DIT,
75 • inside the values of the dynamic section tags DT_NEEDED, DT_RPATH,
77 DT_RUNPATH, DT_AUDIT, and DT_DEPAUDIT of ELF binaries,
78 • in the arguments to the ld.so command line options --audit, --li‐
80 brary-path, and --preload (see below), and
81 • 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 --list-tunables (since glibc 2.33)
141 Print the names and values of all tunables, along with the mini‐
142 mum and maximum allowed values.
143 --preload list (since glibc 2.30)
145 Preload the objects specified in list. The objects in list are
146 delimited by colons or spaces. The objects are preloaded as ex‐
147 plained in the description of the LD_PRELOAD environment vari‐
148 able below.
149 By contrast with LD_PRELOAD, the --preload option provides a way
151 to perform preloading for a single executable without affecting
152 preloading performed in any child process that executes a new
153 program.
154 --verify
156 Verify that program is dynamically linked and this dynamic
157 linker can handle it.
158
160 Various environment variables influence the operation of the dynamic
161 linker.
162 Secure-execution mode
164 For security reasons, if the dynamic linker determines that a binary
165 should be run in secure-execution mode, the effects of some environment
166 variables are voided or modified, and furthermore those environment
167 variables are stripped from the environment, so that the program does
168 not even see the definitions. Some of these environment variables af‐
169 fect the operation of the dynamic linker itself, and are described be‐
170 low. Other environment variables treated in this way include:
171 GCONV_PATH, GETCONF_DIR, HOSTALIASES, LOCALDOMAIN, LOCPATH, MAL‐
172 LOC_TRACE, NIS_PATH, NLSPATH, RESOLV_HOST_CONF, RES_OPTIONS, TMPDIR,
173 and TZDIR.
174 A binary is executed in secure-execution mode if the AT_SECURE entry in
176 the auxiliary vector (see getauxval(3)) has a nonzero value. This en‐
177 try may have a nonzero value for various reasons, including:
178 • The process's real and effective user IDs differ, or the real and
180 effective group IDs differ. This typically occurs as a result of
181 executing a set-user-ID or set-group-ID program.
182 • A process with a non-root user ID executed a binary that conferred
184 capabilities to the process.
185 • A nonzero value may have been set by a Linux Security Module.
187 Environment variables
189 Among the more important environment variables are the following:
190 LD_ASSUME_KERNEL (since glibc 2.2.3)
192 Each shared object can inform the dynamic linker of the minimum
193 kernel ABI version that it requires. (This requirement is en‐
194 coded in an ELF note section that is viewable via readelf -n as
195 a section labeled NT_GNU_ABI_TAG.) At run time, the dynamic
196 linker determines the ABI version of the running kernel and will
197 reject loading shared objects that specify minimum ABI versions
198 that exceed that ABI version.
199 LD_ASSUME_KERNEL can be used to cause the dynamic linker to as‐
201 sume that it is running on a system with a different kernel ABI
202 version. For example, the following command line causes the dy‐
203 namic linker to assume it is running on Linux 2.2.5 when loading
204 the shared objects required by myprog:
205 $ LD_ASSUME_KERNEL=2.2.5 ./myprog
207 On systems that provide multiple versions of a shared object (in
209 different directories in the search path) that have different
210 minimum kernel ABI version requirements, LD_ASSUME_KERNEL can be
211 used to select the version of the object that is used (dependent
212 on the directory search order).
213 Historically, the most common use of the LD_ASSUME_KERNEL fea‐
215 ture was to manually select the older LinuxThreads POSIX threads
216 implementation on systems that provided both LinuxThreads and
217 NPTL (which latter was typically the default on such systems);
218 see pthreads(7).
219 LD_BIND_NOW (since glibc 2.1.1)
221 If set to a nonempty string, causes the dynamic linker to re‐
222 solve all symbols at program startup instead of deferring func‐
223 tion call resolution to the point when they are first refer‐
224 enced. This is useful when using a debugger.
225 LD_LIBRARY_PATH
227 A list of directories in which to search for ELF libraries at
228 execution time. The items in the list are separated by either
229 colons or semicolons, and there is no support for escaping ei‐
230 ther separator. A zero-length directory name indicates the cur‐
231 rent working directory.
232 This variable is ignored in secure-execution mode.
234 Within the pathnames specified in LD_LIBRARY_PATH, the dynamic
236 linker expands the tokens $ORIGIN, $LIB, and $PLATFORM (or the
237 versions using curly braces around the names) as described above
238 in Dynamic string tokens. Thus, for example, the following
239 would cause a library to be searched for in either the lib or
240 lib64 subdirectory below the directory containing the program to
241 be executed:
242 $ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
244 (Note the use of single quotes, which prevent expansion of $ORI‐
246 GIN and $LIB as shell variables!)
247 LD_PRELOAD
249 A list of additional, user-specified, ELF shared objects to be
250 loaded before all others. This feature can be used to selec‐
251 tively override functions in other shared objects.
252 The items of the list can be separated by spaces or colons, and
254 there is no support for escaping either separator. The objects
255 are searched for using the rules given under DESCRIPTION. Ob‐
256 jects are searched for and added to the link map in the left-to-
257 right order specified in the list.
258 In secure-execution mode, preload pathnames containing slashes
260 are ignored. Furthermore, shared objects are preloaded only
261 from the standard search directories and only if they have set-
262 user-ID mode bit enabled (which is not typical).
263 Within the names specified in the LD_PRELOAD list, the dynamic
265 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
266 the versions using curly braces around the names) as described
267 above in Dynamic string tokens. (See also the discussion of
268 quoting under the description of LD_LIBRARY_PATH.)
269 There are various methods of specifying libraries to be pre‐
271 loaded, and these are handled in the following order:
272 (1) The LD_PRELOAD environment variable.
274 (2) The --preload command-line option when invoking the dynamic
276 linker directly.
277 (3) The /etc/ld.so.preload file (described below).
279 LD_TRACE_LOADED_OBJECTS
281 If set (to any value), causes the program to list its dynamic
282 dependencies, as if run by ldd(1), instead of running normally.
283 Then there are lots of more or less obscure variables, many obsolete or
285 only for internal use.
286 LD_AUDIT (since glibc 2.4)
288 A list of user-specified, ELF shared objects to be loaded before
289 all others in a separate linker namespace (i.e., one that does
290 not intrude upon the normal symbol bindings that would occur in
291 the process) These objects can be used to audit the operation of
292 the dynamic linker. The items in the list are colon-separated,
293 and there is no support for escaping the separator.
294 LD_AUDIT is ignored in secure-execution mode.
296 The dynamic linker will notify the audit shared objects at so-
298 called auditing checkpoints—for example, loading a new shared
299 object, resolving a symbol, or calling a symbol from another
300 shared object—by calling an appropriate function within the au‐
301 dit shared object. For details, see rtld-audit(7). The audit‐
302 ing interface is largely compatible with that provided on So‐
303 laris, as described in its Linker and Libraries Guide, in the
304 chapter Runtime Linker Auditing Interface.
305 Within the names specified in the LD_AUDIT list, the dynamic
307 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
308 the versions using curly braces around the names) as described
309 above in Dynamic string tokens. (See also the discussion of
310 quoting under the description of LD_LIBRARY_PATH.)
311 Since glibc 2.13, in secure-execution mode, names in the audit
313 list that contain slashes are ignored, and only shared objects
314 in the standard search directories that have the set-user-ID
315 mode bit enabled are loaded.
316 LD_BIND_NOT (since glibc 2.1.95)
318 If this environment variable is set to a nonempty string, do not
319 update the GOT (global offset table) and PLT (procedure linkage
320 table) after resolving a function symbol. By combining the use
321 of this variable with LD_DEBUG (with the categories bindings and
322 symbols), one can observe all run-time function bindings.
323 LD_DEBUG (since glibc 2.1)
325 Output verbose debugging information about operation of the dy‐
326 namic linker. The content of this variable is one of more of
327 the following categories, separated by colons, commas, or (if
328 the value is quoted) spaces:
329 help Specifying help in the value of this variable does
331 not run the specified program, and displays a help
332 message about which categories can be specified in
333 this environment variable.
334 all Print all debugging information (except statistics
336 and unused; see below).
337 bindings Display information about which definition each sym‐
339 bol is bound to.
340 files Display progress for input file.
342 libs Display library search paths.
344 reloc Display relocation processing.
346 scopes Display scope information.
348 statistics Display relocation statistics.
350 symbols Display search paths for each symbol look-up.
352 unused Determine unused DSOs.
354 versions Display version dependencies.
356 Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution mode,
358 unless the file /etc/suid-debug exists (the content of the file
359 is irrelevant).
360 LD_DEBUG_OUTPUT (since glibc 2.1)
362 By default, LD_DEBUG output is written to standard error. If
363 LD_DEBUG_OUTPUT is defined, then output is written to the path‐
364 name specified by its value, with the suffix "." (dot) followed
365 by the process ID appended to the pathname.
366 LD_DEBUG_OUTPUT is ignored in secure-execution mode.
368 LD_DYNAMIC_WEAK (since glibc 2.1.91)
370 By default, when searching shared libraries to resolve a symbol
371 reference, the dynamic linker will resolve to the first defini‐
372 tion it finds.
373 Old glibc versions (before glibc 2.2), provided a different be‐
375 havior: if the linker found a symbol that was weak, it would re‐
376 member that symbol and keep searching in the remaining shared
377 libraries. If it subsequently found a strong definition of the
378 same symbol, then it would instead use that definition. (If no
379 further symbol was found, then the dynamic linker would use the
380 weak symbol that it initially found.)
381 The old glibc behavior was nonstandard. (Standard practice is
383 that the distinction between weak and strong symbols should have
384 effect only at static link time.) In glibc 2.2, the dynamic
385 linker was modified to provide the current behavior (which was
386 the behavior that was provided by most other implementations at
387 that time).
388 Defining the LD_DYNAMIC_WEAK environment variable (with any
390 value) provides the old (nonstandard) glibc behavior, whereby a
391 weak symbol in one shared library may be overridden by a strong
392 symbol subsequently discovered in another shared library. (Note
393 that even when this variable is set, a strong symbol in a shared
394 library will not override a weak definition of the same symbol
395 in the main program.)
396 Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored in secure-execu‐
398 tion mode.
399 LD_HWCAP_MASK (since glibc 2.1)
401 Mask for hardware capabilities.
402 LD_ORIGIN_PATH (since glibc 2.1)
404 Path where the binary is found.
405 Since glibc 2.4, LD_ORIGIN_PATH is ignored in secure-execution
407 mode.
408 LD_POINTER_GUARD (from glibc 2.4 to glibc 2.22)
410 Set to 0 to disable pointer guarding. Any other value enables
411 pointer guarding, which is also the default. Pointer guarding
412 is a security mechanism whereby some pointers to code stored in
413 writable program memory (return addresses saved by setjmp(3) or
414 function pointers used by various glibc internals) are mangled
415 semi-randomly to make it more difficult for an attacker to hi‐
416 jack the pointers for use in the event of a buffer overrun or
417 stack-smashing attack. Since glibc 2.23, LD_POINTER_GUARD can
418 no longer be used to disable pointer guarding, which is now al‐
419 ways enabled.
420 LD_PROFILE (since glibc 2.1)
422 The name of a (single) shared object to be profiled, specified
423 either as a pathname or a soname. Profiling output is appended
424 to the file whose name is: "$LD_PROFILE_OUTPUT/$LD_PROFILE.pro‐
425 file".
426 Since glibc 2.2.5, LD_PROFILE is ignored in secure-execution
428 mode.
429 LD_PROFILE_OUTPUT (since glibc 2.1)
431 Directory where LD_PROFILE output should be written. If this
432 variable is not defined, or is defined as an empty string, then
433 the default is /var/tmp.
434 LD_PROFILE_OUTPUT is ignored in secure-execution mode; instead
436 /var/profile is always used. (This detail is relevant only be‐
437 fore glibc 2.2.5, since in later glibc versions, LD_PROFILE is
438 also ignored in secure-execution mode.)
439 LD_SHOW_AUXV (since glibc 2.1)
441 If this environment variable is defined (with any value), show
442 the auxiliary array passed up from the kernel (see also getaux‐
443 val(3)).
444 Since glibc 2.3.4, LD_SHOW_AUXV is ignored in secure-execution
446 mode.
447 LD_TRACE_PRELINKING (since glibc 2.4)
449 If this environment variable is defined, trace prelinking of the
450 object whose name is assigned to this environment variable.
451 (Use ldd(1) to get a list of the objects that might be traced.)
452 If the object name is not recognized, then all prelinking activ‐
453 ity is traced.
454 LD_USE_LOAD_BIAS (since glibc 2.3.3)
456 By default (i.e., if this variable is not defined), executables
457 and prelinked shared objects will honor base addresses of their
458 dependent shared objects and (nonprelinked) position-independent
459 executables (PIEs) and other shared objects will not honor them.
460 If LD_USE_LOAD_BIAS is defined with the value 1, both executa‐
461 bles and PIEs will honor the base addresses. If
462 LD_USE_LOAD_BIAS is defined with the value 0, neither executa‐
463 bles nor PIEs will honor the base addresses.
464 Since glibc 2.3.3, this variable is ignored in secure-execution
466 mode.
467 LD_VERBOSE (since glibc 2.1)
469 If set to a nonempty string, output symbol versioning informa‐
470 tion about the program if the LD_TRACE_LOADED_OBJECTS environ‐
471 ment variable has been set.
472 LD_WARN (since glibc 2.1.3)
474 If set to a nonempty string, warn about unresolved symbols.
475 LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
477 According to the Intel Silvermont software optimization guide,
478 for 64-bit applications, branch prediction performance can be
479 negatively impacted when the target of a branch is more than
480 4 GB away from the branch. If this environment variable is set
481 (to any value), the dynamic linker will first try to map exe‐
482 cutable pages using the mmap(2) MAP_32BIT flag, and fall back to
483 mapping without that flag if that attempt fails. NB: MAP_32BIT
484 will map to the low 2 GB (not 4 GB) of the address space.
485 Because MAP_32BIT reduces the address range available for ad‐
487 dress space layout randomization (ASLR), LD_PRE‐
488 FER_MAP_32BIT_EXEC is always disabled in secure-execution mode.
489
491 /lib/ld.so
492 a.out dynamic linker/loader
493 /lib/ld-linux.so.{1,2}
495 ELF dynamic linker/loader
496 /etc/ld.so.cache
498 File containing a compiled list of directories in which to
499 search for shared objects and an ordered list of candidate
500 shared objects. See ldconfig(8).
501 /etc/ld.so.preload
503 File containing a whitespace-separated list of ELF shared ob‐
504 jects to be loaded before the program. See the discussion of
505 LD_PRELOAD above. If both LD_PRELOAD and /etc/ld.so.preload are
506 employed, the libraries specified by LD_PRELOAD are preloaded
507 first. /etc/ld.so.preload has a system-wide effect, causing the
508 specified libraries to be preloaded for all programs that are
509 executed on the system. (This is usually undesirable, and is
510 typically employed only as an emergency remedy, for example, as
511 a temporary workaround to a library misconfiguration issue.)
512 lib*.so*
514 shared objects
515
517 Hardware capabilities
518 Some shared objects are compiled using hardware-specific instructions
519 which do not exist on every CPU. Such objects should be installed in
520 directories whose names define the required hardware capabilities, such
521 as /usr/lib/sse2/. The dynamic linker checks these directories against
522 the hardware of the machine and selects the most suitable version of a
523 given shared object. Hardware capability directories can be cascaded
524 to combine CPU features. The list of supported hardware capability
525 names depends on the CPU. The following names are currently recog‐
526 nized:
527 Alpha ev4, ev5, ev56, ev6, ev67
529 MIPS loongson2e, loongson2f, octeon, octeon2
531 PowerPC
533 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efp‐
534 double, efpsingle, fpu, ic_snoop, mmu, notb, pa6t, power4,
535 power5, power5+, power6x, ppc32, ppc601, ppc64, smt, spe,
536 ucache, vsx
537 SPARC flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
539 s390 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa,
541 stfle, z900, z990, z9-109, z10, zarch
542 x86 (32-bit only)
544 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586,
545 i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse,
546 sse2, tm
547
549 ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5), ca‐
550 pabilities(7), rtld-audit(7), ldconfig(8), sln(8)
551Linux man-pages 6.04 2023-02-05 ld.so(8)