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 format used long ago;
26 ld-linux.so* (/lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2 for
27 glibc2) handles ELF, which everybody has been using for years now.
28 Otherwise, both have the same behavior, and use the same support files
29 and programs as ldd(1), ldconfig(8), and /etc/ld.so.conf.
30 When resolving shared object dependencies, the dynamic linker first
32 inspects each dependency string to see if it contains a slash (this can
33 occur if a shared object pathname containing slashes was specified at
34 link time). If a slash is found, then the dependency string is inter‐
35 preted as a (relative or absolute) pathname, and the shared object is
36 loaded using that pathname.
37 If a shared object dependency does not contain a slash, then it is
39 searched for in the following order:
40 o Using the directories specified in the DT_RPATH dynamic section
42 attribute of the binary if present and DT_RUNPATH attribute does not
43 exist. Use of DT_RPATH is deprecated.
44 o Using the environment variable LD_LIBRARY_PATH, unless the exe‐
46 cutable is being run in secure-execution mode (see below), in which
47 case this variable is ignored.
48 o Using the directories specified in the DT_RUNPATH dynamic section
50 attribute of the binary if present. Such directories are searched
51 only to find those objects required by DT_NEEDED (direct dependen‐
52 cies) entries and do not apply to those objects' children, which
53 must themselves have their own DT_RUNPATH entries. This is unlike
54 DT_RPATH, which is applied to searches for all children in the
55 dependency tree.
56 o From the cache file /etc/ld.so.cache, which contains a compiled list
58 of candidate shared objects previously found in the augmented
59 library path. If, however, the binary was linked with the -z node‐
60 flib linker option, shared objects in the default paths are skipped.
61 Shared objects installed in hardware capability directories (see
62 below) are preferred to other shared objects.
63 o In the default path /lib, and then /usr/lib. (On some 64-bit archi‐
65 tectures, the default paths for 64-bit shared objects are /lib64,
66 and then /usr/lib64.) If the binary was linked with the -z nodeflib
67 linker option, this step is skipped.
68 Rpath token expansion
70 The dynamic linker understands certain token strings in an rpath speci‐
71 fication (DT_RPATH or DT_RUNPATH). Those strings are substituted as
72 follows:
73 $ORIGIN (or equivalently ${ORIGIN})
75 This expands to the directory containing the program or shared
76 object. Thus, an application located in somedir/app could be
77 compiled with
78 gcc -Wl,-rpath,'$ORIGIN/../lib'
80 so that it finds an associated shared object in somedir/lib no
82 matter where somedir is located in the directory hierarchy.
83 This facilitates the creation of "turn-key" applications that do
84 not need to be installed into special directories, but can
85 instead be unpacked into any directory and still find their own
86 shared objects.
87 $LIB (or equivalently ${LIB})
89 This expands to lib or lib64 depending on the architecture
90 (e.g., on x86-64, it expands to lib64 and on x86-32, it expands
91 to lib).
92 $PLATFORM (or equivalently ${PLATFORM})
94 This expands to a string corresponding to the processor type of
95 the host system (e.g., "x86_64"). On some architectures, the
96 Linux kernel doesn't provide a platform string to the dynamic
97 linker. The value of this string is taken from the AT_PLATFORM
98 value in the auxiliary vector (see getauxval(3)).
99
101 --list List all dependencies and how they are resolved.
102 --verify
104 Verify that program is dynamically linked and this dynamic
105 linker can handle it.
106 --inhibit-cache
108 Do not use /etc/ld.so.cache.
109 --library-path path
111 Use path instead of LD_LIBRARY_PATH environment variable setting
112 (see below). The names ORIGIN, LIB, and PLATFORM are inter‐
113 preted as for the LD_LIBRARY_PATH environment variable.
114 --inhibit-rpath list
116 Ignore RPATH and RUNPATH information in object names in list.
117 This option is ignored when running in secure-execution mode
118 (see below).
119 --audit list
121 Use objects named in list as auditors.
122
124 Various environment variables influence the operation of the dynamic
125 linker.
126 Secure-execution mode
128 For security reasons, the effects of some environment variables are
129 voided or modified if the dynamic linker determines that the binary
130 should be run in secure-execution mode. (For details, see the discus‐
131 sion of individual environment variables below.) A binary is executed
132 in secure-execution mode if the AT_SECURE entry in the auxiliary vector
133 (see getauxval(3)) has a nonzero value. This entry may have a nonzero
134 value for various reasons, including:
135 * The process's real and effective user IDs differ, or the real and
137 effective group IDs differ. This typically occurs as a result of
138 executing a set-user-ID or set-group-ID program.
139 * A process with a non-root user ID executed a binary that conferred
141 capabilities to the process.
142 * A nonzero value may have been set by a Linux Security Module.
144 Environment variables
146 Among the more important environment variables are the following:
147 LD_ASSUME_KERNEL (since glibc 2.2.3)
149 Each shared object can inform the dynamic linker of the minimum
150 kernel ABI version that it requires. (This requirement is
151 encoded in an ELF note section that is viewable via readelf -n
152 as a section labeled NT_GNU_ABI_TAG.) At run time, the dynamic
153 linker determines the ABI version of the running kernel and will
154 reject loading shared objects that specify minimum ABI versions
155 that exceed that ABI version.
156 LD_ASSUME_KERNEL can be used to cause the dynamic linker to
158 assume that it is running on a system with a different kernel
159 ABI version. For example, the following command line causes the
160 dynamic linker to assume it is running on Linux 2.2.5 when load‐
161 ing the shared objects required by myprog:
162 $ LD_ASSUME_KERNEL=2.2.5 ./myprog
164 On systems that provide multiple versions of a shared object (in
166 different directories in the search path) that have different
167 minimum kernel ABI version requirements, LD_ASSUME_KERNEL can be
168 used to select the version of the object that is used (dependent
169 on the directory search order).
170 Historically, the most common use of the LD_ASSUME_KERNEL fea‐
172 ture was to manually select the older LinuxThreads POSIX threads
173 implementation on systems that provided both LinuxThreads and
174 NPTL (which latter was typically the default on such systems);
175 see pthreads(7).
176 LD_BIND_NOW (since glibc 2.1.1)
178 If set to a nonempty string, causes the dynamic linker to
179 resolve all symbols at program startup instead of deferring
180 function call resolution to the point when they are first refer‐
181 enced. This is useful when using a debugger.
182 LD_LIBRARY_PATH
184 A list of directories in which to search for ELF libraries at
185 execution time. The items in the list are separated by either
186 colons or semicolons, and there is no support for escaping
187 either separator.
188 This variable is ignored in secure-execution mode.
190 Within the pathnames specified in LD_LIBRARY_PATH, the dynamic
192 linker expands the tokens $ORIGIN, $LIB, and $PLATFORM (or the
193 versions using curly braces around the names) as described above
194 in Rpath token expansion. Thus, for example, the following
195 would cause a library to be searched for in either the lib or
196 lib64 subdirectory below the directory containing the program to
197 be executed:
198 $ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
200 (Note the use of single quotes, which prevent expansion of $ORI‐
202 GIN and $LIB as shell variables!)
203 LD_PRELOAD
205 A list of additional, user-specified, ELF shared objects to be
206 loaded before all others. The items of the list can be sepa‐
207 rated by spaces or colons, and there is no support for escaping
208 either separator. This can be used to selectively override
209 functions in other shared objects. The objects are searched for
210 using the rules given under DESCRIPTION.
211 In secure-execution mode, preload pathnames containing slashes
213 are ignored. Furthermore, shared objects are preloaded only
214 from the standard search directories and only if they have set-
215 user-ID mode bit enabled (which is not typical).
216 Within the names specified in the LD_PRELOAD list, the dynamic
218 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
219 the versions using curly braces around the names) as described
220 above in Rpath token expansion. (See also the discussion of
221 quoting under the description of LD_LIBRARY_PATH.)
222 LD_TRACE_LOADED_OBJECTS
224 If set (to any value), causes the program to list its dynamic
225 dependencies, as if run by ldd(1), instead of running normally.
226 Then there are lots of more or less obscure variables, many obsolete or
228 only for internal use.
229 LD_AUDIT (since glibc 2.4)
231 A colon-separated list of user-specified, ELF shared objects to
232 be loaded before all others in a separate linker namespace
233 (i.e., one that does not intrude upon the normal symbol bindings
234 that would occur in the process) and there is no support for
235 escaping the separator. These objects can be used to audit the
236 operation of the dynamic linker.
237 LD_AUDIT is ignored in secure-execution mode.
239 The dynamic linker will notify the audit shared objects at so-
241 called auditing checkpoints—for example, loading a new shared
242 object, resolving a symbol, or calling a symbol from another
243 shared object—by calling an appropriate function within the
244 audit shared object. For details, see rtld-audit(7). The
245 auditing interface is largely compatible with that provided on
246 Solaris, as described in its Linker and Libraries Guide, in the
247 chapter Runtime Linker Auditing Interface.
248 Within the names specified in the LD_AUDIT list, the dynamic
250 linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
251 the versions using curly braces around the names) as described
252 above in Rpath token expansion. (See also the discussion of
253 quoting under the description of LD_LIBRARY_PATH.)
254 Since glibc 2.13, in secure-execution mode, names in the audit
256 list that contain slashes are ignored, and only shared objects
257 in the standard search directories that have the set-user-ID
258 mode bit enabled are loaded.
259 LD_BIND_NOT (since glibc 2.1.95)
261 If this environment variable is set to a nonempty string, do not
262 update the GOT (global offset table) and PLT (procedure linkage
263 table) after resolving a function symbol. By combining the use
264 of this variable with LD_DEBUG (with the categories bindings and
265 symbols), one can observe all run-time function bindings.
266 LD_DEBUG (since glibc 2.1)
268 Output verbose debugging information about operation of the
269 dynamic linker. The content of this variable is one of more of
270 the following categories, separated by colons, commas, or (if
271 the value is quoted) spaces:
272 help Specifying help in the value of this variable does
274 not run the specified program, and displays a help
275 message about which categories can be specified in
276 this environment variable.
277 all Print all debugging information (except statistics
279 and unused; see below).
280 bindings Display information about which definition each sym‐
282 bol is bound to.
283 files Display progress for input file.
285 libs Display library search paths.
287 reloc Display relocation processing.
289 scopes Display scope information.
291 statistics Display relocation statistics.
293 symbols Display search paths for each symbol look-up.
295 unused Determine unused DSOs.
297 versions Display version dependencies.
299 Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution mode,
301 unless the file /etc/suid-debug exists (the content of the file
302 is irrelevant).
303 LD_DEBUG_OUTPUT (since glibc 2.1)
305 By default, LD_DEBUG output is written to standard error. If
306 LD_DEBUG_OUTPUT is defined, then output is written to the path‐
307 name specified by its value, with the suffix "." (dot) followed
308 by the process ID appended to the pathname.
309 LD_DEBUG_OUTPUT is ignored in secure-execution mode.
311 LD_DYNAMIC_WEAK (since glibc 2.1.91)
313 By default, when searching shared libraries to resolve a symbol
314 reference, the dynamic linker will resolve to the first defini‐
315 tion it finds.
316 Old glibc versions (before 2.2), provided a different behavior:
318 if the linker found a symbol that was weak, it would remember
319 that symbol and keep searching in the remaining shared
320 libraries. If it subsequently found a strong definition of the
321 same symbol, then it would instead use that definition. (If no
322 further symbol was found, then the dynamic linker would use the
323 weak symbol that it initially found.)
324 The old glibc behavior was nonstandard. (Standard practice is
326 that the distinction between weak and strong symbols should have
327 effect only at static link time.) In glibc 2.2, the dynamic
328 linker was modified to provide the current behavior (which was
329 the behavior that was provided by most other implementations at
330 that time).
331 Defining the LD_DYNAMIC_WEAK environment variable (with any
333 value) provides the old (nonstandard) glibc behavior, whereby a
334 weak symbol in one shared library may be overridden by a strong
335 symbol subsequently discovered in another shared library. (Note
336 that even when this variable is set, a strong symbol in a shared
337 library will not override a weak definition of the same symbol
338 in the main program.)
339 Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored in secure-execu‐
341 tion mode.
342 LD_HWCAP_MASK (since glibc 2.1)
344 Mask for hardware capabilities.
345 LD_ORIGIN_PATH (since glibc 2.1)
347 Path where the binary is found.
348 Since glibc 2.4, LD_ORIGIN_PATH is ignored in secure-execution
350 mode.
351 LD_POINTER_GUARD (glibc from 2.4 to 2.22)
353 Set to 0 to disable pointer guarding. Any other value enables
354 pointer guarding, which is also the default. Pointer guarding
355 is a security mechanism whereby some pointers to code stored in
356 writable program memory (return addresses saved by setjmp(3) or
357 function pointers used by various glibc internals) are mangled
358 semi-randomly to make it more difficult for an attacker to
359 hijack the pointers for use in the event of a buffer overrun or
360 stack-smashing attack. Since glibc 2.23, LD_POINTER_GUARD can
361 no longer be used to disable pointer guarding, which is now
362 always enabled.
363 LD_PROFILE (since glibc 2.1)
365 The name of a (single) shared object to be profiled, specified
366 either as a pathname or a soname. Profiling output is appended
367 to the file whose name is: "$LD_PROFILE_OUTPUT/$LD_PROFILE.pro‐
368 file".
369 Since glibc 2.2.5, LD_PROFILE is ignored in secure-execution
371 mode.
372 LD_PROFILE_OUTPUT (since glibc 2.1)
374 Directory where LD_PROFILE output should be written. If this
375 variable is not defined, or is defined as an empty string, then
376 the default is /var/tmp.
377 LD_PROFILE_OUTPUT is ignored in secure-execution mode; instead
379 /var/profile is always used. (This detail is relevant only
380 before glibc 2.2.5, since in later glibc versions, LD_PROFILE is
381 also ignored in secure-execution mode.)
382 LD_SHOW_AUXV (since glibc 2.1)
384 If this environment variable is defined (with any value), show
385 the auxiliary array passed up from the kernel (see also getaux‐
386 val(3)).
387 Since glibc 2.3.4, LD_SHOW_AUXV is ignored in secure-execution
389 mode.
390 LD_TRACE_PRELINKING (since glibc 2.4)
392 If this environment variable is defined, trace prelinking of the
393 object whose name is assigned to this environment variable.
394 (Use ldd(1) to get a list of the objects that might be traced.)
395 If the object name is not recognized, then all prelinking activ‐
396 ity is traced.
397 LD_USE_LOAD_BIAS (since glibc 2.3.3)
399 By default (i.e., if this variable is not defined), executables
400 and prelinked shared objects will honor base addresses of their
401 dependent shared objects and (nonprelinked) position-independent
402 executables (PIEs) and other shared objects will not honor them.
403 If LD_USE_LOAD_BIAS is defined with the value 1, both executa‐
404 bles and PIEs will honor the base addresses. If
405 LD_USE_LOAD_BIAS is defined with the value 0, neither executa‐
406 bles nor PIEs will honor the base addresses.
407 Since glibc 2.3.3, this variable is ignored in secure-execution
409 mode.
410 LD_VERBOSE (since glibc 2.1)
412 If set to a nonempty string, output symbol versioning informa‐
413 tion about the program if the LD_TRACE_LOADED_OBJECTS environ‐
414 ment variable has been set.
415 LD_WARN (since glibc 2.1.3)
417 If set to a nonempty string, warn about unresolved symbols.
418 LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
420 According to the Intel Silvermont software optimization guide,
421 for 64-bit applications, branch prediction performance can be
422 negatively impacted when the target of a branch is more than
423 4 GB away from the branch. If this environment variable is set
424 (to any value), the dynamic linker will first try to map exe‐
425 cutable pages using the mmap(2) MAP_32BIT flag, and fall back to
426 mapping without that flag if that attempt fails. NB: MAP_32BIT
427 will map to the low 2 GB (not 4 GB) of the address space.
428 Because MAP_32BIT reduces the address range available for
430 address space layout randomization (ASLR), LD_PRE‐
431 FER_MAP_32BIT_EXEC is always disabled in secure-execution mode.
432
434 /lib/ld.so
435 a.out dynamic linker/loader
436 /lib/ld-linux.so.{1,2}
437 ELF dynamic linker/loader
438 /etc/ld.so.cache
439 File containing a compiled list of directories in which to
440 search for shared objects and an ordered list of candidate
441 shared objects. See ldconfig(8).
442 /etc/ld.so.preload
443 File containing a whitespace-separated list of ELF shared
444 objects to be loaded before the program. See the discussion of
445 LD_PRELOAD above. If both LD_PRELOAD and /etc/ld.so.preload are
446 employed, the libraries specified by LD_PRELOAD are preloaded
447 first. /etc/ld.so.preload has a system-wide effect, causing the
448 specified libraries to be preloaded for all programs that are
449 executed on the system. (This is usually undesirable, and is
450 typically employed only as an emergency remedy, for example, as
451 a temporary workaround to a library misconfiguration issue.)
452 lib*.so*
453 shared objects
454
456 Hardware capabilities
457 Some shared objects are compiled using hardware-specific instructions
458 which do not exist on every CPU. Such objects should be installed in
459 directories whose names define the required hardware capabilities, such
460 as /usr/lib/sse2/. The dynamic linker checks these directories against
461 the hardware of the machine and selects the most suitable version of a
462 given shared object. Hardware capability directories can be cascaded
463 to combine CPU features. The list of supported hardware capability
464 names depends on the CPU. The following names are currently recog‐
465 nized:
466 Alpha ev4, ev5, ev56, ev6, ev67
468 MIPS loongson2e, loongson2f, octeon, octeon2
470 PowerPC
472 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efp‐
473 double, efpsingle, fpu, ic_snoop, mmu, notb, pa6t, power4,
474 power5, power5+, power6x, ppc32, ppc601, ppc64, smt, spe,
475 ucache, vsx
476 SPARC flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
478 s390 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa,
480 stfle, z900, z990, z9-109, z10, zarch
481 x86 (32-bit only)
483 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586,
484 i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse,
485 sse2, tm
486
488 ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5),
489 capabilities(7), rtld-audit(7), ldconfig(8), sln(8)
490
492 This page is part of release 4.16 of the Linux man-pages project. A
493 description of the project, information about reporting bugs, and the
494 latest version of this page, can be found at
495 https://www.kernel.org/doc/man-pages/.
496GNU 2018-04-30 LD.SO(8)