1LD.SO(8)                   Linux Programmer's Manual                  LD.SO(8)
2
3
4

NAME

6       ld.so, ld-linux.so - dynamic linker/loader
7

SYNOPSIS

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
15       /lib/ld-linux.so.*  [OPTIONS] [PROGRAM [ARGUMENTS]]
16

DESCRIPTION

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
22       Linux binaries require dynamic linking (linking at run time) unless the
23       -static option was given to ld(1) during compilation.
24
25       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
32       When  resolving  shared  object  dependencies, the dynamic linker first
33       inspects 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
39       If a shared object dependency does not contain  a  slash,  then  it  is
40       searched for in the following order:
41
42       o  Using  the  directories  specified  in  the DT_RPATH dynamic section
43          attribute of the binary if present and DT_RUNPATH attribute does not
44          exist.  Use of DT_RPATH is deprecated.
45
46       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
50       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
56          dependency tree.
57
58       o  From the cache file /etc/ld.so.cache, which contains a compiled list
59          of candidate  shared  objects  previously  found  in  the  augmented
60          library  path.  If, however, the binary was linked with the -z node‐
61          flib linker option, shared objects in the default paths are skipped.
62          Shared  objects  installed  in  hardware capability directories (see
63          below) are preferred to other shared objects.
64
65       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
70   Rpath token expansion
71       The dynamic linker understands certain token strings in an rpath speci‐
72       fication (DT_RPATH or DT_RUNPATH).  Those strings  are  substituted  as
73       follows:
74
75       $ORIGIN (or equivalently ${ORIGIN})
76              This  expands  to the directory containing the program or shared
77              object.  Thus, an application located in  somedir/app  could  be
78              compiled with
79
80                  gcc -Wl,-rpath,'$ORIGIN/../lib'
81
82              so  that  it finds an associated shared object in somedir/lib no
83              matter where somedir is  located  in  the  directory  hierarchy.
84              This facilitates the creation of "turn-key" applications that do
85              not need to be  installed  into  special  directories,  but  can
86              instead  be unpacked into any directory and still find their own
87              shared objects.
88
89       $LIB (or equivalently ${LIB})
90              This expands to lib  or  lib64  depending  on  the  architecture
91              (e.g.,  on x86-64, it expands to lib64 and on x86-32, it expands
92              to lib).
93
94       $PLATFORM (or equivalently ${PLATFORM})
95              This expands to a string corresponding to the processor type  of
96              the  host  system  (e.g., "x86_64").  On some architectures, the
97              Linux kernel doesn't provide a platform string  to  the  dynamic
98              linker.   The value of this string is taken from the AT_PLATFORM
99              value in the auxiliary vector (see getauxval(3)).
100

OPTIONS

102       --audit list
103              Use objects named in list as auditors.  The objects in list  are
104              delimited by colons.
105
106       --inhibit-cache
107              Do not use /etc/ld.so.cache.
108
109       --library-path path
110              Use path instead of LD_LIBRARY_PATH environment variable setting
111              (see below).  The names ORIGIN, LIB,  and  PLATFORM  are  inter‐
112              preted as for the LD_LIBRARY_PATH environment variable.
113
114       --inhibit-rpath list
115              Ignore  RPATH  and  RUNPATH information in object names in list.
116              This option is ignored when  running  in  secure-execution  mode
117              (see  below).   The  objects  in list are delimited by colons or
118              spaces.
119
120       --list List all dependencies and how they are resolved.
121
122       --preload list (since glibc 2.30)
123              Preload the objects specified in list.  The objects in list  are
124              delimited  by  colons  or  spaces.  The objects are preloaded as
125              explained in the description of the LD_PRELOAD environment vari‐
126              able below.
127
128              By contrast with LD_PRELOAD, the --preload option provides a way
129              to perform preloading for a single executable without  affecting
130              preloading  performed  in  any child process that executes a new
131              program.
132
133       --verify
134              Verify that program  is  dynamically  linked  and  this  dynamic
135              linker can handle it.
136

ENVIRONMENT

138       Various  environment  variables  influence the operation of the dynamic
139       linker.
140
141   Secure-execution mode
142       For security reasons, if the dynamic linker determines  that  a  binary
143       should be run in secure-execution mode, the effects of some environment
144       variables are voided or modified,  and  furthermore  those  environment
145       variables  are  stripped from the environment, so that the program does
146       not even see the definitions.   Some  of  these  environment  variables
147       affect  the  operation  of the dynamic linker itself, and are described
148       below.  Other  environment  variables  treated  in  this  way  include:
149       GCONV_PATH,   GETCONF_DIR,   HOSTALIASES,  LOCALDOMAIN,  LOCPATH,  MAL‐
150       LOC_TRACE, NIS_PATH, NLSPATH,  RESOLV_HOST_CONF,  RES_OPTIONS,  TMPDIR,
151       and TZDIR.
152
153       A binary is executed in secure-execution mode if the AT_SECURE entry in
154       the auxiliary vector (see getauxval(3))  has  a  nonzero  value.   This
155       entry may have a nonzero value for various reasons, including:
156
157       *  The  process's  real  and effective user IDs differ, or the real and
158          effective group IDs differ.  This typically occurs as  a  result  of
159          executing a set-user-ID or set-group-ID program.
160
161       *  A  process  with a non-root user ID executed a binary that conferred
162          capabilities to the process.
163
164       *  A nonzero value may have been set by a Linux Security Module.
165
166   Environment variables
167       Among the more important environment variables are the following:
168
169       LD_ASSUME_KERNEL (since glibc 2.2.3)
170              Each shared object can inform the dynamic linker of the  minimum
171              kernel  ABI  version  that  it  requires.   (This requirement is
172              encoded in an ELF note section that is viewable  via  readelf -n
173              as  a section labeled NT_GNU_ABI_TAG.)  At run time, the dynamic
174              linker determines the ABI version of the running kernel and will
175              reject  loading shared objects that specify minimum ABI versions
176              that exceed that ABI version.
177
178              LD_ASSUME_KERNEL can be used to  cause  the  dynamic  linker  to
179              assume  that  it  is running on a system with a different kernel
180              ABI version.  For example, the following command line causes the
181              dynamic linker to assume it is running on Linux 2.2.5 when load‐
182              ing the shared objects required by myprog:
183
184                  $ LD_ASSUME_KERNEL=2.2.5 ./myprog
185
186              On systems that provide multiple versions of a shared object (in
187              different  directories  in  the search path) that have different
188              minimum kernel ABI version requirements, LD_ASSUME_KERNEL can be
189              used to select the version of the object that is used (dependent
190              on the directory search order).
191
192              Historically, the most common use of the  LD_ASSUME_KERNEL  fea‐
193              ture was to manually select the older LinuxThreads POSIX threads
194              implementation on systems that provided  both  LinuxThreads  and
195              NPTL  (which  latter was typically the default on such systems);
196              see pthreads(7).
197
198       LD_BIND_NOW (since glibc 2.1.1)
199              If set to a  nonempty  string,  causes  the  dynamic  linker  to
200              resolve  all  symbols  at  program  startup instead of deferring
201              function call resolution to the point when they are first refer‐
202              enced.  This is useful when using a debugger.
203
204       LD_LIBRARY_PATH
205              A  list  of  directories in which to search for ELF libraries at
206              execution time.  The items in the list are separated  by  either
207              colons  or  semicolons,  and  there  is  no support for escaping
208              either separator.
209
210              This variable is ignored in secure-execution mode.
211
212              Within the pathnames specified in LD_LIBRARY_PATH,  the  dynamic
213              linker  expands  the tokens $ORIGIN, $LIB, and $PLATFORM (or the
214              versions using curly braces around the names) as described above
215              in  Rpath  token  expansion.   Thus,  for example, the following
216              would cause a library to be searched for in either  the  lib  or
217              lib64 subdirectory below the directory containing the program to
218              be executed:
219
220                  $ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
221
222              (Note the use of single quotes, which prevent expansion of $ORI‐
223              GIN and $LIB as shell variables!)
224
225       LD_PRELOAD
226              A  list  of additional, user-specified, ELF shared objects to be
227              loaded before all others.  This feature can be  used  to  selec‐
228              tively override functions in other shared objects.
229
230              The  items of the list can be separated by spaces or colons, and
231              there is no support for escaping either separator.  The  objects
232              are  searched  for  using  the  rules  given  under DESCRIPTION.
233              Objects are searched for and added to the link map in the  left-
234              to-right order specified in the list.
235
236              In  secure-execution  mode, preload pathnames containing slashes
237              are ignored.  Furthermore, shared  objects  are  preloaded  only
238              from  the standard search directories and only if they have set-
239              user-ID mode bit enabled (which is not typical).
240
241              Within the names specified in the LD_PRELOAD list,  the  dynamic
242              linker  understands  the tokens $ORIGIN, $LIB, and $PLATFORM (or
243              the versions using curly braces around the names)  as  described
244              above  in  Rpath  token  expansion.  (See also the discussion of
245              quoting under the description of LD_LIBRARY_PATH.)
246
247              There are various methods of specifying  libraries  to  be  pre‐
248              loaded, and these are handled in the following order:
249
250              (1) The LD_PRELOAD environment variable.
251
252              (2) The  --preload command-line option when invoking the dynamic
253                  linker directly.
254
255              (3) The /etc/ld.so.preload file (described below).
256
257       LD_TRACE_LOADED_OBJECTS
258              If set (to any value), causes the program to  list  its  dynamic
259              dependencies, as if run by ldd(1), instead of running normally.
260
261       Then there are lots of more or less obscure variables, many obsolete or
262       only for internal use.
263
264       LD_AUDIT (since glibc 2.4)
265              A list of user-specified, ELF shared objects to be loaded before
266              all  others  in a separate linker namespace (i.e., one that does
267              not intrude upon the normal symbol bindings that would occur  in
268              the process) These objects can be used to audit the operation of
269              the dynamic linker.  The items in the list are  colon-separated,
270              and there is no support for escaping the separator.
271
272              LD_AUDIT is ignored in secure-execution mode.
273
274              The  dynamic  linker will notify the audit shared objects at so-
275              called auditing checkpoints—for example, loading  a  new  shared
276              object,  resolving  a  symbol,  or calling a symbol from another
277              shared object—by calling  an  appropriate  function  within  the
278              audit  shared  object.   For  details,  see  rtld-audit(7).  The
279              auditing interface is largely compatible with that  provided  on
280              Solaris,  as described in its Linker and Libraries Guide, in the
281              chapter Runtime Linker Auditing Interface.
282
283              Within the names specified in the  LD_AUDIT  list,  the  dynamic
284              linker  understands  the tokens $ORIGIN, $LIB, and $PLATFORM (or
285              the versions using curly braces around the names)  as  described
286              above  in  Rpath  token  expansion.  (See also the discussion of
287              quoting under the description of LD_LIBRARY_PATH.)
288
289              Since glibc 2.13, in secure-execution mode, names in  the  audit
290              list  that  contain slashes are ignored, and only shared objects
291              in the standard search directories  that  have  the  set-user-ID
292              mode bit enabled are loaded.
293
294       LD_BIND_NOT (since glibc 2.1.95)
295              If this environment variable is set to a nonempty string, do not
296              update the GOT (global offset table) and PLT (procedure  linkage
297              table)  after resolving a function symbol.  By combining the use
298              of this variable with LD_DEBUG (with the categories bindings and
299              symbols), one can observe all run-time function bindings.
300
301       LD_DEBUG (since glibc 2.1)
302              Output  verbose  debugging  information  about  operation of the
303              dynamic linker.  The content of this variable is one of more  of
304              the  following  categories,  separated by colons, commas, or (if
305              the value is quoted) spaces:
306
307              help        Specifying help in the value of this  variable  does
308                          not  run  the specified program, and displays a help
309                          message about which categories can be  specified  in
310                          this environment variable.
311
312              all         Print  all  debugging information (except statistics
313                          and unused; see below).
314
315              bindings    Display information about which definition each sym‐
316                          bol is bound to.
317
318              files       Display progress for input file.
319
320              libs        Display library search paths.
321
322              reloc       Display relocation processing.
323
324              scopes      Display scope information.
325
326              statistics  Display relocation statistics.
327
328              symbols     Display search paths for each symbol look-up.
329
330              unused      Determine unused DSOs.
331
332              versions    Display version dependencies.
333
334              Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution mode,
335              unless the file /etc/suid-debug exists (the content of the  file
336              is irrelevant).
337
338       LD_DEBUG_OUTPUT (since glibc 2.1)
339              By  default,  LD_DEBUG  output is written to standard error.  If
340              LD_DEBUG_OUTPUT is defined, then output is written to the  path‐
341              name  specified by its value, with the suffix "." (dot) followed
342              by the process ID appended to the pathname.
343
344              LD_DEBUG_OUTPUT is ignored in secure-execution mode.
345
346       LD_DYNAMIC_WEAK (since glibc 2.1.91)
347              By default, when searching shared libraries to resolve a  symbol
348              reference,  the dynamic linker will resolve to the first defini‐
349              tion it finds.
350
351              Old glibc versions (before 2.2), provided a different  behavior:
352              if  the  linker  found a symbol that was weak, it would remember
353              that  symbol  and  keep  searching  in  the   remaining   shared
354              libraries.   If it subsequently found a strong definition of the
355              same symbol, then it would instead use that definition.  (If  no
356              further  symbol was found, then the dynamic linker would use the
357              weak symbol that it initially found.)
358
359              The old glibc behavior was nonstandard.  (Standard  practice  is
360              that the distinction between weak and strong symbols should have
361              effect only at static link time.)  In  glibc  2.2,  the  dynamic
362              linker  was  modified to provide the current behavior (which was
363              the behavior that was provided by most other implementations  at
364              that time).
365
366              Defining  the  LD_DYNAMIC_WEAK  environment  variable  (with any
367              value) provides the old (nonstandard) glibc behavior, whereby  a
368              weak  symbol in one shared library may be overridden by a strong
369              symbol subsequently discovered in another shared library.  (Note
370              that even when this variable is set, a strong symbol in a shared
371              library will not override a weak definition of the  same  symbol
372              in the main program.)
373
374              Since  glibc  2.3.4, LD_DYNAMIC_WEAK is ignored in secure-execu‐
375              tion mode.
376
377       LD_HWCAP_MASK (since glibc 2.1)
378              Mask for hardware capabilities.
379
380       LD_ORIGIN_PATH (since glibc 2.1)
381              Path where the binary is found.
382
383              Since glibc 2.4, LD_ORIGIN_PATH is ignored  in  secure-execution
384              mode.
385
386       LD_POINTER_GUARD (glibc from 2.4 to 2.22)
387              Set  to  0 to disable pointer guarding.  Any other value enables
388              pointer guarding, which is also the default.   Pointer  guarding
389              is  a security mechanism whereby some pointers to code stored in
390              writable program memory (return addresses saved by setjmp(3)  or
391              function  pointers  used by various glibc internals) are mangled
392              semi-randomly to make it  more  difficult  for  an  attacker  to
393              hijack  the pointers for use in the event of a buffer overrun or
394              stack-smashing attack.  Since glibc 2.23,  LD_POINTER_GUARD  can
395              no  longer  be  used  to  disable pointer guarding, which is now
396              always enabled.
397
398       LD_PROFILE (since glibc 2.1)
399              The name of a (single) shared object to be  profiled,  specified
400              either  as a pathname or a soname.  Profiling output is appended
401              to the file whose name is:  "$LD_PROFILE_OUTPUT/$LD_PROFILE.pro‐
402              file".
403
404              Since  glibc  2.2.5,  LD_PROFILE  is ignored in secure-execution
405              mode.
406
407       LD_PROFILE_OUTPUT (since glibc 2.1)
408              Directory where LD_PROFILE output should be  written.   If  this
409              variable  is not defined, or is defined as an empty string, then
410              the default is /var/tmp.
411
412              LD_PROFILE_OUTPUT is ignored in secure-execution  mode;  instead
413              /var/profile  is  always  used.   (This  detail is relevant only
414              before glibc 2.2.5, since in later glibc versions, LD_PROFILE is
415              also ignored in secure-execution mode.)
416
417       LD_SHOW_AUXV (since glibc 2.1)
418              If  this  environment variable is defined (with any value), show
419              the auxiliary array passed up from the kernel (see also  getaux‐
420              val(3)).
421
422              Since  glibc  2.3.4, LD_SHOW_AUXV is ignored in secure-execution
423              mode.
424
425       LD_TRACE_PRELINKING (since glibc 2.4)
426              If this environment variable is defined, trace prelinking of the
427              object  whose  name  is  assigned  to this environment variable.
428              (Use ldd(1) to get a list of the objects that might be  traced.)
429              If the object name is not recognized, then all prelinking activ‐
430              ity is traced.
431
432       LD_USE_LOAD_BIAS (since glibc 2.3.3)
433              By default (i.e., if this variable is not defined),  executables
434              and  prelinked shared objects will honor base addresses of their
435              dependent shared objects and (nonprelinked) position-independent
436              executables (PIEs) and other shared objects will not honor them.
437              If LD_USE_LOAD_BIAS is defined with the value 1,  both  executa‐
438              bles   and   PIEs   will   honor   the   base   addresses.    If
439              LD_USE_LOAD_BIAS is defined with the value 0,  neither  executa‐
440              bles nor PIEs will honor the base addresses.
441
442              Since  glibc 2.3.3, this variable is ignored in secure-execution
443              mode.
444
445       LD_VERBOSE (since glibc 2.1)
446              If set to a nonempty string, output symbol  versioning  informa‐
447              tion  about  the program if the LD_TRACE_LOADED_OBJECTS environ‐
448              ment variable has been set.
449
450       LD_WARN (since glibc 2.1.3)
451              If set to a nonempty string, warn about unresolved symbols.
452
453       LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
454              According to the Intel Silvermont software  optimization  guide,
455              for  64-bit  applications,  branch prediction performance can be
456              negatively impacted when the target of a  branch  is  more  than
457              4 GB  away from the branch.  If this environment variable is set
458              (to any value), the dynamic linker will first try  to  map  exe‐
459              cutable pages using the mmap(2) MAP_32BIT flag, and fall back to
460              mapping without that flag if that attempt fails.  NB:  MAP_32BIT
461              will map to the low 2 GB (not 4 GB) of the address space.
462
463              Because  MAP_32BIT  reduces  the  address  range  available  for
464              address   space    layout    randomization    (ASLR),    LD_PRE‐
465              FER_MAP_32BIT_EXEC is always disabled in secure-execution mode.
466

FILES

468       /lib/ld.so
469              a.out dynamic linker/loader
470
471       /lib/ld-linux.so.{1,2}
472              ELF dynamic linker/loader
473
474       /etc/ld.so.cache
475              File  containing  a  compiled  list  of  directories in which to
476              search for shared objects  and  an  ordered  list  of  candidate
477              shared objects.  See ldconfig(8).
478
479       /etc/ld.so.preload
480              File  containing  a  whitespace-separated  list  of  ELF  shared
481              objects to be loaded before the program.  See the discussion  of
482              LD_PRELOAD above.  If both LD_PRELOAD and /etc/ld.so.preload are
483              employed, the libraries specified by  LD_PRELOAD  are  preloaded
484              first.  /etc/ld.so.preload has a system-wide effect, causing the
485              specified libraries to be preloaded for all  programs  that  are
486              executed  on  the  system.  (This is usually undesirable, and is
487              typically employed only as an emergency remedy, for example,  as
488              a temporary workaround to a library misconfiguration issue.)
489
490       lib*.so*
491              shared objects
492

NOTES

494   Hardware capabilities
495       Some  shared  objects are compiled using hardware-specific instructions
496       which do not exist on every CPU.  Such objects should be  installed  in
497       directories whose names define the required hardware capabilities, such
498       as /usr/lib/sse2/.  The dynamic linker checks these directories against
499       the  hardware of the machine and selects the most suitable version of a
500       given shared object.  Hardware capability directories can  be  cascaded
501       to  combine  CPU  features.   The list of supported hardware capability
502       names depends on the CPU.  The following  names  are  currently  recog‐
503       nized:
504
505       Alpha  ev4, ev5, ev56, ev6, ev67
506
507       MIPS   loongson2e, loongson2f, octeon, octeon2
508
509       PowerPC
510              4xxmac,  altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efp‐
511              double, efpsingle,  fpu,  ic_snoop,  mmu,  notb,  pa6t,  power4,
512              power5,  power5+,  power6x,  ppc32,  ppc601,  ppc64,  smt,  spe,
513              ucache, vsx
514
515       SPARC  flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
516
517       s390   dfp, eimm, esan3, etf3enh,  g5,  highgprs,  hpage,  ldisp,  msa,
518              stfle, z900, z990, z9-109, z10, zarch
519
520       x86 (32-bit only)
521              acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586,
522              i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36,  sep,  ss,  sse,
523              sse2, tm
524

SEE ALSO

526       ld(1),  ldd(1),  pldd(1),  sprof(1),  dlopen(3),  getauxval(3), elf(5),
527       capabilities(7), rtld-audit(7), ldconfig(8), sln(8)
528

COLOPHON

530       This page is part of release 5.07 of the Linux  man-pages  project.   A
531       description  of  the project, information about reporting bugs, and the
532       latest    version    of    this    page,    can     be     found     at
533       https://www.kernel.org/doc/man-pages/.
534
535
536
537GNU                               2019-08-02                          LD.SO(8)
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