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  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
31       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
38       If  a  shared  object  dependency  does not contain a slash, then it is
39       searched for in the following order:
40
41       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
45       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 it is ignored.
48
49       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
57       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
64       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
69   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
74       $ORIGIN (or equivalently ${ORIGIN})
75              This expands to the directory containing the program  or  shared
76              object.   Thus,  an  application located in somedir/app could be
77              compiled with
78
79                  gcc -Wl,-rpath,'$ORIGIN/../lib'
80
81              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
88       $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
93       $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

OPTIONS

101       --list List all dependencies and how they are resolved.
102
103       --verify
104              Verify  that  program  is  dynamically  linked  and this dynamic
105              linker can handle it.
106
107       --inhibit-cache
108              Do not use /etc/ld.so.cache.
109
110       --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
115       --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
120       --audit list
121              Use objects named in list as auditors.
122

ENVIRONMENT

124       Various environment variables influence the operation  of  the  dynamic
125       linker.
126
127   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
136       *  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
140       *  A  process  with a non-root user ID executed a binary that conferred
141          capabilities to the process.
142
143       *  A nonzero value may have been set by a Linux Security Module.
144
145   Environment variables
146       Among the more important environment variables are the following:
147
148       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
157              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
163                  $ LD_ASSUME_KERNEL=2.2.5 ./myprog
164
165              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
171              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
177       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
183       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.  Similar to the PATH environment variable.
187
188              This variable is ignored in secure-execution mode.
189
190              Within  the  pathnames specified in LD_LIBRARY_PATH, the dynamic
191              linker expands the tokens $ORIGIN, $LIB, and $PLATFORM  (or  the
192              versions using curly braces around the names) as described above
193              in Rpath token expansion.   Thus,  for  example,  the  following
194              would  cause  a  library to be searched for in either the lib or
195              lib64 subdirectory below the directory containing the program to
196              be executed:
197
198                  $ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
199
200              (Note the use of single quotes, which prevent expansion of $ORI‐
201              GIN and $LIB as shell variables!)
202
203       LD_PRELOAD
204              A list of additional, user-specified, ELF shared objects  to  be
205              loaded  before  all  others.  The items of the list can be sepa‐
206              rated by spaces or colons.  This  can  be  used  to  selectively
207              override  functions  in  other  shared objects.  The objects are
208              searched for using the rules given under DESCRIPTION.
209
210              In secure-execution mode, preload pathnames  containing  slashes
211              are  ignored.   Furthermore,  shared  objects are preloaded only
212              from the standard search directories and only if they have  set-
213              user-ID mode bit enabled (which is not typical).
214
215              Within  the  names specified in the LD_PRELOAD list, the dynamic
216              linker understands the tokens $ORIGIN, $LIB, and  $PLATFORM  (or
217              the  versions  using curly braces around the names) as described
218              above in Rpath token expansion.  (See  also  the  discussion  of
219              quoting under the description of LD_LIBRARY_PATH.)
220
221       LD_TRACE_LOADED_OBJECTS
222              If  set  (to  any value), causes the program to list its dynamic
223              dependencies, as if run by ldd(1), instead of running normally.
224
225       Then there are lots of more or less obscure variables, many obsolete or
226       only for internal use.
227
228       LD_AUDIT (since glibc 2.4)
229              A  colon-separated list of user-specified, ELF shared objects to
230              be loaded before all  others  in  a  separate  linker  namespace
231              (i.e., one that does not intrude upon the normal symbol bindings
232              that would occur in the process).  These objects can be used  to
233              audit the operation of the dynamic linker.
234
235              LD_AUDIT is ignored in secure-execution mode.
236
237              The  dynamic  linker will notify the audit shared objects at so-
238              called auditing checkpoints—for example, loading  a  new  shared
239              object,  resolving  a  symbol,  or calling a symbol from another
240              shared object—by calling  an  appropriate  function  within  the
241              audit  shared  object.   For  details,  see  rtld-audit(7).  The
242              auditing interface is largely compatible with that  provided  on
243              Solaris,  as described in its Linker and Libraries Guide, in the
244              chapter Runtime Linker Auditing Interface.
245
246              Within the names specified in the  LD_AUDIT  list,  the  dynamic
247              linker  understands  the tokens $ORIGIN, $LIB, and $PLATFORM (or
248              the versions using curly braces around the names)  as  described
249              above  in  Rpath  token  expansion.  (See also the discussion of
250              quoting under the description of LD_LIBRARY_PATH.)
251
252              Since glibc 2.13, in secure-execution mode, names in  the  audit
253              list  that  contain slashes are ignored, and only shared objects
254              in the standard search directories  that  have  the  set-user-ID
255              mode bit enabled are loaded.
256
257       LD_BIND_NOT (since glibc 2.1.95)
258              If this environment variable is set to a nonempty string, do not
259              update the GOT (global offset table) and PLT (procedure  linkage
260              table)  after resolving a function symbol.  By combining the use
261              of this variable with LD_DEBUG (with the categories bindings and
262              symbols), one can observe all run-time function bindings.
263
264       LD_DEBUG (since glibc 2.1)
265              Output  verbose  debugging  information  about  operation of the
266              dynamic linker.  The content of this variable is one of more  of
267              the  following  categories,  separated by colons, commas, or (if
268              the value is quoted) spaces:
269
270              help        Specifying help in the value of this  variable  does
271                          not  run  the specified program, and displays a help
272                          message about which categories can be  specified  in
273                          this environment variable.
274
275              all         Print  all  debugging information (except statistics
276                          and unused; see below).
277
278              bindings    Display information about which definition each sym‐
279                          bol is bound to.
280
281              files       Display progress for input file.
282
283              libs        Display library search paths.
284
285              reloc       Display relocation processing.
286
287              scopes      Display scope information.
288
289              statistics  Display relocation statistics.
290
291              symbols     Display search paths for each symbol look-up.
292
293              unused      Determine unused DSOs.
294
295              versions    Display version dependencies.
296
297              Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution mode,
298              unless the file /etc/suid-debug exists (the content of the  file
299              is irrelevant).
300
301       LD_DEBUG_OUTPUT (since glibc 2.1)
302              By  default,  LD_DEBUG  output is written to standard error.  If
303              LD_DEBUG_OUTPUT is defined, then output is written to the  path‐
304              name  specified by its value, with the suffix "." (dot) followed
305              by the process ID appended to the pathname.
306
307              LD_DEBUG_OUTPUT is ignored in secure-execution mode.
308
309       LD_DYNAMIC_WEAK (since glibc 2.1.91)
310              By default, when searching shared libraries to resolve a  symbol
311              reference,  the dynamic linker will resolve to the first defini‐
312              tion it finds.
313
314              Old glibc versions (before 2.2), provided a different  behavior:
315              if  the  linker  found a symbol that was weak, it would remember
316              that  symbol  and  keep  searching  in  the   remaining   shared
317              libraries.   If it subsequently found a strong definition of the
318              same symbol, then it would instead use that definition.  (If  no
319              further  symbol was found, then the dynamic linker would use the
320              weak symbol that it initially found.)
321
322              The old glibc behavior was nonstandard.  (Standard  practice  is
323              that the distinction between weak and strong symbols should have
324              effect only at static link time.)  In  glibc  2.2,  the  dynamic
325              linker  was  modified to provide the current behavior (which was
326              the behavior that was provided by most other implementations  at
327              that time).
328
329              Defining  the  LD_DYNAMIC_WEAK  environment  variable  (with any
330              value) provides the old (nonstandard) glibc behavior, whereby  a
331              weak  symbol in one shared library may be overridden by a strong
332              symbol subsequently discovered in another shared library.  (Note
333              that even when this variable is set, a strong symbol in a shared
334              library will not override a weak definition of the  same  symbol
335              in the main program.)
336
337              Since  glibc  2.3.4, LD_DYNAMIC_WEAK is ignored in secure-execu‐
338              tion mode.
339
340       LD_HWCAP_MASK (since glibc 2.1)
341              Mask for hardware capabilities.
342
343       LD_ORIGIN_PATH (since glibc 2.1)
344              Path where the binary is found.
345
346              Since glibc 2.4, LD_ORIGIN_PATH is ignored  in  secure-execution
347              mode.
348
349       LD_POINTER_GUARD (glibc from 2.4 to 2.22)
350              Set  to  0 to disable pointer guarding.  Any other value enables
351              pointer guarding, which is also the default.   Pointer  guarding
352              is  a security mechanism whereby some pointers to code stored in
353              writable program memory (return addresses saved by setjmp(3)  or
354              function  pointers  used by various glibc internals) are mangled
355              semi-randomly to make it  more  difficult  for  an  attacker  to
356              hijack  the pointers for use in the event of a buffer overrun or
357              stack-smashing attack.  Since glibc 2.23,  LD_POINTER_GUARD  can
358              no  longer  be  used  to  disable pointer guarding, which is now
359              always enabled.
360
361       LD_PROFILE (since glibc 2.1)
362              The name of a (single) shared object to be  profiled,  specified
363              either  as a pathname or a soname.  Profiling output is appended
364              to the file whose name is:  "$LD_PROFILE_OUTPUT/$LD_PROFILE.pro‐
365              file".
366
367              Since  glibc  2.2.5,  LD_PROFILE  is ignored in secure-execution
368              mode.
369
370       LD_PROFILE_OUTPUT (since glibc 2.1)
371              Directory where LD_PROFILE output should be  written.   If  this
372              variable  is not defined, or is defined as an empty string, then
373              the default is /var/tmp.
374
375              LD_PROFILE_OUTPUT is ignored in secure-execution  mode;  instead
376              /var/profile  is  always  used.   (This  detail is relevant only
377              before glibc 2.2.5, since in later glibc versions, LD_PROFILE is
378              also ignored in secure-execution mode.)
379
380       LD_SHOW_AUXV (since glibc 2.1)
381              If  this  environment variable is defined (with any value), show
382              the auxiliary array passed up from the kernel (see also  getaux‐
383              val(3)).
384
385              Since  glibc  2.3.4, LD_SHOW_AUXV is ignored in secure-execution
386              mode.
387
388       LD_TRACE_PRELINKING (since glibc 2.4)
389              If this environment variable is defined, trace prelinking of the
390              object  whose  name  is  assigned  to this environment variable.
391              (Use ldd(1) to get a list of the objects that might be  traced.)
392              If the object name is not recognized, then all prelinking activ‐
393              ity is traced.
394
395       LD_USE_LOAD_BIAS (since glibc 2.3.3)
396              By default (i.e., if this variable is not defined),  executables
397              and  prelinked shared objects will honor base addresses of their
398              dependent shared objects and (nonprelinked) position-independent
399              executables (PIEs) and other shared objects will not honor them.
400              If LD_USE_LOAD_BIAS is defined with the value 1,  both  executa‐
401              bles   and   PIEs   will   honor   the   base   addresses.    If
402              LD_USE_LOAD_BIAS is defined with the value 0,  neither  executa‐
403              bles nor PIEs will honor the base addresses.
404
405              Since  glibc 2.3.3, this variable is ignored in secure-execution
406              mode.
407
408       LD_VERBOSE (since glibc 2.1)
409              If set to a nonempty string, output symbol  versioning  informa‐
410              tion  about  the program if the LD_TRACE_LOADED_OBJECTS environ‐
411              ment variable has been set.
412
413       LD_WARN (since glibc 2.1.3)
414              If set to a nonempty string, warn about unresolved symbols.
415
416       LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
417              According to the Intel Silvermont software  optimization  guide,
418              for  64-bit  applications,  branch prediction performance can be
419              negatively impacted when the target of a  branch  is  more  than
420              4 GB  away from the branch.  If this environment variable is set
421              (to any value), the dynamic linker will first try  to  map  exe‐
422              cutable pages using the mmap(2) MAP_32BIT flag, and fall back to
423              mapping without that flag if that attempt fails.  NB:  MAP_32BIT
424              will map to the low 2 GB (not 4 GB) of the address space.
425
426              Because  MAP_32BIT  reduces  the  address  range  available  for
427              address   space    layout    randomization    (ASLR),    LD_PRE‐
428              FER_MAP_32BIT_EXEC is always disabled in secure-execution mode.
429

FILES

431       /lib/ld.so
432              a.out dynamic linker/loader
433       /lib/ld-linux.so.{1,2}
434              ELF dynamic linker/loader
435       /etc/ld.so.cache
436              File  containing  a  compiled  list  of  directories in which to
437              search for shared objects  and  an  ordered  list  of  candidate
438              shared objects.  See ldconfig(8).
439       /etc/ld.so.preload
440              File  containing  a  whitespace-separated  list  of  ELF  shared
441              objects to be loaded before the program.  See the discussion  of
442              LD_PRELOAD above.  If both LD_PRELOAD and /etc/ld.so.preload are
443              employed, the libraries specified by  LD_PRELOAD  are  preloaded
444              first.  /etc/ld.so.preload has a system-wide effect, causing the
445              specified libraries to be preloaded for all  programs  that  are
446              executed  on  the  system.  (This is usually undesirable, and is
447              typically employed only as an emergency remedy, for example,  as
448              a temporary workaround to a library misconfiguration issue.)
449       lib*.so*
450              shared objects
451

NOTES

453   Hardware capabilities
454       Some  shared  objects are compiled using hardware-specific instructions
455       which do not exist on every CPU.  Such objects should be  installed  in
456       directories whose names define the required hardware capabilities, such
457       as /usr/lib/sse2/.  The dynamic linker checks these directories against
458       the  hardware of the machine and selects the most suitable version of a
459       given shared object.  Hardware capability directories can  be  cascaded
460       to  combine  CPU  features.   The list of supported hardware capability
461       names depends on the CPU.  The following  names  are  currently  recog‐
462       nized:
463
464       Alpha  ev4, ev5, ev56, ev6, ev67
465
466       MIPS   loongson2e, loongson2f, octeon, octeon2
467
468       PowerPC
469              4xxmac,  altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efp‐
470              double, efpsingle,  fpu,  ic_snoop,  mmu,  notb,  pa6t,  power4,
471              power5,  power5+,  power6x,  ppc32,  ppc601,  ppc64,  smt,  spe,
472              ucache, vsx
473
474       SPARC  flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
475
476       s390   dfp, eimm, esan3, etf3enh,  g5,  highgprs,  hpage,  ldisp,  msa,
477              stfle, z900, z990, z9-109, z10, zarch
478
479       x86 (32-bit only)
480              acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586,
481              i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36,  sep,  ss,  sse,
482              sse2, tm
483

SEE ALSO

485       ld(1),  ldd(1),  pldd(1),  sprof(1),  dlopen(3),  getauxval(3), elf(5),
486       capabilities(7), rtld-audit(7), ldconfig(8), sln(8)
487

COLOPHON

489       This page is part of release 4.15 of the Linux  man-pages  project.   A
490       description  of  the project, information about reporting bugs, and the
491       latest    version    of    this    page,    can     be     found     at
492       https://www.kernel.org/doc/man-pages/.
493
494
495
496GNU                               2017-09-15                          LD.SO(8)
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