ld.so(8)



LD.SO(8)                   Linux Programmer's Manual                  LD.SO(8)

NAME
       ld.so, ld-linux.so* - dynamic linker/loader

SYNOPSIS
       The dynamic linker can be run either indirectly by running some dynami-
       cally linked program or library (in which case no command-line  options
       to  the  dynamic linker can be passed and, in the ELF case, the dynamic
       linker which is stored in the .interp section of the  program  is  exe-
       cuted) or directly by running:

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

DESCRIPTION
       The  programs ld.so and ld-linux.so* find and load the shared libraries
       needed by a program, prepare the program to run, and then run it.

       Linux binaries require dynamic linking (linking at run time) unless the
       -static option was given to ld(1) during compilation.

       The  program  ld.so handles a.out binaries, a format used long ago; ld-
       linux.so* handles ELF (/lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2
       for  glibc2), which everybody has been using for years now.  Otherwise,
       both have the same behavior, and use the same support  files  and  pro-
       grams ldd(1), ldconfig(8) and /etc/ld.so.conf.

       When  resolving library dependencies, the dynamic linker first inspects
       each dependency string to see if it contains a slash (this can occur if
       a  library pathname containing slashes was specified at link time).  If
       a slash is found, then the dependency string is interpreted as a (rela-
       tive  or absolute) pathname, and the library is loaded using that path-
       name.

       If a library dependency does not contain a slash, then it  is  searched
       for in the following order:

       o  (ELF  only)  Using the directories specified in the DT_RPATH dynamic
          section attribute of the binary if present and DT_RUNPATH  attribute
          does not exist.  Use of DT_RPATH is deprecated.

       o  Using  the environment variable LD_LIBRARY_PATH.  Except if the exe-
          cutable is a set-user-ID/set-group-ID binary, in which  case  it  is
          ignored.

       o  (ELF only) Using the directories specified in the DT_RUNPATH dynamic
          section attribute of the binary if present.

       o  From the cache file /etc/ld.so.cache, which contains a compiled list
          of  candidate  libraries  previously  found in the augmented library
          path.  If, however, the binary  was  linked  with  the  -z  nodeflib
          linker  option,  libraries in the default library paths are skipped.
          Libraries installed in hardware capability directories  (see  below)
          are preferred to other libraries.

       o  In  the  default  path  /lib,  and then /usr/lib.  If the binary was
          linked with the -z nodeflib linker option, this step is skipped.

   Rpath token expansion
       ld.so understands certain strings in an rpath  specification  (DT_RPATH
       or DT_RUNPATH); those strings are substituted as follows

       $ORIGIN (or equivalently ${ORIGIN})
              This  expands  to  the directory containing the application exe-
              cutable.  Thus, an application located in somedir/app  could  be
              compiled with

                  gcc -Wl,-rpath,'$ORIGIN/../lib'

              so  that it finds an associated shared library in somedir/lib no
              matter where somedir is  located  in  the  directory  hierarchy.
              This facilitates the creation of "turn-key" applications that do
              not need to be  installed  into  special  directories,  but  can
              instead  be unpacked into any directory and still find their own
              shared libraries.

       $LIB (or equivalently ${LIB})
              This expands to lib  or  lib64  depending  on  the  architecture
              (e.g.,  on x86-64, it expands to lib64 and on x86-32, it expands
              to lib).

       $PLATFORM (or equivalently ${PLATFORM})
              This expands to a string corresponding to the processor type  of
              the  host  system  (e.g., "x86_64").  On some architectures, the
              Linux kernel doesn't provide a platform string  to  the  dynamic
              linker.   The value of this string is taken from the AT_PLATFORM
              value in the auxiliary vector (see getauxval(3)).

OPTIONS
       --list List all dependencies and how they are resolved.

       --verify
              Verify that program  is  dynamically  linked  and  this  dynamic
              linker can handle it.

       --library-path PATH
              Use PATH instead of LD_LIBRARY_PATH environment variable setting
              (see below).

       --inhibit-rpath LIST
              Ignore RPATH and RUNPATH information in object  names  in  LIST.
              This option is ignored if ld.so is set-user-ID or set-group-ID.

       --audit LIST
              Use objects named in LIST as auditors.

HARDWARE CAPABILITIES
       Some  libraries are compiled using hardware-specific instructions which
       do not exist on every CPU.   Such  libraries  should  be  installed  in
       directories whose names define the required hardware capabilities, such
       as /usr/lib/sse2/.  The dynamic linker checks these directories against
       the  hardware of the machine and selects the most suitable version of a
       given library.  Hardware capability directories can be cascaded to com-
       bine  CPU  features.   The  list of supported hardware capability names
       depends on the CPU.  The following names are currently recognized:

       Alpha  ev4, ev5, ev56, ev6, ev67

       MIPS   loongson2e, loongson2f, octeon, octeon2

       PowerPC
              4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp,  efp-
              double,  efpsingle,  fpu,  ic_snoop,  mmu,  notb,  pa6t, power4,
              power5,  power5+,  power6x,  ppc32,  ppc601,  ppc64,  smt,  spe,
              ucache, vsx

       SPARC  flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2

       s390   dfp,  eimm,  esan3,  etf3enh,  g5,  highgprs, hpage, ldisp, msa,
              stfle, z900, z990, z9-109, z10, zarch

       x86 (32-bit only)
              acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586,
              i686,  mca,  mmx,  mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse,
              sse2, tm

ENVIRONMENT
       Among the more important environment variables are the following:

       LD_ASSUME_KERNEL
              (glibc since 2.2.3) Each shared library can inform  the  dynamic
              linker  of  the  minimum  kernel  ABI  version that it requires.
              (This requirement is encoded in an  ELF  note  section  that  is
              viewable  via  readelf -n  as a section labeled NT_GNU_ABI_TAG.)
              At run time, the dynamic linker determines the  ABI  version  of
              the running kernel and will reject loading shared libraries that
              specify minimum ABI versions that exceed that ABI version.

              LD_ASSUME_KERNEL can be used to  cause  the  dynamic  linker  to
              assume  that  it  is running on a system with a different kernel
              ABI version.  For example, the following command line causes the
              dynamic linker to assume it is running on Linux 2.2.5 when load-
              ing the shared libraries required by myprog:

                  $ LD_ASSUME_KERNEL=2.2.5 ./myprog

              On systems that provide multiple versions of  a  shared  library
              (in  different directories in the search path) that have differ-
              ent minimum kernel ABI  version  requirements,  LD_ASSUME_KERNEL
              can  be  used  to select the version of the library that is used
              (dependent on the directory search  order).   Historically,  the
              most  common use of the LD_ASSUME_KERNEL feature was to manually
              select the older LinuxThreads POSIX  threads  implementation  on
              systems  that  provided both LinuxThreads and NPTL (which latter
              was typically the default on such systems); see pthreads(7).

       LD_BIND_NOT
              (glibc since 2.2) Don't update the Global Offset Table (GOT) and
              Procedure Linkage Table (PLT) when resolving a symbol.

       LD_BIND_NOW
              (libc5;  glibc  since 2.1.1) If set to a nonempty string, causes
              the dynamic linker to resolve all  symbols  at  program  startup
              instead  of deferring function call resolution to the point when
              they are first referenced.  This is useful when using  a  debug-
              ger.

       LD_LIBRARY_PATH
              A colon-separated list of directories in which to search for ELF
              libraries at execution-time.  Similar to  the  PATH  environment
              variable.  Ignored in set-user-ID and set-group-ID programs.

       LD_PRELOAD
              A list of additional, user-specified, ELF shared libraries to be
              loaded before all others.  The items of the list  can  be  sepa-
              rated  by  spaces  or  colons.   This can be used to selectively
              override functions in other shared libraries.  The libraries are
              searched  for using the rules given under DESCRIPTION.  For set-
              user-ID/set-group-ID ELF binaries, preload pathnames  containing
              slashes are ignored, and libraries in the standard search direc-
              tories are loaded only if  the  set-user-ID  permission  bit  is
              enabled on the library file.

       LD_TRACE_LOADED_OBJECTS
              (ELF  only)  If  set to a nonempty string, causes the program to
              list its dynamic library dependencies,  as  if  run  by  ldd(1),
              instead of running normally.

       Then there are lots of more or less obscure variables, many obsolete or
       only for internal use.

       LD_AOUT_LIBRARY_PATH
              (libc5) Version of LD_LIBRARY_PATH for a.out binaries only.  Old
              versions of ld-linux.so.1 also supported LD_ELF_LIBRARY_PATH.

       LD_AOUT_PRELOAD
              (libc5) Version of LD_PRELOAD for a.out binaries only.  Old ver-
              sions of ld-linux.so.1 also supported LD_ELF_PRELOAD.

       LD_AUDIT
              (glibc since 2.4) A colon-separated list of user-specified,  ELF
              shared  objects  to  be  loaded  before all others in a separate
              linker namespace (i.e., one that does not intrude upon the  nor-
              mal  symbol  bindings  that  would occur in the process).  These
              libraries can be used to audit  the  operation  of  the  dynamic
              linker.   LD_AUDIT is ignored for set-user-ID/set-group-ID bina-
              ries.

              The dynamic linker will notify the audit libraries at  so-called
              auditing   checkpoints--for  example,  loading  a  new  library,
              resolving a symbol, or calling  a  symbol  from  another  shared
              object--by  calling  an  appropriate  function  within the audit
              library.  For details, see rtld-audit(7).  The  auditing  inter-
              face  is  largely  compatible  with that provided on Solaris, as
              described in its Linker and Libraries Guide, in the chapter Run-
              time Linker Auditing Interface.

       LD_BIND_NOT
              (glibc since 2.1.95) Do not update the GOT (global offset table)
              and PLT (procedure linkage table) after resolving a symbol.

       LD_DEBUG
              (glibc since 2.1) Output verbose debugging information about the
              dynamic  linker.  If set to all prints all debugging information
              it has, if set to help prints a help message about  which  cate-
              gories  can  be  specified  in this environment variable.  Since
              glibc 2.3.4, LD_DEBUG is  ignored  for  set-user-ID/set-group-ID
              binaries.

       LD_DEBUG_OUTPUT
              (glibc  since 2.1) File in which LD_DEBUG output should be writ-
              ten.  The default is standard error.  LD_DEBUG_OUTPUT is ignored
              for set-user-ID/set-group-ID binaries.

       LD_DYNAMIC_WEAK
              (glibc  since  2.1.91)  Allow  weak  symbols  to  be  overridden
              (reverting to old glibc behavior).  For security reasons,  since
              glibc  2.3.4,  LD_DYNAMIC_WEAK  is  ignored for set-user-ID/set-
              group-ID binaries.

       LD_HWCAP_MASK
              (glibc since 2.1) Mask for hardware capabilities.

       LD_KEEPDIR
              (a.out only)(libc5) Don't ignore the directory in the  names  of
              a.out  libraries  to  be loaded.  Use of this option is strongly
              discouraged.

       LD_NOWARN
              (a.out only)(libc5) Suppress warnings about a.out libraries with
              incompatible minor version numbers.

       LD_ORIGIN_PATH
              (glibc  since  2.1) Path where the binary is found (for non-set-
              user-ID programs).   For  security  reasons,  since  glibc  2.4,
              LD_ORIGIN_PATH is ignored for set-user-ID/set-group-ID binaries.

       LD_POINTER_GUARD
              (glibc  since  2.4)  Set  to 0 to disable pointer guarding.  Any
              other value enables pointer guarding, which is also the default.
              Pointer  guarding  is a security mechanism whereby some pointers
              to code stored in  writable  program  memory  (return  addresses
              saved  by  setjmp(3)  or function pointers used by various glibc
              internals) are mangled semi-randomly to make it  more  difficult
              for an attacker to hijack the pointers for use in the event of a
              buffer overrun or stack-smashing attack.

       LD_PROFILE
              (glibc since 2.1) The name of a (single)  shared  object  to  be
              profiled, specified either as a pathname or a soname.  Profiling
              output is appended to the file whose name is:  "$LD_PROFILE_OUT-
              PUT/$LD_PROFILE.profile".

       LD_PROFILE_OUTPUT
              (glibc  since  2.1)  Directory where LD_PROFILE output should be
              written.  If this variable is not defined, or is defined  as  an
              empty  string,  then the default is /var/tmp.  LD_PROFILE_OUTPUT
              is ignored for  set-user-ID  and  set-group-ID  programs,  which
              always use /var/profile.

       LD_SHOW_AUXV
              (glibc  since  2.1) Show auxiliary array passed up from the ker-
              nel.  For security reasons, since glibc 2.3.5,  LD_SHOW_AUXV  is
              ignored for set-user-ID/set-group-ID binaries.

       LD_USE_LOAD_BIAS
              By  default  (i.e., if this variable is not defined) executables
              and prelinked shared objects will honor base addresses of  their
              dependent libraries and (nonprelinked) position-independent exe-
              cutables (PIEs) and other shared objects will  not  honor  them.
              If  LD_USE_LOAD_BIAS  is defined wit the value, both executables
              and PIEs will honor the base addresses.  If LD_USE_LOAD_BIAS  is
              defined  with  the  value  0,  neither executables nor PIEs will
              honor the base addresses.  This variable is ignored by set-user-
              ID and set-group-ID programs.

       LD_VERBOSE
              (glibc  since  2.1)  If  set to a nonempty string, output symbol
              versioning   information    about    the    program    if    the
              LD_TRACE_LOADED_OBJECTS environment variable has been set.

       LD_WARN
              (ELF  only)(glibc since 2.1.3) If set to a nonempty string, warn
              about unresolved symbols.

       LDD_ARGV0
              (libc5) argv[0] to be used by ldd(1) when none is present.

FILES
       /lib/ld.so
              a.out dynamic linker/loader
       /lib/ld-linux.so.{1,2}
              ELF dynamic linker/loader
       /etc/ld.so.cache
              File containing a compiled  list  of  directories  in  which  to
              search for libraries and an ordered list of candidate libraries.
       /etc/ld.so.preload
              File  containing  a  whitespace-separated  list  of  ELF  shared
              libraries to be loaded before the program.
       lib*.so*
              shared libraries

NOTES
       The ld.so functionality is available  for  executables  compiled  using
       libc  version  4.4.3  or greater.  ELF functionality is available since
       Linux 1.1.52 and libc5.

SEE ALSO
       ldd(1), sprof(1), getauxval(3), rtld-audit(7), ldconfig(8), sln(8)

COLOPHON
       This page is part of release 3.71 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       http://www.kernel.org/doc/man-pages/.

GNU                               2014-07-08                          LD.SO(8)

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