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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 dynamically 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 executed) 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  pro-
       gram, 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 programs 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 (relative or absolute) pathname, and the library  is  loaded  using  that
       pathname.

       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 dep-
          recated.

       o  Using the environment variable LD_LIBRARY_PATH.  Except if the  executable  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 pre-
          ferred 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  executable.   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 plat-
              form 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 combine  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,  efpdouble,  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
       There are four important environment variables.

       LD_ASSUME_KERNEL
              (glibc since 2.2.3) Each shared library can inform the dynamic linker of the  mini-
              mum  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 run-
              ning 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
              loading 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  direc-
              tories  in the search path) that have different minimum kernel ABI version require-
              ments, 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  reso-
              lution  to  the  point when they are first referenced.  This is useful when using a
              debugger.

       LD_LIBRARY_PATH
              A colon-separated list of directories in which to search for ELF libraries at  exe-
              cution-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 separated 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 directories 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  versions  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 normal 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 binaries.

              The dynamic linker will notify the audit libraries  at  so-called  auditing  check-
              points--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 interface is largely com-
              patible with that provided on Solaris, as described in  its  Linker  and  Libraries
              Guide, in the chapter Runtime 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 categories 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 written.  The default is
              standard output.  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 mecha-
              nism 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) Shared object to be profiled, specified either as a pathname or a
              soname.  Profiling output is written 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  kernel.   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 executables (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), sln(1), getauxval(3), rtld-audit(7), ldconfig(8)

COLOPHON
       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and    information    about    reporting    bugs,    can    be    found     at
       http://www.kernel.org/doc/man-pages/.



GNU                                         2013-07-15                                   LD.SO(8)

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