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 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 compila-
       tion.

       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.

       The shared libraries needed by the program are 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 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.

       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.

   $ORIGIN and rpath
       ld.so understands the string $ORIGIN (or equivalently ${ORIGIN}) in an rpath specification (DT_RPATH  or  DT_RUNPATH)  to
       mean  the directory containing the application executable.  Thus, an application located in somedir/app could be compiled
       with gcc -Wl,-rpath,'$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.

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
              Override 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.

ENVIRONMENT
       There are four important environment variables.

       LD_BIND_NOW
              (libc5; glibc since 2.1.1) If set to a nonempty string, causes the dynamic linker to resolve all symbols  at  pro-
              gram  startup  instead of deferring function call resolution 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 execution-time.  Similar to the PATH
              environment variable.

       LD_PRELOAD
              A  whitespace-separated  list  of additional, user-specified, ELF shared libraries to be loaded before all others.
              This can be used to selectively override functions in other shared libraries.   For  set-user-ID/set-group-ID  ELF
              binaries, only libraries in the standard search directories that are also set-user-ID will be loaded.

       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 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 interface is largely compatible with that
              provided on Solaris, as described in its Linker and Libraries Guide, in the chapter Runtime Linker Auditing Inter-
              face.

       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  debug-
              ging  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 where LD_DEBUG output should be fed into, 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 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) Shared object to be profiled, specified either as a pathname or a soname.  Profiling  output  is
              written to the file whose name is: "$LD_PROFILE_OUTPUT/$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  querying
              information about the program (i.e., either LD_TRACE_LOADED_OBJECTS 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), rtld-audit(7), ldconfig(8)

COLOPHON
       This  page  is  part of release 3.25 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                                                        2009-01-12                                                   LD.SO(8)