ld.so

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 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.

       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 non-empty 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
              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 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 non-empty 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 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 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 (non-prelinked) 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 non-empty string, output symbol
              versioning information about the program if querying information
              about the program (i.e., either LD_TRACE_LOADED_OBJECTS has been
              set, or --list or --verify options have been given to the
              dynamic linker).

       LD_WARN
              (ELF only)(glibc since 2.1.3) If set to a non-empty 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 3.22 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)