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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 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.

       The shared libraries needed by the program are searched for in the fol‐
       lowing 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.

       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 ${ORIGIN}) in an
       rpath specification (DT_RPATH or DT_RUNPATH) to mean the directory con‐
       taining	the  application  executable.  Thus, an application located in
       somedir/app could be compiled with gcc  -Wl,-rpath,'$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 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.

       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 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 ver‐
	      sions of ld-linux.so.1 also supported LD_ELF_PRELOAD.

       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 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_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  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	 (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), ldconfig(8)

COLOPHON
       This  page  is  part of release 3.15 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				  2008-10-27			      LD.SO(8)
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