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

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

       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]]

       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

       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.

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

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

       --library-path PATH
	      Override	LD_LIBRARY_PATH	 environment  variable	setting	  (see

       --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.

       There are four important environment variables.

	      (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‐

	      A colon-separated list of directories in which to search for ELF
	      libraries	 at  execution-time.   Similar to the PATH environment

	      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.

	      (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.

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

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

	      (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‐

	      The  dynamic linker will notify the audit libraries at so-called
	      auditing checkpoints—for example, loading a new library, resolv‐
	      ing  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 Audit‐
	      ing Interface.

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

	      (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

	      (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.

	      (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.

	      (glibc since 2.1) Mask for hardware capabilities.

	      (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

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

	      (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.

	      (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.

	      (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".

	      (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.

	      (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.

	      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.

	      (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).

	      (ELF only)(glibc since 2.1.3) If set to a non-empty string, warn
	      about unresolved symbols.

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

	      a.out dynamic linker/loader
	      ELF dynamic linker/loader
	      File containing a compiled  list	of  directories	 in  which  to
	      search for libraries and an ordered list of candidate libraries.
	      File  containing	a  whitespace  separated  list	of  ELF shared
	      libraries to be loaded before the program.
	      shared libraries

       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.

       ldd(1), rtld-audit(7), ldconfig(8)

       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)

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