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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.

       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 (rela‐
       tive  or absolute) pathname, and the library is loaded using that path‐
       name.

       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 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.
	  Libraries installed in hardware capability directories  (see	below)
	  are preferred 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 exe‐
	      cutable.	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 platform 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 com‐
       bine  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,  efp‐
	      double,  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  minimum	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 running 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 load‐
	      ing 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 directories in the search path) that have differ‐
	      ent minimum kernel ABI  version  requirements,  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 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.	 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	 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.

       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 ver‐
	      sions 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  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‐
	      ries.

	      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.

       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 in which LD_DEBUG output should be writ‐
	      ten.   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 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  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 (nonprelinked) position-independent exe‐
	      cutables	(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), getauxval(3), rtld-audit(7), ldconfig(8), sln(8)

COLOPHON
       This  page  is  part of release 3.54 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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