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

NAME,* - 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/*  [OPTIONS] [PROGRAM [ARGUMENTS]]

       The  programs and* 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 handles a.out binaries, a format used long ago; ld-* handles ELF (/lib/ for libc5, /lib/
       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/

       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-

       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

       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/,	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 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)).

       --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
	      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 is set-user-ID or set-group-ID.

       --audit LIST
	      Use objects named	in LIST	as auditors.

       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

	      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

       There are four important	environment variables.

	      (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).

	      (glibc since 2.2)	Don't update the Global	Offset Table (GOT) and
	      Procedure	Linkage	Table (PLT) when resolving a symbol.

	      (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-

	      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.

	      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 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.

	      (libc5) Version of LD_LIBRARY_PATH for a.out binaries only.  Old
	      versions of	also supported LD_ELF_LIBRARY_PATH.

	      (libc5) Version of LD_PRELOAD for	a.out binaries only.  Old ver-
	      sions of 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,
	      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	inter-
	      face is largely compatible with that  provided  on  Solaris,  as
	      described	in its Linker and Libraries Guide, in the chapter Run-
	      time Linker Auditing 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 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.

	      (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 (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.

	      (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.

	      (ELF only)(glibc since 2.1.3) If set to a	nonempty 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 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), sln(1), getauxval(3), rtld-audit(7), ldconfig(8)

       This page is part of release 3.53 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at

GNU				  2013-07-15			      LD.SO(8)


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