Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
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  in-
	      stead  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  di-
       rectories  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 de-
       pends 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

       Among the more important	environment variables are the following:

	      (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  as-
	      sume  that it is running on a system with	a different kernel ABI
	      version.	For example, the following command line	causes the dy-
	      namic linker to assume it	is running on Linux 2.2.5 when loading
	      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 in-
	      stead  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  en-
	      abled 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),  in-
	      stead 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 li-
	      braries 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, re-
	      solving a	symbol,	or calling a symbol from  another  shared  ob-
	      ject--by	calling	 an  appropriate function within the audit li-
	      brary.  For details, see rtld-audit(7).  The auditing  interface
	      is  largely  compatible  with  that  provided on Solaris,	as de-
	      scribed 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 error.  LD_DEBUG_OUTPUT is ignored
	      for set-user-ID/set-group-ID binaries.

	      (glibc  since  2.1.91)  Allow weak symbols to be overridden (re-
	      verting 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)	The name of a (single)	shared	object	to  be
	      profiled,	specified either as a pathname or a soname.  Profiling
	      output is	appended to the	file whose name	is:  "$LD_PROFILE_OUT-

	      (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  al-
	      ways 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 li-
	      braries 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.

       ld(1), ldd(1), pldd(1),	sprof(1),  dlopen(3),  getauxval(3),  rtld-au-
       dit(7), ldconfig(8), sln(8)

       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at

GNU				  2014-10-02			      LD.SO(8)


Want to link to this manual page? Use this URL:

home | help