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 shared object (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
       executed) 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 objects
       (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* (/lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2 for glibc2)
       handles ELF, 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 shared object dependencies,  the	dynamic	 linker	 first
       inspects each dependency string to see if it contains a slash (this can
       occur if a shared object pathname containing slashes was	 specified  at
       link  time).  If a slash is found, then the dependency string is inter-
       preted as a (relative or absolute) pathname, and the shared  object  is
       loaded using that pathname.

       If  a  shared  object  dependency  does not contain a slash, then it is
       searched for in the following order:

       o  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  (unless  the  exe-
	  cutable is being run in secure-execution mode; see below).  in which
	  case it is ignored.

       o  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	shared	objects	 previously  found  in	the  augmented
	  library  path.  If, however, the binary was linked with the -z node-
	  flib linker option, shared objects in the default paths are skipped.
	  Shared  objects  installed  in  hardware capability directories (see
	  below) are preferred to other shared objects.

       o  In the default path /lib, and then /usr/lib.	(On some 64-bit archi-
	  tectures,  the  default  paths for 64-bit shared objects are /lib64,
	  and then /usr/lib64.)	 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 program  or	shared
	      object.	Thus,  an  application located in somedir/app could be
	      compiled with

		  gcc -Wl,-rpath,'$ORIGIN/../lib'

	      so that it finds an associated shared object 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 objects.

       $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.

       --inhibit-cache
	      Do not use /etc/ld.so.cache.

       --library-path path
	      Use path instead of LD_LIBRARY_PATH environment variable setting
	      (see  below).   The  names  ORIGIN, LIB, and PLATFORM are inter-
	      preted as for the LD_LIBRARY_PATH environment variable.

       --inhibit-rpath list
	      Ignore RPATH and RUNPATH information in object  names  in	 list.
	      This  option  is	ignored	 when running in secure-execution mode
	      (see below).

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

ENVIRONMENT
       Various environment variables influence the operation  of  the  dynamic
       linker.

   Secure-execution mode
       For  security  reasons,	the  effects of some environment variables are
       voided or modified if the dynamic linker	 determines  that  the	binary
       should  be run in secure-execution mode.	 This determination is made by
       checking whether the AT_SECURE  entry  in  the  auxiliary  vector  (see
       getauxval(3)) has a nonzero value.  This entry may have a nonzero value
       for various reasons, including:

       *  The process's real and effective user IDs differ, or	the  real  and
	  effective  group  IDs	 differ.  This typically occurs as a result of
	  executing a set-user-ID or set-group-ID program.

       *  A process with a non-root user ID executed a binary  that  conferred
	  permitted or effective capabilities.

       *  A nonzero value may have been set by a Linux Security Module.

   Environment variables
       Among the more important environment variables are the following:

       LD_ASSUME_KERNEL (since glibc 2.2.3)
	      Each  shared object 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 objects 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 objects required by myprog:

		  $ LD_ASSUME_KERNEL=2.2.5 ./myprog

	      On systems that provide multiple versions of a shared object (in
	      different directories in the search path)	 that  have  different
	      minimum kernel ABI version requirements, LD_ASSUME_KERNEL can be
	      used to select the version of the object 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_NOW (since glibc 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 refer-
	      enced.  This is useful when using a debugger.

       LD_LIBRARY_PATH
	      A list of directories in which to search for  ELF	 libraries  at
	      execution-time.	The  items in the list are separated by either
	      colons or semicolons.  Similar to the PATH environment variable.
	      This variable is ignored in secure-execution mode.

	      Within  the  pathnames specified in LD_LIBRARY_PATH, the dynamic
	      linker expands the strings $ORIGIN, $LIB, and $PLATFORM (or  the
	      versions using curly braces around the names) as described above
	      in Rpath token expansion.	  Thus,	 for  example,	the  following
	      would  cause  a  library to be searched for in either the lib or
	      lib64 subdirectory below the directory containing the program to
	      be executed:

		  $ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog

	      (Note  the use of single quotes, which prevent expansion of ORI-
	      GIN and LIB as shell variables!)

       LD_PRELOAD
	      A list of additional, user-specified, ELF shared objects	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 objects.  The objects are
	      searched for using the rules given under DESCRIPTION.

	      In secure-execution mode, preload pathnames  containing  slashes
	      are  ignored,  and  only	shared	objects in the standard search
	      directories that have  the  set-user-ID  mode  bit  enabled  are
	      loaded.

	      Within  the  names specified in the LD_PRELOAD list, the dynamic
	      linker understands the strings $ORIGIN, $LIB, and $PLATFORM  (or
	      the  versions  using curly braces around the names) as described
	      above in Rpath token expansion.

       LD_TRACE_LOADED_OBJECTS
	      If set (to any value), causes the program to  list  its  dynamic
	      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_AUDIT (since glibc 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 normal symbol bindings
	      that  would occur in the process).  These objects can be used to
	      audit the operation of the dynamic linker.  LD_AUDIT is  ignored
	      in secure-execution mode.

	      The  dynamic  linker will notify the audit shared objects at so-
	      called auditing checkpoints--for example, loading a  new	shared
	      object,  resolving  a  symbol,  or calling a symbol from another
	      shared object--by calling an  appropriate	 function  within  the
	      audit  shared  object.   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 Auditing Interface.

	      Within the names specified in the	 LD_AUDIT  list,  the  dynamic
	      linker  understands the strings $ORIGIN, $LIB, and $PLATFORM (or
	      the versions using curly braces around the names)	 as  described
	      above in Rpath token expansion.

	      Since  glibc  2.13, in secure-execution mode, names in the audit
	      list that contain slashes are ignored, and only  shared  objects
	      in  the  standard	 search	 directories that have the set-user-ID
	      mode bit enabled are loaded.

       LD_BIND_NOT (since glibc 2.1.95)
	      If this environment variable is set to a nonempty string, do not
	      update  the GOT (global offset table) and PLT (procedure linkage
	      table) after resolving a function symbol.	 By combining the  use
	      of this variable with LD_DEBUG (with the categories bindings and
	      symbols), one can observe all run-time function bindings.

       LD_DEBUG (since glibc 2.1)
	      Output verbose debugging information about the  dynamic  linker.
	      If  set  to  all,	 print all debugging information, Setting this
	      variable to help does not run the specified  program,  and  dis-
	      plays  a help message about which categories can be specified in
	      this environment variable.  The categories are:

	      bindings	  Display information about which definition each sym-
			  bol is bound to.

	      files	  Display progress for input file.

	      libs	  Display library search paths.

	      reloc	  Display relocation processing.

	      scopes	  Display scope information.

	      statistics  Display relocation statistics.

	      symbols	  Display search paths for each symbol look-up.

	      unused	  Determine unused DSOs.

	      versions	  Display version dependencies.

	      The value in LD_DEBUG can specify multiple categories, separated
	      by colons, commas, or (if the value is quoted) spaces.

	      Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution mode,
	      unless  the file /etc/suid-debug exists (the content of the file
	      is irrelevant).

       LD_DEBUG_OUTPUT (since glibc 2.1)
	      File in which LD_DEBUG output should be written.	The default is
	      standard	error.	LD_DEBUG_OUTPUT is ignored in secure-execution
	      mode.

       LD_DYNAMIC_WEAK (since glibc 2.1.91)
	      If this environment variable is defined (with any value),	 allow
	      weak symbols to be overridden (reverting to old glibc behavior).
	      Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored  in	 secure-execu-
	      tion mode.

       LD_HWCAP_MASK (since glibc 2.1)
	      Mask for hardware capabilities.

       LD_ORIGIN_PATH (since glibc 2.1)
	      Path where the binary is found.  Since glibc 2.4, LD_ORIGIN_PATH
	      is ignored in secure-execution mode.

       LD_POINTER_GUARD (glibc from 2.4 to 2.22)
	      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.	Since glibc 2.23, LD_POINTER_GUARD can
	      no longer be used to disable  pointer  guarding,	which  is  now
	      always enabled.

       LD_PROFILE (since glibc 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: "$LD_PROFILE_OUTPUT/$LD_PROFILE.pro-
	      file".

       LD_PROFILE_OUTPUT (since glibc 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  in
	      secure-execution mode; instead /var/profile is always used.

       LD_SHOW_AUXV (since glibc 2.1)
	      If  this	environment variable is defined (with any value), show
	      the auxiliary array passed up from the kernel (see also  getaux-
	      val(3)).	 Since glibc 2.3.5, LD_SHOW_AUXV is ignored in secure-
	      execution mode.

       LD_TRACE_PRELINKING (since glibc 2.4)
	      If this environment variable is defined, trace prelinking of the
	      object  whose  name  is  assigned	 to this environment variable.
	      (Use ldd(1) to get a list of the objects that might be  traced.)
	      If the object name is not recognized, then all prelinking activ-
	      ity is traced.

       LD_USE_LOAD_BIAS (since glibc 2.3.3)
	      By default (i.e., if this variable is not defined),  executables
	      and  prelinked shared objects will honor base addresses of their
	      dependent shared objects and (nonprelinked) position-independent
	      executables (PIEs) and other shared objects will not honor them.
	      If LD_USE_LOAD_BIAS is defined with the value 1,	both  executa-
	      bles   and   PIEs	  will	 honor	 the   base   addresses.    If
	      LD_USE_LOAD_BIAS is defined with the value 0,  neither  executa-
	      bles  nor	 PIEs will honor the base addresses.  This variable is
	      ignored in secure-execution mode.

       LD_VERBOSE (since glibc 2.1)
	      If set to a nonempty string, output symbol  versioning  informa-
	      tion  about  the program if the LD_TRACE_LOADED_OBJECTS environ-
	      ment variable has been set.

       LD_WARN (since glibc 2.1.3)
	      If set to a nonempty string, warn about unresolved symbols.

       LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
	      According to the Intel Silvermont software  optimization	guide,
	      for  64-bit  applications,  branch prediction performance can be
	      negatively impacted when the target of a branch is more than 4GB
	      away  from  the branch.  If this environment variable is set (to
	      any value), ld.so will first try to map executable  pages	 using
	      the  mmap(2)  MAP_32BIT  flag,  and fall back to mapping without
	      that flag if that attempt fails.	NB: MAP_32BIT will map to  the
	      low  2GB	(not  4GB)  of	the  address space.  Because MAP_32BIT
	      reduces the address range available  for	address	 space	layout
	      randomization  (ASLR),  LD_PREFER_MAP_32BIT_EXEC	is always dis-
	      abled in secure-execution mode.

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  shared  objects  and	 an  ordered list of candidate
	      shared objects.
       /etc/ld.so.preload
	      File  containing	a  whitespace-separated	 list  of  ELF	shared
	      objects to be loaded before the program.
       lib*.so*
	      shared objects

NOTES
   Hardware capabilities
       Some  shared  objects are compiled using hardware-specific instructions
       which do not exist on every CPU.	 Such objects 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 shared object.  Hardware capability directories can  be  cascaded
       to  combine  CPU	 features.   The list of supported hardware capability
       names depends on the CPU.  The following	 names	are  currently	recog-
       nized:

       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

SEE ALSO
       ld(1),  ldd(1),	pldd(1),  sprof(1), dlopen(3), getauxval(3), capabili-
       ties(7), rtld-audit(7), ldconfig(8), sln(8)

COLOPHON
       This page is part of release 4.10 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
       https://www.kernel.org/doc/man-pages/.



GNU				  2017-03-13			      LD.SO(8)