ld.so(8)



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* 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 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  (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  (unless  the  exe-
          cutable is being run in secure-execution mode; see below).  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  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 (glibc since 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
              (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  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  shared objects in the standard search directories
              are loaded only if the set-user-ID mode bit is  enabled  on  the
              shared object file.

              Within the pathnames specified in LD_PRELOAD, 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
              (ELF only) 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_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
              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.

       LD_BIND_NOT
              (glibc  since  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 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  in  secure-execution mode,
              unless the file /etc/suid-debug exists (the content of the  file
              is irrelevant).

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

       LD_DYNAMIC_WEAK
              (glibc  since  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-execution mode.

       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.  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  vari-
              ous  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
              (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: "$LD_PROFILE_OUT-
              PUT/$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  in  secure-execution  mode; instead /var/profile is
              always used.

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

       LD_TRACE_PRELINKING
              (glibc  since 2.4) If this environment variable is defined (with
              any value),  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 activity is traced.

       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 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
              (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.

       LD_PREFER_MAP_32BIT_EXEC
              (x86-64 only)(glibc since 2.23) According to the  Intel  Silver-
              mont  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_PRE-
              FER_MAP_32BIT_EXEC is always disabled in secure-execution mode.

       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  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
       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.

   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 4.06 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                               2016-05-09                          LD.SO(8)

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