ld-linux.so <root
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 dynamically 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 programs ldd(1), ldconfig(8), and /etc/ld.so.conf.

       When  resolving  shared object dependencies, the dynamic linker first inspects each depen‐
       dency 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 interpreted 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 dep‐
          recated.

       o  Using the environment variable LD_LIBRARY_PATH (unless the executable 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 nodeflib 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  architectures,  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 plat‐
              form 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).

       --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  effec‐
          tive 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 ver‐
              sion.

              LD_ASSUME_KERNEL can be used to cause the dynamic linker to assume that it is  run‐
              ning  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
              loading 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 directo‐
              ries in the search path) that have different minimum kernel  ABI  version  require‐
              ments,  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  reso‐
              lution  to  the  point when they are first referenced.  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.

       LD_PRELOAD
              A list of additional, user-specified, ELF shared objects to be  loaded  before  all
              others.   The  items of the list can be separated 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  stan‐
              dard  search  directories are loaded only if the set-user-ID mode bit is enabled on
              the shared object file.

       LD_TRACE_LOADED_OBJECTS
              (ELF only) If set (to any value), causes the program to list its dynamic  dependen‐
              cies, 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  versions  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 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 call‐
              ing 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  categories  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 written.  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  (for  non-set-user-ID  programs).
              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  secu‐
              rity  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
              (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: "$LD_PROFILE_OUTPUT/$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 in secure-execution mode; instead /var/pro‐
              file 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 exe‐
              cutables 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 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  Silvermont  software  opti‐
              mization guide, for 64-bit applications, branch prediction performance can be nega‐
              tively 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.

       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 fea‐
       tures.  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, efpdouble, 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), capabilities(7), rtld-audit(7),
       ldconfig(8), sln(8)

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

GNU                                         2015-12-28                                   LD.SO(8)

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