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* (/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 directo-
              ries that have the set-user-ID mode bit enabled are loaded.

              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
              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.

       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 symbol.

       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 (with any value),  trace
              prelinking of the object whose name is assigned to this environ-
              ment 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 (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 4.08 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                               2016-10-08                          LD.SO(8)

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