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 programs 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.  Such directories are searched
          only to find those objects required by DT_NEEDED  (direct  dependen-
          cies)  entries  and  do  not apply to those objects' children, which
          must themselves have their own DT_RUNPATH entries.  This  is  unlike
          DT_RPATH,  which  is  applied  to  searches  for all children in the
          dependency tree.

       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
       The dynamic linker understands certain token strings in an rpath speci-
       fication  (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.  (For details, see the discus-
       sion of individual environment variables below.)  A binary is  executed
       in secure-execution mode if 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  fea-
              ture 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 tokens $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.   Furthermore,  shared  objects are preloaded only
              from the standard search directories and only if they have  set-
              user-ID mode bit enabled (which is not typical).

              Within  the  names specified in the LD_PRELOAD list, the dynamic
              linker understands the tokens $ORIGIN, $LIB, and  $PLATFORM  (or
              the  versions  using curly braces around the names) as described
              above in Rpath token expansion.  (See  also  the  discussion  of
              quoting under the description of LD_LIBRARY_PATH.)

       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 tokens $ORIGIN, $LIB, and $PLATFORM (or
              the versions using curly braces around the names)  as  described
              above  in  Rpath  token  expansion.  (See also the discussion of
              quoting under the description of LD_LIBRARY_PATH.)

              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  operation of the
              dynamic linker.  The content of this variable is one of more  of
              the  following  categories,  separated by colons, commas, or (if
              the value is quoted) spaces:

              help        Specifying help in the value of this  variable  does
                          not  run  the specified program, and displays a help
                          message about which categories can be  specified  in
                          this environment variable.

              all         Print  all  debugging information (except statistics
                          and unused; see below).

              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.

              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)
              By  default,  LD_DEBUG  output is written to standard error.  If
              LD_DEBUG_OUTPUT is defined, then output is written to the  path-
              name  specified by its value, with the suffix "." (dot) followed
              by the process ID appended to the pathname.

              LD_DEBUG_OUTPUT is ignored in secure-execution mode.

       LD_DYNAMIC_WEAK (since glibc 2.1.91)
              By default, when searching shared libraries to resolve a  symbol
              reference,  the dynamic linker will resolve to the first defini-
              tion it finds.

              Old glibc versions (before 2.2), provided a different  behavior:
              if  the  linker  found a symbol that was weak, it would remember
              that  symbol  and  keep  searching  in  the   remaining   shared
              libraries.   If it subsequently found a strong definition of the
              same symbol, then it would instead use that definition.  (If  no
              further  symbol was found, then the dynamic linker would use the
              weak symbol that it initially found.)

              The old glibc behavior was nonstandard.  (Standard  practice  is
              that the distinction between weak and strong symbols should have
              effect only at static link time.)  In  glibc  2.2,  the  dynamic
              linker  was  modified to provide the current behavior (which was
              the behavior that was provided by most other implementations  at
              that time).

              Defining  the  LD_DYNAMIC_WEAK  environment  variable  (with any
              value) provides the old (nonstandard) glibc behavior, whereby  a
              weak  symbol in one shared library may be overridden by a strong
              symbol subsequently discovered in another shared library.  (Note
              that even when this variable is set, a strong symbol in a shared
              library will not override a weak definition of the  same  symbol
              in the main program.)

              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".

              Since  glibc  2.2.5,  LD_PROFILE  is ignored in secure-execution
              mode.

       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.   (This  detail is relevant only
              before glibc 2.2.5, since in later glibc versions, LD_PROFILE is
              also ignored in secure-execution mode.)

       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  2.3.4, 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.

              Since  glibc 2.3.3, 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), the dynamic linker 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.

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.  See ldconfig(8).
       /etc/ld.so.preload
              File  containing  a  whitespace-separated  list  of  ELF  shared
              objects to be loaded before the program.  See the discussion  of
              LD_PRELOAD above.  If both LD_PRELOAD and /etc/ld.so.preload are
              employed, the libraries specified by  LD_PRELOAD  are  preloaded
              first.  /etc/ld.so.preload has a system-wide effect, causing the
              specified libraries to be preloaded for all  programs  that  are
              executed  on  the  system.  (This is usually undesirable, and is
              typically employed only as an emergency remedy, for example,  as
              a temporary workaround to a library misconfiguration issue.)
       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), elf(5),
       capabilities(7), rtld-audit(7), ldconfig(8), sln(8)

COLOPHON
       This page is part of release 4.12 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                               2017-05-03                          LD.SO(8)

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