ld-linux

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  binary  format  used  long
       ago.    The   program   ld-linux.so*   (/lib/ld-linux.so.1  for  libc5,
       /lib/ld-linux.so.2 for glibc2) handles binaries that are  in  the  more
       modern  ELF  format.  Both programs 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 in-
       spects 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 at-
          tribute 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 this variable 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  de-
          pendency tree.

       o  From the cache file /etc/ld.so.cache, which contains a compiled list
          of candidate shared objects previously found in  the  augmented  li-
          brary 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 be-
          low) 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  in-
              stead  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
       --audit list
              Use objects named in list as auditors.  The objects in list  are
              delimited by colons.

       --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).   The  objects  in list are delimited by colons or
              spaces.

       --list List all dependencies and how they are resolved.

       --preload list (since glibc 2.30)
              Preload the objects specified in list.  The objects in list  are
              delimited by colons or spaces.  The objects are preloaded as ex-
              plained in the description of the LD_PRELOAD  environment  vari-
              able below.

              By contrast with LD_PRELOAD, the --preload option provides a way
              to perform preloading for a single executable without  affecting
              preloading  performed  in  any child process that executes a new
              program.

       --verify
              Verify that program  is  dynamically  linked  and  this  dynamic
              linker can handle it.

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

   Secure-execution mode
       For security reasons, if the dynamic linker determines  that  a  binary
       should be run in secure-execution mode, the effects of some environment
       variables are voided or modified,  and  furthermore  those  environment
       variables  are  stripped from the environment, so that the program does
       not even see the definitions.  Some of these environment variables  af-
       fect  the operation of the dynamic linker itself, and are described be-
       low.   Other  environment  variables  treated  in  this  way   include:
       GCONV_PATH,   GETCONF_DIR,   HOSTALIASES,  LOCALDOMAIN,  LOCPATH,  MAL-
       LOC_TRACE, NIS_PATH, NLSPATH,  RESOLV_HOST_CONF,  RES_OPTIONS,  TMPDIR,
       and TZDIR.

       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  en-
       try 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
          capabilities to the process.

       *  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 en-
              coded 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  as-
              sume  that it is running on a system with a different kernel ABI
              version.  For example, the following command line causes the dy-
              namic 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  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  re-
              solve  all symbols at program startup instead of deferring func-
              tion 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, and there is no support for escaping ei-
              ther separator.

              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.  This feature can be  used  to  selec-
              tively override functions in other shared objects.

              The  items of the list can be separated by spaces or colons, and
              there is no support for escaping either separator.  The  objects
              are  searched  for using the rules given under DESCRIPTION.  Ob-
              jects are searched for and added to the link map in the left-to-
              right order specified in the list.

              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.)

              There are various methods of specifying  libraries  to  be  pre-
              loaded, and these are handled in the following order:

              (1) The LD_PRELOAD environment variable.

              (2) The  --preload command-line option when invoking the dynamic
                  linker directly.

              (3) The /etc/ld.so.preload file (described below).

       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 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.  The items in the list are  colon-separated,
              and there is no support for escaping the separator.

              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 au-
              dit  shared object.  For details, see rtld-audit(7).  The audit-
              ing interface is largely compatible with that  provided  on  So-
              laris,  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 dy-
              namic 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  li-
              braries.   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  hi-
              jack  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 al-
              ways 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 be-
              fore 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
              4 GB  away from the branch.  If this environment variable is set
              (to any value), the dynamic linker will first try  to  map  exe-
              cutable 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 2 GB (not 4 GB) of the address space.

              Because  MAP_32BIT  reduces  the address range available for ad-
              dress    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 ob-
              jects 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), ca-
       pabilities(7), rtld-audit(7), ldconfig(8), sln(8)

COLOPHON
       This page is part of release 5.05 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                               2019-08-02                          LD.SO(8)
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