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NAME,* - dynamic linker/loader


       The dynamic linker can be run either indirectly by running some dynamically linked program
       or library (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/*  [OPTIONS] [PROGRAM [ARGUMENTS]]



       Linux binaries require dynamic linking (linking at run time) unless the -static option was
       given to ld(1) during compilation.

       The program handles a.out binaries, a format used long ago;* handles ELF
       (/lib/  for  libc5,  /lib/ 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/

       When  resolving  library  dependencies,  the dynamic linker first inspects each dependency
       string to see if it contains a slash (this can occur  if  a  library  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 library  is  loaded  using  that

       If a library dependency does not contain a slash, then it is searched for in the following

       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

       o  Using the environment variable LD_LIBRARY_PATH.  Except if the  executable  is  a  set-
          user-ID/set-group-ID binary, 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/, which  contains  a  compiled  list  of  candidate
          libraries  previously found in the augmented library path.  If, however, the binary was
          linked with the -z nodeflib linker option, libraries in the default library  paths  are
          skipped.   Libraries  installed  in  hardware  capability  directories  (see below) are
          preferred to other libraries.

       o  In the default path /lib, and then /usr/lib.  If the binary  was  linked  with  the  -z
          nodeflib linker option, this step is skipped.

   Rpath token expansion  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  application  executable.   Thus,  an
              application located in somedir/app could be compiled with


              so  that  it  finds  an  associated  shared  library 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

       $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)).


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

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

       --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 if is set-user-ID or set-group-ID.

       --audit LIST
              Use objects named in LIST as auditors.


       Some libraries are compiled using hardware-specific instructions which  do  not  exist  on
       every  CPU.   Such  libraries  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 library.  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 recognized:

       Alpha  ev4, ev5, ev56, ev6, ev67

       MIPS   loongson2e, loongson2f, octeon, octeon2

              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


       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


       There are four important environment variables.

              (glibc since 2.2.3) Each shared library  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 libraries 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 loading the shared libraries required by myprog:

                  $ LD_ASSUME_KERNEL=2.2.5 ./myprog

              On systems that provide  multiple  versions  of  a  shared  library  (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  library
              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

              (glibc  since 2.2) Don't update the Global Offset Table (GOT) and Procedure Linkage
              Table (PLT) when resolving a symbol.

              (libc5; glibc since 2.1.1) If set to a nonempty string, causes the  dynamic  linker
              to  resolve  all  symbols  at  program  startup  instead of deferring function call
              resolution to the point when they are first referenced.  This is useful when  using
              a debugger.

              A  colon-separated  list  of  directories  in  which to search for ELF libraries at
              execution-time.  Similar to the PATH environment variable.  Ignored in  set-user-ID
              and set-group-ID programs.

              A  list of additional, user-specified, ELF shared libraries 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 libraries.  The libraries
              are searched for using the rules given  under  DESCRIPTION.   For  set-user-ID/set-
              group-ID  ELF  binaries,  preload  pathnames  containing  slashes  are ignored, and
              libraries in the standard search directories are loaded  only  if  the  set-user-ID
              permission bit is enabled on the library file.

              (ELF  only)  If  set  to  a nonempty string, causes the program to list its dynamic
              library 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

              (libc5)  Version  of  LD_LIBRARY_PATH  for  a.out  binaries  only.  Old versions of
     also supported LD_ELF_LIBRARY_PATH.

              (libc5)  Version  of  LD_PRELOAD  for  a.out  binaries  only.   Old   versions   of
     also supported LD_ELF_PRELOAD.

              (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
              libraries can be used to audit the operation of the dynamic  linker.   LD_AUDIT  is
              ignored for set-user-ID/set-group-ID binaries.

              The   dynamic  linker  will  notify  the  audit  libraries  at  so-called  auditing
              checkpoints—for example, loading a new library, resolving a symbol,  or  calling  a
              symbol  from  another  shared  object—by calling an appropriate function within the
              audit library.  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.

              (glibc since 2.1.95) Do not update the GOT (global offset table) and PLT (procedure
              linkage table) after resolving a symbol.

              (glibc  since  2.1)  Output verbose debugging information about the dynamic linker.
              message  about  which  categories  can  be  specified in this environment variable.
              Since glibc 2.3.4, LD_DEBUG is ignored for set-user-ID/set-group-ID binaries.

              (glibc since 2.1) File in which LD_DEBUG output should be written.  The default  is
              standard output.  LD_DEBUG_OUTPUT is ignored for set-user-ID/set-group-ID binaries.

              (glibc  since  2.1.91)  Allow weak symbols to be overridden (reverting to old glibc
              behavior).  For security reasons, since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored for

              (glibc since 2.1) Mask for hardware capabilities.

              (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.

              (a.out only)(libc5) Suppress warnings about a.out libraries with incompatible minor
              version numbers.

              (glibc  since  2.1)  Path where the binary is found (for non-set-user-ID programs).
              For security reasons, since glibc 2.4,  LD_ORIGIN_PATH  is  ignored  for  set-user-
              ID/set-group-ID binaries.

              (glibc  since  2.4)  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.

              (glibc since 2.1) Shared object to be profiled, specified either as a pathname or a
              soname.    Profiling   output   is   written   to   the   file   whose   name   is:

              (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 for set-user-ID and set-group-ID  programs,
              which always use /var/profile.

              (glibc  since  2.1)  Show  auxiliary array passed up from the kernel.  For security
              reasons, since glibc 2.3.5, LD_SHOW_AUXV is  ignored  for  set-user-ID/set-group-ID

              By default (i.e., if this variable is not defined) executables and prelinked shared
              objects will honor base addresses of their dependent libraries  and  (nonprelinked)
              position-independent  executables  (PIEs)  and  other shared objects will not honor
              them.  If LD_USE_LOAD_BIAS is defined wit the value, both executables 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  by

              about the program if the LD_TRACE_LOADED_OBJECTS environment variable has been set.


              (libc5) argv[0] to be used by ldd(1) when none is present.


              a.out dynamic linker/loader
              ELF dynamic linker/loader
              File containing a compiled list of directories in which to search for libraries and
              an ordered list of candidate libraries.
              File containing a whitespace-separated list of ELF shared libraries  to  be  loaded
              before the program.
              shared libraries


       The 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.


       ldd(1), getauxval(3), rtld-audit(7), ldconfig(8), sln(8)


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