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NAME, - dynamic linker/loader


       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/*  [OPTIONS] [PROGRAM [ARGUMENTS]]


       The programs and* 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  handles  a.out  binaries,  a  format  used  long  ago;*
       (/lib/ for libc5, /lib/ 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 as ldd(1), ldconfig(8), and /etc/

       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 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  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 executable 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 dependencies) 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/, 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
       The dynamic linker understands certain token 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)).


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

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

              Do not use /etc/

       --library-path path
              Use  path instead of LD_LIBRARY_PATH environment variable setting (see below).  The
              names ORIGIN,  LIB,  and  PLATFORM  are  interpreted  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.


       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 discussion 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

       *  A  process with a non-root user ID executed a binary that conferred capabilities to the

       *  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

              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 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 referenced.  This is useful when using a debugger.

              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  $ORIGIN  and  $LIB  as
              shell variables!)

              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.
              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.)

              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

       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

              bindings    Display information about which definition each symbol 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 pathname 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 definition 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-execution 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:

              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 getauxval(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
              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
              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.

              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 information about the program
              if the LD_TRACE_LOADED_OBJECTS environment 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 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 2 GB (not  4 GB)  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  disabled  in  secure-
              execution mode.


              a.out dynamic linker/loader
              ELF dynamic linker/loader
              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).
              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/ are employed, the  libraries  specified  by  LD_PRELOAD  are
              preloaded   first.   /etc/  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.)
              shared objects


   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 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

       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


       ld(1),  ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5), capabilities(7), rtld-
       audit(7), ldconfig(8), sln(8)


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