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NAME

       ld.so, ld-linux.so - dynamic linker/loader

SYNOPSIS

       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/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 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:

       (1)  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.

       (2)  Using  the  environment  variable  LD_LIBRARY_PATH,  unless  the  executable is being run in secure-
            execution mode (see below), in which case this variable is ignored.

       (3)  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.

       (4)  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
            nodefaultlib 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.

       (5)  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
            nodefaultlib linker option, this step is skipped.

   Dynamic string tokens
       In several places, the dynamic linker expands dynamic string tokens:

       •  In the environment variables LD_LIBRARY_PATH, LD_PRELOAD, and LD_AUDIT,

       •  inside  the  values  of  the  dynamic  section  tags  DT_NEEDED,  DT_RPATH,  DT_RUNPATH, DT_AUDIT, and
          DT_DEPAUDIT of ELF binaries,

       •  in the arguments to the ld.so command line options --audit, --library-path, and --preload (see below),
          and

       •  in the filename arguments to the dlopen(3) and dlmopen(3) functions.

       The substituted tokens are 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)).

       Note  that  the  dynamic string tokens have to be quoted properly when set from a shell, to prevent their
       expansion as shell or environment variables.

OPTIONS

       --argv0 string (since glibc 2.33)
              Set argv[0] to the value string before running the program.

       --audit list
              Use objects named in list as auditors.  The objects in list are delimited by colons.

       --glibc-hwcaps-mask list
              only search built-in subdirectories if in list.

       --glibc-hwcaps-prepend list
              Search glibc-hwcaps subdirectories in list.

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

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

       --list-diagnostics (since glibc 2.33)
              Print system diagnostic information in a machine-readable format, such  as  some  internal  loader
              variables,  the  auxiliary  vector  (see  getauxval(3)),  and  the environment variables.  On some
              architectures, the command might print additional information (like the cpu features used  in  GNU
              indirect  function  selection  on  x86).   --list-tunables  (since glibc 2.33) Print the names and
              values of all tunables, along with the minimum and maximum allowed values.

       --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 explained in the description of the LD_PRELOAD environment variable
              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  affect  the  operation of the dynamic linker itself, and are described
       below.  Other environment variables treated in this way include:  GCONV_PATH,  GETCONF_DIR,  HOSTALIASES,
       LOCALDOMAIN,   LD_AUDIT,  LD_DEBUG,  LD_DEBUG_OUTPUT,  LD_DYNAMIC_WEAK,  LD_HWCAP_MASK,  LD_LIBRARY_PATH,
       LD_ORIGIN_PATH, LD_PRELOAD,  LD_PROFILE,  LD_SHOW_AUXV,  LOCALDOMAIN,  LOCPATH,  MALLOC_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 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 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 (from glibc 2.2.3 to glibc 2.36)
              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 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.

       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  either
              separator.  A zero-length directory name indicates the current working directory.

              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
              Dynamic  string tokens.  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!)

       LD_PRELOAD
              A list of additional, user-specified, ELF shared objects to be loaded  before  all  others.   This
              feature can be used to selectively 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.   Objects
              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  Dynamic  string  tokens.   (See also the discussion of quoting under the description of
              LD_LIBRARY_PATH.)

              There are various methods of specifying libraries to be preloaded, 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  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  Dynamic  string  tokens.   (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 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 glibc 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 (from glibc 2.1 to glibc 2.38)
              Mask for hardware capabilities.  Since glibc 2.26, the option might be ignored if glibc  does  not
              support tunables.

       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 (from glibc 2.4 to glibc 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.profile.

              Since glibc 2.2.5, LD_PROFILE uses a different default path 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.

       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 (from glibc 2.4 to glibc 2.35)
              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 (from glibc 2.3.3 to glibc 2.35)
              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.

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

   Legacy Hardware capabilities (from glibc 2.5 to glibc 2.37)
       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

       PowerPC
              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

       The  legacy  hardware  capabilities support has the drawback that each new feature added grows the search
       path exponentially, because it has to be added to every combination of the other existing features.

       For instance, on x86 32-bit, if the hardware supports i686 and sse2, the resulting search  path  will  be
       i686/sse2:i686:sse2:..     A    new    capability    newcap    will    set    the    search    path    to
       newcap/i686/sse2:newcap/i686:newcap/sse2:newcap:i686/sse2:i686:sse2:.

   glibc Hardware capabilities (from glibc 2.33)
       glibc 2.33 added a new hardware capability scheme,
              where under each CPU architecture, certain levels can be defined,  grouping  support  for  certain
              features  or  special instructions.  Each architecture level has a fixed set of paths that it adds
              to the dynamic linker search list, depending on the hardware  of  the  machine.   Since  each  new
              architecture level is not combined with previously existing ones, the new scheme does not have the
              drawback of growing the dynamic linker search list uncontrollably.

       For instance, on x86 64-bit, if the hardware supports  x86_64-v3  (for  instance  Intel  Haswell  or  AMD
       Excavator),  the  resulting  search  path  will  be  glibc-hwcaps/x86-64-v3:glibc-hwcaps/x86-64-v2:.  The
       following paths are currently supported, in priority order.

       PowerPC (64-bit little-endian only)
              power10, power9

       s390 (64-bit only)
              z16, z15, z14, z13

       x86 (64-bit only)
              x86-64-v4, x86-64-v3, x86-64-v2

       glibc 2.37 removed support for the legacy hardware capabilities.

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)