ld.so(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT | FILES | NOTES | SEE ALSO | COLOPHON

LD.SO(8)                Linux Programmer's Manual               LD.SO(8)

NAME         top

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

SYNOPSIS         top

       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         top

       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:

       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 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 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/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 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.

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

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

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

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

       o  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         top

       --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.

       --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.

       --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         top

       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, 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 (since glibc 2.2.3)
              Each shared object can inform the dynamic linker of the
              minimum kernel ABI version that it requires.  (This
              requirement is encoded in an ELF note section that is
              viewable via readelf -n as a section labeled
              NT_GNU_ABI_TAG.)  At run time, the dynamic linker
              determines the ABI version of the running kernel and will
              reject loading shared objects that specify minimum ABI
              versions that exceed that ABI version.

              LD_ASSUME_KERNEL can be used to cause the dynamic linker
              to assume that it is running on a system with a different
              kernel ABI version.  For example, the following command
              line causes the dynamic linker to assume it is running on
              Linux 2.2.5 when 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 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:
              "$LD_PROFILE_OUTPUT/$LD_PROFILE.profile".

              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.

FILES         top

       /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         top

   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

       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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.11 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                            2021-03-22                       LD.SO(8)

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