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

NAME         top,* - 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/*  [OPTIONS] [PROGRAM [ARGUMENTS]]

DESCRIPTION         top

       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; ld-* (/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 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.

       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 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 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)).

OPTIONS         top

       --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.

ENVIRONMENT         top

       Various environment variables influence the operation of the dynamic

   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.  This determination is made
       by checking whether 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
          permitted or effective capabilities.

       *  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.

              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 strings $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, and only shared objects in the
              standard search directories that have the set-user-ID mode bit
              enabled are loaded.

              Within the pathnames specified in LD_PRELOAD, the dynamic
              linker understands the strings $ORIGIN, $LIB, and $PLATFORM
              (or the versions using curly braces around the names) as
              described above in Rpath token expansion.

              If set (to any value), causes the program to list its dynamic
              dependencies, as if run by ldd(1), instead of running

       Then there are lots of more or less obscure variables, many obsolete
       or only for internal use.

       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.

       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 symbol.

       LD_DEBUG (since glibc 2.1)
              Output verbose debugging information about the dynamic linker.
              If set to all, print all debugging information, Setting this
              variable to help does not run the specified program, and
              displays a help message about which categories can be
              specified in this environment variable.  The categories are:

              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.

              The value in LD_DEBUG can specify multiple categories,
              separated by colons, commas, or (if the value is quoted)

              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)
              File in which LD_DEBUG output should be written.  The default
              is standard error.  LD_DEBUG_OUTPUT is ignored in secure-
              execution mode.

       LD_DYNAMIC_WEAK (since glibc 2.1.91)
              If this environment variable is defined (with any value),
              allow weak symbols to be overridden (reverting to old glibc
              behavior).  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 (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 2.3.5, LD_SHOW_AUXV is ignored in
              secure-execution mode.

       LD_TRACE_PRELINKING (since glibc 2.4)
              If this environment variable is defined (with any value),
              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.
              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
              4GB away from the branch.  If this environment variable is set
              (to any value), 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 2GB (not 4GB) 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

              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.
              File containing a whitespace-separated list of ELF shared
              objects to be loaded before the program.
              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

              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),
       capabilities(7), rtld-audit(7), ldconfig(8), sln(8)

COLOPHON         top

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       latest version of this page, can be found at

GNU                              2016-10-08                         LD.SO(8)