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 format used long ago; ld-
       linux.so* handles ELF (/lib/ld-linux.so.1 for libc5, /lib/ld-
       linux.so.2 for glibc2), which everybody has been using for years now.
       Otherwise, both have the same behavior, and use the same support
       files and programs ldd(1), ldconfig(8), and /etc/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  (ELF only) Using the directories specified in the DT_RPATH dynamic
          section attribute of the binary if present and DT_RUNPATH
          attribute does not exist.  Use of DT_RPATH is 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  (ELF only) Using the directories specified in the DT_RUNPATH
          dynamic section attribute of the binary if present.

       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.

   Rpath token expansion
       ld.so 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
              Verify that program is dynamically linked and this dynamic
              linker can handle it.

       --inhibit-cache
              Do not use /etc/ld.so.cache.

       --library-path path
              Use path instead of LD_LIBRARY_PATH environment variable
              setting (see below).

       --inhibit-rpath list
              Ignore RPATH and RUNPATH information in object names in list.
              This option is ignored 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
       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.  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 (glibc since 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
              (libc5; glibc since 2.1.1) If set to a nonempty string, causes
              the dynamic linker to resolve all symbols at program startup
              instead of deferring function call resolution to the point
              when they are first referenced.  This is useful when using a
              debugger.

       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.  Similar to the PATH environment
              variable.  This variable is ignored in secure-execution mode.

       LD_PRELOAD
              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 shared objects in the
              standard search directories are loaded only if the set-user-ID
              mode bit is enabled on the shared object file.

       LD_TRACE_LOADED_OBJECTS
              (ELF only) 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_AOUT_LIBRARY_PATH
              (libc5) Version of LD_LIBRARY_PATH for a.out binaries only.
              Old versions of ld-linux.so.1 also supported
              LD_ELF_LIBRARY_PATH.

       LD_AOUT_PRELOAD
              (libc5) Version of LD_PRELOAD for a.out binaries only.  Old
              versions of ld-linux.so.1 also supported LD_ELF_PRELOAD.

       LD_AUDIT
              (glibc since 2.4) A colon-separated list of user-specified,
              ELF shared objects to be loaded before all others in a
              separate linker namespace (i.e., one that does not intrude
              upon the normal symbol bindings that would occur in the
              process).  These 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
              (glibc since 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
              (glibc since 2.1) Output verbose debugging information about
              the dynamic linker.  If set to all prints all debugging
              information it has, if set to help prints a help message about
              which categories can be specified in this environment
              variable.  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
              (glibc since 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
              (glibc since 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
              (glibc since 2.1) Mask for hardware capabilities.

       LD_KEEPDIR
              (a.out only)(libc5) Don't ignore the directory in the names of
              a.out libraries to be loaded.  Use of this option is strongly
              discouraged.

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

       LD_ORIGIN_PATH
              (glibc since 2.1) Path where the binary is found (for non-set-
              user-ID programs).  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
              (glibc since 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".

       LD_PROFILE_OUTPUT
              (glibc since 2.1) Directory where LD_PROFILE output should be
              written.  If this variable is not defined, or is defined as an
              empty string, then the default is /var/tmp.  LD_PROFILE_OUTPUT
              is ignored in secure-execution mode; instead /var/profile is
              always used.

       LD_SHOW_AUXV
              (glibc since 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
              (glibc since 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
              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
              (glibc since 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
              (ELF only)(glibc since 2.1.3) If set to a nonempty string,
              warn about unresolved symbols.

       LD_PREFER_MAP_32BIT_EXEC
              (x86-64 only)(glibc since 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), ld.so 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.

       LDD_ARGV0
              (libc5) argv[0] to be used by ldd(1) when none is present.

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.
       /etc/ld.so.preload
              File containing a whitespace-separated list of ELF shared
              objects to be loaded before the program.
       lib*.so*
              shared objects

NOTES         top

       The ld.so functionality is available for executables compiled using
       libc version 4.4.3 or greater.  ELF functionality is available since
       Linux 1.1.52 and libc5.

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

COLOPHON         top

       This page is part of release 4.05 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                              2015-12-28                         LD.SO(8)