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* (/lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2 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/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 it is ignored.

       o  Using the directories specified in the DT_RUNPATH dynamic section
          attribute of the binary if present.  Such directories are searched
          only to find those objects required by DT_NEEDED (direct
          dependencies) entries and do not apply to those objects' children,
          which must themselves have their own DT_RUNPATH entries.  This is
          unlike DT_RPATH, which is applied to searches for all children in
          the dependency tree.

       o  From the cache file /etc/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
       The dynamic linker understands certain token strings in an rpath
       specification (DT_RPATH or DT_RUNPATH).  Those strings are
       substituted as follows:

       $ORIGIN (or equivalently ${ORIGIN})
              This expands to the directory containing the program or shared
              object.  Thus, an application located in somedir/app could be
              compiled with

                  gcc -Wl,-rpath,'$ORIGIN/../lib'

              so that it finds an associated shared object in somedir/lib no
              matter where somedir is located in the directory hierarchy.
              This facilitates the creation of "turn-key" applications that
              do not need to be installed into special directories, but can
              instead be unpacked into any directory and still find their
              own shared objects.

       $LIB (or equivalently ${LIB})
              This expands to lib or lib64 depending on the architecture
              (e.g., on x86-64, it expands to lib64 and on x86-32, it
              expands to lib).

       $PLATFORM (or equivalently ${PLATFORM})
              This expands to a string corresponding to the processor type
              of the host system (e.g., "x86_64").  On some architectures,
              the Linux kernel doesn't provide a platform string to the
              dynamic linker.  The value of this string is taken from the
              AT_PLATFORM value in the auxiliary vector (see getauxval(3)).

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).  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
       linker.

   Secure-execution mode
       For security reasons, the effects of some environment variables are
       voided or modified if the dynamic linker determines that the binary
       should be run in secure-execution mode.  (For details, see the
       discussion of individual environment variables below.)  A binary is
       executed in secure-execution mode if the AT_SECURE entry in the
       auxiliary vector (see getauxval(3)) has a nonzero value.  This entry
       may have a nonzero value for various reasons, including:

       *  The process's real and effective user IDs differ, or the real and
          effective group IDs differ.  This typically occurs as a result of
          executing a set-user-ID or set-group-ID 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 dif‐
              ferent minimum kernel ABI version requirements, LD_ASSUME_KER‐
              NEL 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 fea‐
              ture was to manually select the older LinuxThreads POSIX
              threads implementation on systems that provided both Linux‐
              Threads 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 ref‐
              erenced.  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 vari‐
              able.

              This variable is ignored in secure-execution mode.

              Within the pathnames specified in LD_LIBRARY_PATH, the dynamic
              linker expands the tokens $ORIGIN, $LIB, and $PLATFORM (or the
              versions using curly braces around the names) as described
              above in Rpath token expansion.  Thus, for example, the fol‐
              lowing 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.  The items of the list can be sepa‐
              rated by spaces or colons.  This can be used to selectively
              override functions in other shared objects.  The objects are
              searched for using the rules given under DESCRIPTION.

              In secure-execution mode, preload pathnames containing slashes
              are ignored.  Furthermore, shared objects are preloaded only
              from the standard search directories and only if they have
              set-user-ID mode bit enabled (which is not typical).

              Within the names specified in the LD_PRELOAD list, the dynamic
              linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
              the versions using curly braces around the names) as described
              above in Rpath token expansion.  (See also the discussion of
              quoting under the description of LD_LIBRARY_PATH.)

       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 nor‐
              mally.

       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 bind‐
              ings that would occur in the process).  These objects can be
              used to audit the operation of the dynamic linker.

              LD_AUDIT is ignored in secure-execution mode.

              The dynamic linker will notify the audit shared objects at so-
              called auditing checkpoints—for example, loading a new shared
              object, resolving a symbol, or calling a symbol from another
              shared object—by calling an appropriate function within the
              audit shared object.  For details, see rtld-audit(7).  The
              auditing interface is largely compatible with that provided on
              Solaris, as described in its Linker and Libraries Guide, in
              the chapter Runtime Linker Auditing Interface.

              Within the names specified in the LD_AUDIT list, the dynamic
              linker understands the tokens $ORIGIN, $LIB, and $PLATFORM (or
              the versions using curly braces around the names) as described
              above in Rpath token expansion.  (See also the discussion of
              quoting under the description of LD_LIBRARY_PATH.)

              Since glibc 2.13, in secure-execution mode, names in the audit
              list that contain slashes are ignored, and only shared objects
              in the standard search directories that have the set-user-ID
              mode bit enabled are loaded.

       LD_BIND_NOT (since glibc 2.1.95)
              If this environment variable is set to a nonempty string, do
              not update the GOT (global offset table) and PLT (procedure
              linkage table) after resolving a function symbol.  By combin‐
              ing the use of this variable with LD_DEBUG (with the cate‐
              gories 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 sym‐
              bol reference, the dynamic linker will resolve to the first
              definition it finds.

              Old glibc versions (before 2.2), provided a different behav‐
              ior: 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 defini‐
              tion of the same symbol, then it would instead use that defi‐
              nition.  (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 imple‐
              mentations 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-execu‐
              tion 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 inter‐
              nals) 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: "$LD_PROFILE_OUT‐
              PUT/$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 vari‐
              able.  (Use ldd(1) to get a list of the objects that might be
              traced.)  If the object name is not recognized, then all pre‐
              linking activity is traced.

       LD_USE_LOAD_BIAS (since glibc 2.3.3)
              By default (i.e., if this variable is not defined), executa‐
              bles 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 exe‐
              cutables nor PIEs will honor the base addresses.

              Since glibc 2.3.3, this variable is ignored in secure-execu‐
              tion mode.

       LD_VERBOSE (since glibc 2.1)
              If set to a nonempty string, output symbol versioning informa‐
              tion about the program if the LD_TRACE_LOADED_OBJECTS environ‐
              ment 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_PRE‐
              FER_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 4.13 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                              2017-09-15                         LD.SO(8)

Pages that refer to this page: ldd(1)memusage(1)pldd(1)sprof(1)execve(2)prctl(2)uselib(2)dladdr(3)dlinfo(3)dl_iterate_phdr(3)dlopen(3)dlsym(3)getauxval(3)lttng-ust(3)lttng-ust-cyg-profile(3)environ(7)rtld-audit(7)execstack(8)ldconfig(8)prelink(8)sln(8)