ld.so

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



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

SYNOPSIS
       The dynamic linker can be run either indirectly by running some
       dynamically linked program or shared object (in which case no command-
       line options to the dynamic linker can be passed and, in the ELF case,
       the dynamic linker which is stored in the .interp section of the program
       is executed) or directly by running:

       /lib/ld-linux.so.*  [OPTIONS] [PROGRAM [ARGUMENTS]]

DESCRIPTION
       The programs ld.so and ld-linux.so* find and load the shared objects
       (shared libraries) needed by a program, prepare the program to run, and
       then run it.

       Linux binaries require dynamic linking (linking at run time) unless the
       -static option was given to ld(1) during compilation.

       The program ld.so handles a.out binaries, a binary format used long ago.
       The program ld-linux.so* (/lib/ld-linux.so.1 for libc5,
       /lib/ld-linux.so.2 for glibc2) handles binaries that are in the more
       modern ELF format.  Both programs have the same behavior, and use the
       same support files and programs (ldd(1), ldconfig(8), and
       /etc/ld.so.conf).

       When resolving shared object dependencies, the dynamic linker first
       inspects each dependency string to see if it contains a slash (this can
       occur if a shared object pathname containing slashes was specified at
       link time).  If a slash is found, then the dependency string is
       interpreted as a (relative or absolute) pathname, and the shared object
       is loaded using that pathname.

       If a shared object dependency does not contain a slash, then it is
       searched for in the following order:

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

COLOPHON
       This page is part of release 5.09 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                                2020-08-13                           LD.SO(8)