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 library (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 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.
The shared libraries needed by the program are 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. Except if the
executable is a set-user-ID/set-group-ID binary, 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 libraries previously found in the augmented library
path. If, however, the binary was linked with the -z nodeflib
linker option, libraries in the default library paths are skipped.
o In the default path /lib, and then /usr/lib. If the binary was
linked with the -z nodeflib linker option, this step is skipped.
$ORIGIN and rpath
ld.so understands the string $ORIGIN (or equivalently ${ORIGIN}) in an
rpath specification (DT_RPATH or DT_RUNPATH) to mean the directory
containing the application executable. Thus, an application located in
somedir/app could be compiled with gcc -Wl,-rpath,'$ORIGIN/../lib' so
that it finds an associated shared library 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 libraries.
OPTIONS
--list List all dependencies and how they are resolved.
--verify
Verify that program is dynamically linked and this dynamic
linker can handle it.
--library-path PATH
Override 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 if ld.so is set-user-ID or set-group-ID.
ENVIRONMENT
There are four important environment variables.
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 colon-separated list of directories in which to search for ELF
libraries at execution-time. Similar to the PATH environment
variable.
LD_PRELOAD
A whitespace-separated list of additional, user-specified, ELF
shared libraries to be loaded before all others. This can be
used to selectively override functions in other shared
libraries. For set-user-ID/set-group-ID ELF binaries, only
libraries in the standard search directories that are also set-
user-ID will be loaded.
LD_TRACE_LOADED_OBJECTS
(ELF only) If set to a nonempty string, causes the program to
list its dynamic library 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
libraries can be used to audit the operation of the dynamic
linker. LD_AUDIT is ignored for set-user-ID/set-group-ID
binaries.
The dynamic linker will notify the audit libraries at so-called
auditing checkpoints—for example, loading a new library,
resolving a symbol, or calling a symbol from another shared
object—by calling an appropriate function within the audit
library. 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) 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 for set-user-ID/set-group-ID
binaries.
LD_DEBUG_OUTPUT
(glibc since 2.1) File where LD_DEBUG output should be fed into,
default is standard output. LD_DEBUG_OUTPUT is ignored for set-
user-ID/set-group-ID binaries.
LD_DYNAMIC_WEAK
(glibc since 2.1.91) Allow weak symbols to be overridden
(reverting to old glibc behavior). For security reasons, since
glibc 2.3.4, LD_DYNAMIC_WEAK is ignored for set-user-ID/set-
group-ID binaries.
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). For security reasons, since glibc 2.4,
LD_ORIGIN_PATH is ignored for set-user-ID/set-group-ID binaries.
LD_POINTER_GUARD
(glibc since 2.4) 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.
LD_PROFILE
(glibc since 2.1) Shared object to be profiled, specified either
as a pathname or a soname. Profiling output is written 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 for set-user-ID and set-group-ID programs, which
always use /var/profile.
LD_SHOW_AUXV
(glibc since 2.1) Show auxiliary array passed up from the
kernel. For security reasons, since glibc 2.3.5, LD_SHOW_AUXV
is ignored for set-user-ID/set-group-ID binaries.
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 libraries and (nonprelinked) position-independent
executables (PIEs) and other shared objects will not honor them.
If LD_USE_LOAD_BIAS is defined wit the value, 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 by set-user-
ID and set-group-ID programs.
LD_VERBOSE
(glibc since 2.1) If set to a nonempty string, output symbol
versioning information about the program if querying information
about the program (i.e., either LD_TRACE_LOADED_OBJECTS has been
set.
LD_WARN
(ELF only)(glibc since 2.1.3) If set to a nonempty string, warn
about unresolved symbols.
LDD_ARGV0
(libc5) argv[0] to be used by ldd(1) when none is present.
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 libraries and an ordered list of candidate libraries.
/etc/ld.so.preload
File containing a whitespace separated list of ELF shared
libraries to be loaded before the program.
lib*.so*
shared libraries
NOTES
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.
SEE ALSO
ldd(1), rtld-audit(7), ldconfig(8)
COLOPHON
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.