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LD.SO(8) Linux Programmer's Manual LD.SO(8)
ld.so, ld-linux.so* - dynamic linker/loader
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 run-
ning:
/lib/ld-linux.so.* [OPTIONS] [PROGRAM [ARGUMENTS]]
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.
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.
--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.
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, or --list or
--verify options have been given to the dynamic linker).
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.
/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
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.
ldd(1), rtld-audit(7), ldconfig(8)
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/.
GNU 2009-01-12 LD.SO(8)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface