LD.SO(8) LD.SO(8)
NAME
ld.so/ld-linux.so - dynamic linker/loader
DESCRIPTION
ld.so loads the shared libraries needed by a program, prepares the pro-
gram to run, and then runs it. Unless explicitly specified via the
-static option to ld during compilation, all Linux programs are incom-
plete and require further linking at run time.
The necessary shared libraries needed by the program are searched for
in the following order
o Using the environment variable LD_LIBRARY_PATH
(LD_AOUT_LIBRARY_PATH for a.out programs). Except if the exe-
cutable is a setuid/setgid binary, in which case it is ignored.
o From the cache file /etc/ld.so.cache which contains a compiled
list of candidate libraries previously found in the augmented
library path.
o In the default path /lib, and then /usr/lib.
COMMAND LINE 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).
--ignore-rpath LIST
Ignore RPATH and RUNPATH information in object names in LIST.
This option has been supported by glibc2 for about one hour.
Then it was renamed into:
--inhibit-rpath LIST
ENVIRONMENT
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 setuid/setgid ELF binaries, only libraries in
the standard search directories that are also setgid will be
loaded.
LD_TRACE_LOADED_OBJECTS
If present, causes the program to list its dynamic library
dependencies, as if run by ldd, instead of running normally.
LD_BIND_NOW
If present, causes the dynamic linker to resolve all symbols at
program startup instead of when they are first referenced.
LD_BIND_NOT
Do not update the GOT (global offset table) and PLT (procedure
linkage table) after resolving a symbol.
LD_AOUT_LIBRARY_PATH
A colon-separated list of directories in which to search for
a.out libraries at execution-time. Similar to the PATH environ-
ment variable.
LD_AOUT_PRELOAD
The name of an additional, user-specified, a.out shared library
to be loaded after all others. This can be used to selectively
override functions in other shared libraries.
LD_NOWARN
Suppress warnings about a.out libraries with incompatible minor
version numbers.
LD_WARN
If set to non-empty string, warn about unresolved symbols.
LD_KEEPDIR
Don't ignore the directory in the names of a.out libraries to be
loaded. Use of this option is strongly discouraged.
LD_DEBUG
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.
LD_DEBUG_OUTPUT
File where LD_DEBUG output should be fed into, default is stan-
dard output. LD_DEBUG_OUTPUT is ignored for setuid/setgid bina-
ries.
LD_VERBOSE
If set to non-empty string, output symbol versioning information
about the program if querying information about the program (ie.
either LD_TRACE_LOADED_OBJECTS has been set, or --list or --ver-
ify options have been given to the dynamic linker).
LD_PROFILE
Shared object to be profiled.
LD_PROFILE_OUTPUT
File where LD_PROFILE output should be stored, default is stan-
dard output. LD_PROFILE_OUTPUT is ignored for setuid/setgid
binaries.
LD_ASSUME_KERNEL
Every DSO (Dynamic Shared Object, aka shared library) can tell
the dynamic linker in glibc which minimum OS ABI version is
needed. The information about the minimum OS ABI version is
encoded in a ELF note section usually named .note.ABI-tag. This
is used to determine which library to load when multiple version
of the same library is installed on the system. The
LD_ASSUME_KERNEL environment variable overrides the kernel ver-
sion used by the dynamic linker to determine which library to
load.
RPATH TOKEN EXPANSION
The runtime linker provides a number of tokens that can be used in an
rpath specification (DT_RPATH or DT_RUNPATH).
$ORIGIN
ld.so understands the string $ORIGIN (or equivalently ${ORIGIN})
in an rpath specification to mean the directory containing the
application executable. Thus, an application located in
somedir/app could be compiled with gcc -Wl,-rpath,'$ORI-
GIN/../lib' so that it finds an associated shared library in
somedir/lib no matter where somedir is located in the directory
hierarchy.
$PLATFORM
The string $PLATFORM (or equivalently ${PLATFORM}) in an rpath
specifcation expands to the processor type of the current
machine. Please note that on some architectures the Linux kernel
doesn't provide a platform string to the dynamic linker.
$LIB The string $LIB (or equivalently ${LIB}) in an rpath corresponds
to the system libraries directory, which is /lib for the native
architecture on FHS compliant GNU/Linux systems.
FILES
/lib/ld.so a.out dynamic linker/loader
/lib/ld-linux.so.* 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.
libraries and an ordered list of candidate
libraries.
/etc/ld.so.nohwcap When this file is present the dynamic linker will
load the non-optimized version of a library, even
if the CPU supports the optimized version.
lib*.so* shared libraries
SEE ALSO
ldd(1), ldconfig(8).
BUGS
Currently ld.so has no means of unloading and searching for compatible
or newer version of libraries.
ld.so functionality is only available for executables compiled using
libc version 4.4.3 or greater.
AUTHORS
David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus Tor-
valds, Lars Wirzenius and Mitch D'Souza (not necessarily in that
order).
26 May 2007 LD.SO(8)
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