ld.so - phpMan

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

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

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 during compilation.

The program ld.so handles a.out binaries, a format used long ago; ld-linux.so* han-
dles 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 behaviour,
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 various places:

o (ELF only) Using the DT_RPATH dynamic section attribute of the binary if
present and DT_RUNPATH attribute does not exist. Use of DT_RPATH is depre-
cated.

o Using the environment variable LD_LIBRARY_PATH. Except if the executable is
a setuid/setgid binary, in which case it is ignored.

o (ELF only) Using 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 can-
didate libraries previously found in the augmented library path. If, how-
ever, the binary was linked with -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
-z nodeflib linker option, this step is skipped.

SYNOPSIS
The dynamic linker can be run either indirectly through 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]]

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
There are four important environment variables.

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 bina-
ries, only libraries in the standard search directories that are also setuid
will be loaded.

LD_BIND_NOW
(libc5; glibc since 2.1.1) If set to non-empty string, causes the dynamic
linker to resolve all symbols at program startup instead of deferring func-
tion call resolval to the point when they are first referenced. This is
useful when using a debugger.

LD_TRACE_LOADED_OBJECTS
(ELF only) If set to non-empty string, causes the program to list its
dynamic library 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_WARN
(ELF only)(glibc since 2.1.3) If set to non-empty string, warn about unre-
solved symbols.

LD_NOWARN
(a.out only)(libc5) Suppress warnings about a.out libraries with incompati-
ble minor version numbers.

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.

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 setuid/setgid binaries.

LD_VERBOSE
(glibc since 2.1) 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 --verify options
have been given to the dynamic linker).

LD_PROFILE
(glibc since 2.1) Shared object to be profiled.

LD_PROFILE_OUTPUT
(glibc since 2.1) File where LD_PROFILE output should be stored, default is
standard output. LD_DEBUG_OUTPUT is ignored for setuid/setgid binaries.

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_SHOW_AUXV
(glibc since 2.1) Show auxiliary array passed up from the kernel.

LD_HWCAP_MASK
(glibc since 2.1) Mask for hardware capabilities.

LD_ORIGIN_PATH
(glibc since 2.1) Path where the binary is found (for non-setuid programs).

LD_DYNAMIC_WEAK
(glibc since 2.1.91) Allow weak symbols to be overridden (reverting to old
glibc behaviour).

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.

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. libraries and an ordered list of candidate
libraries.
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.