.TH "LD.SO" 8 "26 May 2007"
.SH NAME
ld.so/ld-linux.so \- dynamic linker/loader
.SH DESCRIPTION
.B ld.so
loads the shared libraries needed by a program, prepares the program
to run, and then runs it.
Unless explicitly specified via the
.B \-static
option to
.B ld
during compilation, all Linux programs are incomplete and require 
further linking at run time.
.PP
The necessary shared libraries needed by the program are searched for 
in the following order
.IP o
Using the environment variable
.B LD_LIBRARY_PATH
.RB ( LD_AOUT_LIBRARY_PATH
for a.out programs).
Except if the executable is a setuid/setgid binary, in which case it
is ignored.
.IP o
From the cache file
.BR /etc/ld.so.cache
which contains a compiled list of candidate libraries previously found
in the augmented library path.
.IP o
In the default path
.BR /lib ,
and then
.BR /usr/lib .
.SH COMMAND LINE OPTIONS
.TP
.B \-\-list
List all dependencies and how they are resolved.
.TP
.B \-\-verify
Verify that program is dynamically linked and this dynamic linker can handle
it.
.TP
.B \-\-library\-path PATH
Override
.B LD_LIBRARY_PATH
environment variable setting (see below).
.TP
.B \-\-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:
.TP
.B \-\-inhibit\-rpath LIST
.SH ENVIRONMENT
.TP
.B LD_LIBRARY_PATH
A colon-separated list of directories in which to search for
ELF libraries at execution-time.
Similar to the 
.B PATH
environment variable.
.TP
.B 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.
.TP
.B LD_TRACE_LOADED_OBJECTS
If present, causes the program to list its dynamic library dependencies,
as if run by ldd, instead of running normally.
.TP
.B LD_BIND_NOW
If present, causes the dynamic linker to resolve all symbols at program
startup instead of when they are first referenced.
.TP
.B LD_BIND_NOT 
Do not update the GOT (global offset table) and PLT (procedure linkage table) after resolving a symbol.
.TP
.B LD_AOUT_LIBRARY_PATH
A colon-separated list of directories in which to search for
a.out libraries at execution-time.
Similar to the 
.B PATH
environment variable.
.TP
.B 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.
.TP
.B LD_NOWARN
Suppress warnings about a.out libraries with incompatible minor 
version numbers.
.TP
.B LD_WARN
If set to non-empty string, warn about unresolved symbols.
.TP
.B LD_KEEPDIR
Don't ignore the directory in the names of a.out libraries to be loaded.
Use of this option is strongly discouraged.
.TP
.B 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.
.TP
.B LD_DEBUG_OUTPUT
File where LD_DEBUG output should be fed into, default is standard
output. LD_DEBUG_OUTPUT is ignored for setuid/setgid binaries.
.TP
.B LD_VERBOSE
If set to non-empty string, output symbol versioning information
about the program if querying information about the program (ie. either
.B LD_TRACE_LOADED_OBJECTS 
has been set, or --list or --verify options have been given to the dynamic linker).
.TP
.B LD_PROFILE 
Shared object to be profiled. 
.TP
.B
LD_PROFILE_OUTPUT 
File where 
.B LD_PROFILE
output should be stored, default is standard output. LD_PROFILE_OUTPUT is ignored for setuid/setgid binaries.
.TP
.B 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
.B LD_ASSUME_KERNEL
environment variable overrides the kernel version used by the dynamic linker to determine which
library to load.
.SH RPATH TOKEN EXPANSION
The runtime linker provides a number of tokens that can be used in an rpath 
specification (\fBDT_RPATH\fR or \fBDT_RUNPATH\fR).
.TP
.B $ORIGIN
ld.so understands the string \fB$ORIGIN\fR (or equivalently \fB${ORIGIN}\fR) 
in an rpath specification to mean the directory containing the application
executable. Thus, an application located in somedir/app could be compiled
with gcc -Wl,-rpath,'$ORIGIN/../lib' so that it finds an associated shared
library in somedir/lib no matter where somedir is located in the directory 
hierarchy.
.TP
.B $PLATFORM
The string \fB$PLATFORM\fR (or equivalently \fB${PLATFORM}\fR) 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.
.TP
.B $LIB
The string \fB$LIB\fR (or equivalently \fB${LIB}\fR) in an rpath corresponds
to the system libraries directory, which is /lib for the native architecture
on FHS compliant GNU/Linux systems.
.SH FILES
.PD 0
.TP 20
.B /lib/ld.so
a.out dynamic linker/loader
.TP 20
.B /lib/ld-linux.so.*
ELF dynamic linker/loader
.TP
.B /etc/ld.so.cache
File containing a compiled list of directories in which to search for
libraries and an ordered list of candidate libraries.
.TP
.B /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.
.TP
.B /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.
.TP
.B lib*.so*
shared libraries
.PD
.SH SEE ALSO
.BR ldd (1),
.BR ldconfig (8).
.SH BUGS
.LP
Currently
.B ld.so
has no means of unloading and searching for compatible or newer version of
libraries.
.PP
.B ld.so
functionality is only available for executables compiled using libc version
4.4.3 or greater.
.SH AUTHORS
David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
Torvalds, Lars Wirzenius and Mitch D'Souza (not necessarily in that order).