.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. Libraries installed in hardware capabilities
directories (see below) are prefered to other libraries.
.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
There are four important environment variables.
.TP
.B LD_BIND_NOW
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.
.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 set to a nonempty string, causes the program to list its dynamic library
dependencies, as if run by
.BR ldd (1),
instead of running normally.
.LP
Then there are lots of more or less obscure variables,
many obsolete or only for internal use.
.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_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.
.TP
.B LD_AUDIT
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.
.B 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\(emfor example,
loading a new library, resolving a symbol,
or calling a symbol from another shared object\(emby
calling an appropriate function within the audit library.
For details, see
.BR rtld-audit (7).
The auditing interface is largely compatible with that provided on Solaris,
as described in its
.IR "Linker and Libraries Guide" ,
in the chapter
.IR "Runtime Linker Auditing Interface" .
.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_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_DYNAMIC_WEAK
Allow weak symbols to be overridden (reverting to old glibc behavior).
For security reasons, since glibc 2.3.4,
.B LD_DYNAMIC_WEAK
is ignored for set-user-ID/set-group-ID binaries.
.TP
.B LD_HWCAP_MASK
Mask for hardware capabilities.
.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_NOWARN
Suppress warnings about a.out libraries with incompatible minor 
version numbers.
.TP
.B LD_ORIGIN_PATH
Path where the binary is found (for non-set-user-ID programs).
For security reasons, since glibc 2.4,
.B LD_ORIGIN_PATH
is ignored for set-user-ID/set-group-ID binaries.
.TP
.B LD_POINTER_GUARD
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
.BR 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.
.TP
.B LD_PROFILE
Shared object to be profiled.
.TP
.B
LD_PROFILE_OUTPUT
File where
.B LD_PROFILE
output should be written.
If this variable is not defined, or is defined as an empty string,
then the default is
.IR /var/tmp .
.B LD_PROFILE_OUTPUT
is ignored for set-user-ID and set-group-ID programs,
which always use
.IR /var/profile .
.TP
.B LD_SHOW_AUXV
Show auxiliary array passed up from the kernel.
For security reasons, since glibc 2.3.5,
.B LD_SHOW_AUXV
is ignored for set-user-ID/set-group-ID binaries.
.TP
.B 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
.B LD_USE_LOAD_BIAS
is defined wit the value, both executables and PIEs
will honor the base addresses.
If
.B 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.
.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_WARN
If set to non-empty string, warn about unresolved symbols.
.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 HARDWARE CAPABILITIES
Libraries might be compiled using hardware-specific instructions which do not
exist on all CPU. Such libraries should be installed in directories whose name
defines the hardware capabilities such as \fB/usr/lib/sse2/\fR. The dynamic 
linker checks these directories against the hardware of the machine and selects
the best suitable version of a given library. Hardware capabilities directories
could be cascaded to combine CPU features. Hardware capabilities depends on
the CPU. The following names are currently recognized:
.TP
.B Alpha
ev4, ev5, ev56, ev6, ev67
.TP
.B MIPS
loongson2e, loongson2f, octeon, octeon2
.TP
.B PowerPC
4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
ppc64, smt, spe, ucache, vsx
.TP
.B SPARC
flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
.TP
.B s390
dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
z900, z990, z9-109, z10, zarch
.TP
.TP
.B x86 (32-bit only)
acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx, 
mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
.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).