| NAME | SYNOPSIS | DESCRIPTION | OPTIONS | HARDWARE CAPABILITIES | ENVIRONMENT | FILES | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
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 running:
/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.
When resolving library dependencies, the dynamic linker first
inspects each dependency string to see if it contains a slash (this
can occur if a library pathname containing slashes was specified at
link time). If a slash is found, then the dependency string is
interpreted as a (relative or absolute) pathname, and the library is
loaded using that pathname.
If a library dependency does not contain a slash, then it is 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. Libraries installed in hardware capability directories
(see below) are preferred to other libraries.
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
Use PATH instead of 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.
--audit LIST
Use objects named in LIST as auditors.
Some libraries are compiled using hardware-specific instructions
which do not exist on every CPU. Such libraries should be installed
in directories whose names define the required hardware capabilities,
such as /usr/lib/sse2/. The dynamic linker checks these directories
against the hardware of the machine and selects the most suitable
version of a given library. Hardware capability directories can be
cascaded to combine CPU features. The list of supported hardware
capability names depends on the CPU. The following names are
currently recognized:
Alpha ev4, ev5, ev56, ev6, ev67
MIPS loongson2e, loongson2f, octeon, octeon2
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
SPARC flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
s390 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa,
stfle, z900, z990, z9-109, z10, zarch
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
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. Ignored in set-user-ID and set-group-ID
programs.
LD_PRELOAD
A list of additional, user-specified, ELF shared libraries to
be loaded before all others. The items of the list can be
separated by spaces or colons. This can be used to
selectively override functions in other shared libraries. The
libraries are searched for using the rules given under
DESCRIPTION. For set-user-ID/set-group-ID ELF binaries,
preload pathnames containing slashes are ignored, and
libraries in the standard search directories are loaded only
if the set-user-ID permission bit is enabled on the library
file.
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
LD_TRACE_LOADED_OBJECTS variable has been set.
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), sln(1), getauxval(3), rtld-audit(7), ldconfig(8)
This page is part of release 3.51 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 2013-02-10 LD.SO(8)
HTML rendering created 2013-05-17 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project
Hosting by jambit GmbH