LD.SO(8) Linux Programmer's Manual LD.SO(8)
NAME
ld.so, ld-linux.so* - dynamic linker/loader
SYNOPSIS
The dynamic linker can be run either indirectly by running some dynami-
cally 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 exe-
cuted) or directly by running:
/lib/ld-linux.so.* [OPTIONS] [PROGRAM [ARGUMENTS]]
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(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 pro-
grams 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 (rela-
tive or absolute) pathname, and the library is loaded using that path-
name.
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 exe-
cutable 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.
Rpath token expansion
ld.so understands certain strings in an rpath specification (DT_RPATH
or DT_RUNPATH); those strings are substituted as follows
$ORIGIN (or equivalently ${ORIGIN})
This expands to the directory containing the application exe-
cutable. Thus, an application located in somedir/app could be
compiled with
gcc -Wl,-rpath,'$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.
$LIB (or equivalently ${LIB})
This expands to lib or lib64 depending on the architecture
(e.g., on x86-64, it expands to lib64 and on x86-32, it expands
to lib).
$PLATFORM (or equivalently ${PLATFORM})
This expands to a string corresponding to the processor type of
the host system (e.g., "x86_64"). On some architectures, the
Linux kernel doesn't provide a platform string to the dynamic
linker. The value of this string is taken from the AT_PLATFORM
value in the auxiliary vector (see getauxval(3)).
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
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.
HARDWARE CAPABILITIES
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 com-
bine 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, efp-
double, 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
ENVIRONMENT
There are four important environment variables.
LD_ASSUME_KERNEL
(glibc since 2.2.3) Each shared library can inform the dynamic
linker of the minimum kernel ABI version that it requires.
(This requirement is encoded in an ELF note section that is
viewable via readelf -n as a section labeled NT_GNU_ABI_TAG.)
At run time, the dynamic linker determines the ABI version of
the running kernel and will reject loading shared libraries that
specify minimum ABI versions that exceed that ABI version.
LD_ASSUME_KERNEL can be used to cause the dynamic linker to
assume that it is running on a system with a different kernel
ABI version. For example, the following command line causes the
dynamic linker to assume it is running on Linux 2.2.5 when load-
ing the shared libraries required by myprog:
$ LD_ASSUME_KERNEL=2.2.5 ./myprog
On systems that provide multiple versions of a shared library
(in different directories in the search path) that have differ-
ent minimum kernel ABI version requirements, LD_ASSUME_KERNEL
can be used to select the version of the library that is used
(dependent on the directory search order). Historically, the
most common use of the LD_ASSUME_KERNEL feature was to manually
select the older LinuxThreads POSIX threads implementation on
systems that provided both LinuxThreads and NPTL (which latter
was typically the default on such systems); see pthreads(7).
LD_BIND_NOT
(glibc since 2.2) Don't update the Global Offset Table (GOT) and
Procedure Linkage Table (PLT) when resolving a symbol.
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 debug-
ger.
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 sepa-
rated 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 direc-
tories 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 ver-
sions 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 nor-
mal 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 bina-
ries.
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 inter-
face is largely compatible with that provided on Solaris, as
described in its Linker and Libraries Guide, in the chapter Run-
time 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 cate-
gories 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 in which LD_DEBUG output should be writ-
ten. The 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 ker-
nel. 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 exe-
cutables (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 the
LD_TRACE_LOADED_OBJECTS environment 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.
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.
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.
SEE ALSO
ldd(1), sln(1), getauxval(3), rtld-audit(7), ldconfig(8)
COLOPHON
This page is part of release 3.53 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 2013-07-15 LD.SO(8)
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