LD.SO(8)LD.SO(8)NAME
ld.so/ld-linux.so - dynamic linker/loader
DESCRIPTIONld.so loads the shared libraries needed by a program, prepares the pro‐
gram to run, and then runs it. Unless explicitly specified via the
-static option to ld during compilation, all Linux programs are incom‐
plete and require further linking at run time.
The necessary shared libraries needed by the program are searched for
in the following order
o Using the environment variable LD_LIBRARY_PATH
(LD_AOUT_LIBRARY_PATH for a.out programs). Except if the exe‐
cutable is a setuid/setgid binary, in which case it is ignored.
o From the cache file /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.
o In the default path /lib, and then /usr/lib.
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_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 refer‐
enced. 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.
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.
LD_TRACE_LOADED_OBJECTS
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
A colon-separated list of directories in which to search for
a.out libraries at execution-time. Similar to the PATH environ‐
ment variable.
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.
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
LD_ASSUME_KERNEL environment variable overrides the kernel ver‐
sion used by the dynamic linker to determine which library to
load.
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. 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, resolv‐
ing 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 Audit‐
ing Interface.
LD_BIND_NOT
Do not update the GOT (global offset table) and PLT (procedure
linkage table) after resolving a symbol.
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.
LD_DEBUG_OUTPUT
File where LD_DEBUG output should be fed into, default is stan‐
dard output. LD_DEBUG_OUTPUT is ignored for setuid/setgid bina‐
ries.
LD_DYNAMIC_WEAK
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 bina‐
ries.
LD_HWCAP_MASK
Mask for hardware capabilities.
LD_KEEPDIR
Don't ignore the directory in the names of a.out libraries to be
loaded. Use of this option is strongly discouraged.
LD_NOWARN
Suppress warnings about a.out libraries with incompatible minor
version numbers.
LD_ORIGIN_PATH
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
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
Shared object to be profiled.
LD_PROFILE_OUTPUT
File where LD_PROFILE output should be written. If this vari‐
able 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
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 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
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 --ver‐
ify options have been given to the dynamic linker).
LD_WARN
If set to non-empty string, warn about unresolved symbols.
RPATH TOKEN EXPANSION
The runtime linker provides a number of tokens that can be used in an
rpath specification (DT_RPATH or DT_RUNPATH).
$ORIGIN
ld.so understands the string $ORIGIN (or equivalently ${ORIGIN})
in an rpath specification to mean the directory containing the
application executable. Thus, an application located in
somedir/app could be compiled with gcc -Wl,-rpath,'$ORI‐
GIN/../lib' so that it finds an associated shared library in
somedir/lib no matter where somedir is located in the directory
hierarchy.
$PLATFORM
The string $PLATFORM (or equivalently ${PLATFORM}) 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.
$LIB The string $LIB (or equivalently ${LIB}) in an rpath corresponds
to the system libraries directory, which is /lib for the native
architecture on FHS compliant GNU/Linux systems.
HARDWARE CAPABILITIES
Libraries might be compiled using hardware-specific instructions which
do not exist on all CPU. Such libraries should be installed in directo‐
ries whose name defines the hardware capabilities such as
/usr/lib/sse2/. 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:
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
FILES
/lib/ld.so a.out dynamic linker/loader
/lib/ld-linux.so.* ELF dynamic linker/loader
/etc/ld.so.cache File containing a compiled list of directo‐
ries 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.
/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 opti‐
mized version.
lib*.so* shared libraries
SEE ALSOldd(1), ldconfig(8).
BUGS
Currently ld.so has no means of unloading and searching for com‐
patible or newer version of libraries.
ld.so functionality is only available for executables compiled
using libc version 4.4.3 or greater.
AUTHORS
David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
Torvalds, Lars Wirzenius and Mitch D'Souza (not necessarily in
that order).
26 May 2007 LD.SO(8)
