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ld-linux.so(8) - phpMan
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 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]]
DESCRIPTION
The programs ld.so and ld-linux.so* find and load the shared libraries needed by a pro‐
gram, 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 dep‐
recated.
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 pre‐
ferred 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 ${ORIGIN})
This expands to the directory containing the application executable. Thus, an
application located in somedir/app could be compiled with
gcc -Wl,-rpath,'$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.
$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 plat‐
form 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 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
ENVIRONMENT
Among the more important environment variables are the following:
LD_ASSUME_KERNEL
(glibc since 2.2.3) Each shared library can inform the dynamic linker of the mini‐
mum 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 run‐
ning 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
loading 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 direc‐
tories in the search path) that have different minimum kernel ABI version require‐
ments, 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 reso‐
lution 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 exe‐
cution-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 check‐
points—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 com‐
patible 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 in which LD_DEBUG output should be written. The default is
standard error. 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 mecha‐
nism 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) The name of a (single) shared object to be profiled, specified
either as a pathname or a soname. Profiling output is appended 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 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
ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), rtld-audit(7), ldconfig(8),
sln(8)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
GNU 2014-10-02 LD.SO(8)
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