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 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.
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 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 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 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 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
directories in the search path) that have different 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 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 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 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) 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/.