Ubuntu Manpage: ld.so, ld-linux.so* - dynamic linker/loader

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

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 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
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 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.

COLOPHON

       This  page  is  part  of  release 3.54 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/.