ld.so(8) — Linux manual pages

ld.so
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SYNOPSIS

The dynamic linker can be run either indirectly through 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.

The shared libraries needed by the program are searched for in various places:

(ELF only) Using the DT_RPATH dynamic section attribute of the binary if present and DT_RUNPATH attribute does not exist. Use of DT_RPATH is deprecated.
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.
(ELF only) Using the DT_RUNPATH dynamic section attribute of the binary if present.
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 −z nodeflib linker option, libraries in the default library paths are skipped.
In the default path /lib, and then /usr/lib. If the binary was linked with −z nodeflib linker option, this step is skipped.

$ORIGIN and rpath

ld.so understands the string $ORIGIN (or equivalently ${ORIGIN}) in an rpath specification (DT_RPATH or DT_RUNPATH) to mean the directory containing the application executable. Thus, an application located in somedir/app could be compiled with gcc -Wl,-rpath,'$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.

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−pathPATH: Override LD_LIBRARY_PATH environment variable setting (see below).
−−inhibit−rpathLIST: Ignore RPATH and RUNPATH information in object names in LIST.

ENVIRONMENT

There are four important environment variables.

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 set-user-ID/set-group-ID ELF binaries, only libraries in the standard search directories that are also set-user-ID will be loaded.
LD_BIND_NOW: (libc5; glibc since 2.1.1) If set to nonempty string, causes the dynamic linker to resolve all symbols at program startup instead of deferring function call resolval to the point when they are first referenced. This is useful when using a debugger.
LD_TRACE_LOADED_OBJECTS: (ELF only) If set to 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_WARN: (ELF only)(glibc since 2.1.3) If set to nonempty string, warn about unresolved symbols.
LD_NOWARN: (a.out only)(libc5) Suppress warnings about a.out libraries with incompatible minor version numbers.
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.
LD_DEBUG_OUTPUT: (glibc since 2.1) File where LD_DEBUG output should be fed into, default is standard output. LD_DEBUG_OUTPUT is ignored for set-user-ID/set-group-ID binaries.
LD_VERBOSE: (glibc since 2.1) If set to nonempty string, output symbol versioning information about the program if querying information about the program (i.e. either LD_TRACE_LOADED_OBJECTS has been set, or −−list or −−verify options have been given to the dynamic linker).
LD_PROFILE: (glibc since 2.1) Shared object to be profiled.
LD_PROFILE_OUTPUT: (glibc since 2.1) File where LD_PROFILE output should be stored, default is standard output. LD_PROFILE_OUTPUT is ignored for set-user-ID/set-group-ID binaries.
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_SHOW_AUXV: (glibc since 2.1) Show auxiliary array passed up from the kernel.
LD_HWCAP_MASK: (glibc since 2.1) Mask for hardware capabilities.
LD_ORIGIN_PATH: (glibc since 2.1) Path where the binary is found (for non-set-user-ID programs).
LD_DYNAMIC_WEAK: (glibc since 2.1.91) Allow weak symbols to be overridden (reverting to old glibc behavior).
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.
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.

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