LD.SO(8)

NAME
ld.so/ld-linux.so - dynamic linker/loader

DESCRIPTION
ld.so loads the shared libraries needed by a program, prepares the program to run, and then runs it. Unless explicitly specified via the -static option to ld during compilation, all Linux programs are incomplete 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 executable 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 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.

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 environment 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 version 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, 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
    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 standard output. LD_DEBUG_OUTPUT is ignored for setuid/setgid binaries.

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 binaries.

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 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
    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
    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 --verify 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 ${ORIGIN}) in an rpath specification to mean the directory containing the application executable. Thus, an application located in somedir/app could be compiled with gcc -Wl,-rpath,'$ORIGIN/../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 directories 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, 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

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 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. 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 optimized version.
lib*.so*
    shared libraries

SEE ALSO
ldd(1), ldconfig(8).

BUGS

Currently ld.so has no means of unloading and searching for compatible 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).