Originální popis anglicky:
ld.so, ld-linux.so* - dynamic linker/loader
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 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 behaviour, 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:
- o
- (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.
- o
- Using the environment variable LD_LIBRARY_PATH.
Except if the executable is a setuid/setgid binary, in which case it is
ignored.
- o
- (ELF only) Using 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 -z nodeflib
linker option, libraries in the default library paths are skipped.
- o
- In the default path /lib, and then /usr/lib.
If the binary was linked with -z nodeflib linker option, this step
is skipped.
Návod, kniha: Linux Programmer's Manual
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]]
- --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
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
setuid/setgid ELF binaries, only libraries in the standard search
directories that are also setuid will be loaded.
- LD_BIND_NOW
- (libc5; glibc since 2.1.1) If set to non-empty 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 non-empty 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 non-empty 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
setuid/setgid binaries.
- LD_VERBOSE
- (glibc since 2.1) 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_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_DEBUG_OUTPUT is ignored
for setuid/setgid 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-setuid programs).
- LD_DYNAMIC_WEAK
- (glibc since 2.1.91) Allow weak symbols to be overridden
(reverting to old glibc behaviour).
- 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.
- /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. libraries and an ordered list
of candidate libraries.
- lib*.so*
- shared libraries
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.
ldd(1),
ldconfig(8)
