ldconfig

Dynamic linker run-time bindings management command

Description

The ldconfig command is primarily used to search for shareable dynamic libraries (in the format lib*.so*) in the default directories /lib and /usr/lib, as well as in the directories listed in the dynamic library configuration file /etc/ld.so.conf. It then creates the necessary links and cache files required by the dynamic loader (ld.so). The cache file defaults to /etc/ld.so.cache, which stores an ordered list of dynamic library names. To make dynamic libraries shareable across the system, you need to run the ldconfig management command, which is located in the /sbin directory.

ldconfig is typically run at system startup, but it must be run manually whenever a new dynamic library is installed.

Syntax

ldconfig [-v|--verbose] [-n] [-N] [-X] [-f CONF] [-C CACHE] [-r ROOT] [-l] [-p|--print-cache] [-c FORMAT] [--format=FORMAT] [-V] -?|--[help|--usage] path... 

Options

-v, --verbose: Display the directories being scanned, the dynamic libraries found, and the names of the links created.
-n: Only scan directories specified on the command line; do not scan default directories (/lib, /usr/lib) or directories listed in /etc/ld.so.conf.
-N: Do not rebuild the cache file (/etc/ld.so.cache). Unless -X is also specified, ldconfig still updates the links.
-X: Do not update the links. Unless -N is also specified, the cache file is still updated.
-f CONF: Use CONF as the configuration file instead of the default /etc/ld.so.conf.
-C CACHE: Use CACHE as the cache file instead of the default /etc/ld.so.cache.
-r ROOT: Change the root directory to ROOT (implemented using the chroot function). The configuration file /etc/ld.so.conf will then be looked for in ROOT/etc/ld.so.conf.
-l: Expert mode. Normally, ldconfig automatically creates links when searching for dynamic libraries; in expert mode, links must be set manually.
-p, --print-cache: Print the names of all shared libraries currently stored in the cache file.
-c FORMAT, --format=FORMAT: Specify the cache file format: old, new, or compat (the default).
-V: Print the version of ldconfig and exit.
-?, --help, --usage: Display help information and exit.

Notes on ldconfig:

1. Adding files to /lib and /usr/lib does not require modifying /etc/ld.so.conf, but you must run ldconfig afterwards, otherwise the libraries won't be found.
2. When adding libraries to directories other than the two mentioned above, you must modify /etc/ld.so.conf and then run ldconfig.
3. For example, if you install MySQL to /usr/local/mysql, and it has many libraries in /usr/local/mysql/lib, you need to add the line /usr/local/mysql/lib to /etc/ld.so.conf and then run ldconfig so the new libraries can be found at runtime.
4. If you want to place libraries outside these directories but don't want to (or can't) modify /etc/ld.so.conf, you can export the environment variable LD_LIBRARY_PATH. This is generally a temporary solution.
5. ldconfig operations are related to program execution, not compilation. During compilation, you still need to use -L flags as appropriate.
6. In short, it's best to run ldconfig after any changes related to libraries to avoid unexpected results.
7. Files like libdb-4.3.so contain library name information in their headers (detectable with the strings command). Simply renaming a file to impersonate another library (e.g., libdb-4.8.so) will not work. You should specify the library name in the Makefile during compilation.