The Linux documentation project has a nice Program Library HOWTO explaining several concepts around libraries on Linux.

These are some notes I took while reading through it, mostly to gain a better understanding about shared libraries.

A static library self-contained archive containing object files, has the .a suffix. When compiling a program and linking against it, all code referenced by the program will be included in the final executable.

This will make a bigger binary, with an impossible to update dependency, but it’s also handy because no dependencies.

Shared libraries are libraries which are loaded automatically by programs when they start. They use specific binary formats, and there are several rules and options and how to use them.

Dynamic libraries (DLLs) are essentially “plug-ins”: libraries that loaded programmatically by the code via a specifig API. Share the same format as shared libraries, the only difference is how they’re used.

More details about shared libraries

Shared libraries have a soname and a real name.

The soname is in a format like lib${NAME}.so.${NUMBER}

The real name adds version and optionally release number at the end, like so: lib${NAME}.so.${NUMBER}.{VERSION}.{RELEASE}

Example:

• soname: libjack.so.0
• fully qualified soname: /usr/lib/x86_64-linux-gnu/libjack.so.0 (symlinks to the real name file)
• real name: /usr/lib/x86_64-linux-gnu/libjack.so.0.1.0 (actual ELF file)
• linker name: libjack.so (symlink to soname)

Useful programs and config files:

ldconfig:

a program that helps to setup all those library links when installing a library

ldd:

a program that displays which shared libraries are used by a given binary

ld:

the linker, last step when compiling a program. it combines a bunch of object files into a single file w/ references resolved, which can be either an executable, or a shared library, or a static library, in different formats.

/lib/ld-linux.so.X:

The loader, Linux will run this automatically when running an ELF binary, It will find the shared libraries from the correct places and load them. Uses cache file /etc/ld.so.cache, which is kept up-to-date by ldconfig whenever a library is installed.

/etc/ld.so.conf:

configures behavior of the loader

/etc/ld.so.preload:

Any libraries here will be preloaded and take precedence over the standard set. This can be used for emergency patches.

Environment variables

LD_LIBRARY_PATH

Allows overriding standard set of libraries, the loader will search here first if it’s set. Handy for development and testing, but not used for installation.

Alternatively, one can do: /lib/ld-linux.so.2 --library-path $LIBRARY_PATH /path/to/executable

LD_PRELOAD

Like LD_LIBRARY_PATH, but will also preload the libraries here. Basically, same as /etc/ld.so.preload, except works in user-level

LD_DEBUG

Used to debug the loader behavior.

• LD_DEBUG=files $COMMAND will show processing of files and libraries, what is loaded and in what order.
• LD_DEBUG=bindings $COMMAND will show information about symbols binding
• LD_DEBUG=libs $COMMAND will show library search paths
• LD_DEBUG=versions $COMMAND will show dependencies and versions