Shared Libraries

• A shared library (also known as a shared object, or as a dynamically linked library) is similar to a archive in that it is a grouping of object files.
• The most fundamental difference is that when a shared library is linked into a program, the final executable does not actually contain the code that is present in the shared library. Instead, the executable contains a reference to the shared library.
• Thus, the library is "shared" among all the programs that link with it.
• A second important difference is that a shared library is not a collection of object files. Instead, the object files that compose the shared library are combined into a single object file so that a program that links against a shared library always includes all of the code in the library, rather than just those portions that are needed.
• To create a shared library, you must compile the objects that will make up the library using the -fPIC option to the compiler, like this:

  $ gcc -c -fPIC test1.c
  

  Then you combine the object files into a shared library, like this:

  $ gcc -shared -fPIC -o libtest.so test1.o test2.o
  

• Shared libraries use the extension .so, which stands for shared object. Like static archives, the name always begins with lib to indicate that the file is a library.

  $ gcc -o app app.o -L. -ltest
  

• Suppose that both libtest.a and libtest.so are available. The linker searches each directory (first those specified with $-L$ options, and then those in the standard directories).When the linker finds a directory that contains either libtest.a or libtest.so, the linker stops search directories.
• If only one of the two variants is present in the directory, the linker chooses that variant. Otherwise, the linker chooses the shared library version, unless you explicitly instruct it otherwise.

  $ gcc -static -o app app.o -L. -ltest
  

• Using LD_LIBRARY_PATH
  • When you link a program with a shared library, the linker does not put the full path to the shared library in the resulting executable. Instead, it places only the name of the shared library.
  • Suppose that you use this:

    $ gcc -o app app.o -L. -ltest -Wl,-rpath,/usr/local/lib
    

    Then, when app is run, the system will search /usr/local/lib for any required shared libraries.
  • Another solution to this problem is to set the LD_LIBRARY_PATH environment variable when running the program. Like the $PATH$ environment variable.
  • LD_LIBRARY_PATH is a colon-separated list of directories. For example, if LD_LIBRARY_PATH is $/usr/local/lib:/opt/lib$, then $/usr/local/lib$ and $/opt/lib$ will be searched before the standard $/lib$ and $/usr/lib$ directories.