Differences between static and dynamic libraries

The Starry Night. Vincent van Gogh

Why do libraries exist?

Libraries exist in computer science because developers need to save time and efforts by reusing code, that means that if there are proven and confident (without bugs) functions, a programmer doesn’t need to reinvent the wheel, just add that code to new ideas.

Libraries are collections of functions which can be called from a program, and this can be done by embedding those libraries in the linking process when compiling the program. Libraries can come from your own creations or from open source code.

What is a static library in C?

A static library is a group of object files linked directly into the final executable of a program, that will remain unchanged unless the program is recompiled. The group of object files before the linking lives in an archive (.a) file written in machine code that is obtained from (.o) files which are obtained from (.c) files where are the codes of the functions. That implies that are needed three steps to generate an executable file with a static library into it.

Another useful step is to generate an index to the contents of the (.a) file before the compiling because this will reduce the time of the process. That index will be in the (.a) file and helps in the finding of the symbols of the functions.

What is a dynamic library in C?

A dynamic library (also called a shared library) consists of routines that are loaded into your application at run time. When you compile a program that uses a dynamic library, the library does not become part of your executable — it remains as a separate unit. On Linux, dynamic libraries typically have a .so (shared object) extension. The main advantage of using dynamically linked libraries is that the size of executable programs is dramatically reduced. Perhaps a bigger advantage is that the dynamic library can be upgraded to a newer version without replacing all of the executables that use it.

How to create and use a dynamic library?

A shared library (on Linux) is a collection of object files. In dynamic linking, object files are not combined with programs at compile time, also, they are not copied permanently into the final executable file.

At first, we need to have all of our (.c) files in the current directory with a header file (.h) that contains the prototypes of our functions.

Then, we have to use two commands to compile those files:

gcc -fPIC -Wall -Werror -c *.c && gcc -shared -o liball.so *.o

the options for this process are as follows:

• fPIC : This is a requirement for shared libraries that stands for “Position Independent Code”.
• -Wall -Werror : standard error checks you should use
• -c: compiles source files without linking.
• *.c: selects all of your c files
• -shared: Creates a shared library with a prefix of lib and a suffix of .so which stands for shared object.
• -o: Places output in a file

To use a shared library is not as straightforward as the static library was. Once you create a shared library you will have to install it.

Usage:

• Use this command to see the functions in your libraries.
• -D: This refers to the symbols in the data initialization section
• Update your LD_LIBRARY_PATH to include the directory where your library can be found

nm -D libholberton.so

• Compile your program and include -L. to include the path to your library
• Exclude the lib prefix and .so suffix to include your library

gcc -L. -o main.c program.c -lholberton

If you need more information about dynamically linked libraries:

A.1 - Static and dynamic libraries

http://cs-fundamentals.com/c-programming/static-and-dynamic-linking-in-c.php