What are they?
Programmers are lazy. They go through great lengths to automate and streamline the tedious everyday tasks that, when repeated over and over again, take up an increasing amount of time and keystrokes. One of the things programmers use the most are functions. C has a handy little trick up it’s sleeve when it comes to calling functions more efficiently, namely using libraries both static and dynamic. They are both ways to organize functions into a tidy, organized bunch that can be easily called upon when needed. The biggest difference boils down to this: static libraries are compiled into a program while dynamic libraries are shared and therefore exist outside, independent of the code being compiled.
Both have their advantages and disadvantages. For starters, the nature of compiling the library into the output file means that any change to a static library involves a second step of re-compiling. Meanwhile, dynamic libraries are easy to modify and pass on to the necessary files. However dynamic libraries require more processing power at compile time, and also carry a higher risk of failure, as one bad file at execution will render the whole library useless. Static libraries come out on top in this regard as they are processed much faster at compilation.
Creating a library:
To create a dynamic library, first you need your functions, preferably in individual .c files. Summon the compiler with:
gcc *.c -c -fPIC
which calls for every file ending in .c that hasn’t been linked (that’s the ‘-c’ part) regardless of where they are located in memory (and that’s the ‘-fPIC’ part). This will generate one object file (.o) for every .c file compiled. These files need to be compiled as well, so we call gcc *.o -shared -o liball.so. This commands takes every .o file and creates a dynamic library (-shared) where liball is a generic name for the .so file it outputs.
To create a static library: follow the same steps to create the object files and then simply archive the library using:
ar rcs liball.a *.o
‘ar’ is the GNU archive program and rcs breaks down to the following: r- rewrite if existant, c- create if non-existant, and s- sort it.
Using a dynamic library:
To use a dynamic library we must call it on compile as follows:
gcc -L. example.c -lholberton -o example
In this example -L tells it to expect library files and -lholberton tells it to expect them to come from a libholberton.so file.