What's a File! An A file contains a library of functions and headers that may be referenced by a C/C++ source file. It may store only a few functions or may include an entire library of functions, such as a 3D modeling engine. A files are typically created by the GNU ar utility. A static libraries are more common in Unix-like systems than in Windows systems. In Windows, static libraies typically use the ".lib" extension instead of the ".a" extension.NOTE: Using static libraries may result.