Files with the .C extension contain source code in text format for programs, functions, and objects in C or C++. It can be a single program - or only a part of one that creates a software project together with other files.

The C language is usually used for system programming in the Unix and Linux family of systems, while C++ is an object-oriented version where many companion programs are created. Some compilers accept the convention that files with the lowercase extension C contain source code in C, while those with the uppercase extension concern C++ - but this is not a common practice. The starting point of a program stored in C, the function main() is usually stored in a file named main.c.