### 26.4 Header Files ¶

A header file is a file of C code, typically containing C declarations and macro definitions (see Macros), to be shared between several source files. You request the use of a header file in your program by including it, with the C preprocessing directive `#include`.

Header files serve two purposes.

- System header files declare the interfaces to parts of the operating system. You include them in your program to supply the definitions and declarations that you need to invoke system calls and libraries.
- Program-specific header files contain declarations for interfaces between the source files of a particular program. It is a good idea to create a header file for related declarations and macro definitions if all or most of them are needed in several different source files.

Including a header file produces the same results as copying the header file into each source file that needs it. Such copying would be time-consuming and error-prone. With a header file, the related declarations appear in only one place. If they need to be changed, you can change them in one place, and programs that include the header file will then automatically use the new version when next recompiled. The header file eliminates the labor of finding and changing all the copies as well as the risk that a failure to change one copy will result in inconsistencies within a program.

In C, the usual convention is to give header files names that end with `.h`. It is most portable to use only letters, digits, dashes, and underscores in header file names, and at most one dot.

The operation of including another source file isn’t actually limited to the sort of code we put into header files. You can put any sort of C code into a separate file, then use `#include` to copy it virtually into other C source files. But that is a strange thing to do.