A header file is a C file that should contain the following:

• external function declarations
• macro definitions (preprocessor #defines)
• structure, union, and enumeration declarations
• typedef declarations
• global variable declarations

Header files are generally used to save changing a lot of separate function declarations when you change something.

See also https://c-faq.com/cpp/hfiles.html

To protect against multiple and circular includes, include this in your header file:

#ifndef MYHEADER_H
#define MYHEADER_H
 
// ... header content ...
 
#endif