<string.h>

The <string.h> include file declares the string manipulation functions:

 
Note: 1 These functions are available for C++ programs. They are available for C only when the program defines the __cplusplus__strings__ macro.

The <string.h> include file also defines the macro NULL, and the type size_t.

For more information about NULL and the type size_t, see <stddef.h>.