Header files are text files included in a source file
during compilation. To include a header file in a compilation, the
#include preprocessor directive must be used in the
source file. See Chapter 8 for more
information on this directive. The entire header file, regardless of
content, is substituted for the #include preprocessor
directive.
A header file can contain other #include preprocessor
directives to include another file. You can
nest #include directives to any depth.
Header files can include any legal C source code. They are most
often used to include external variable declarations, macro
definitions, type definitions, and function declarations. Groups
of logically related functions are commonly declared together in a
header file, such as the C library input and output functions listed
in the stdio.h header file. Header files traditionally
have a .h suffix (stdio.h , for example).
The names of header files must not include the ', \, ", or /* characters, because the use of these punctuation characters in a header file is undefined.
When referenced in a program, header names are surrounded by angle brackets or double quotation marks, as shown in the following example:
#include <math.h> /* or */ #include "local.h"
Chapter 8 explains the difference between the two formats. The algorithm the compiler uses for finding the named files is discussed in Section B.37. Chapter 9 describes the library routines in each of the ANSI standard header files.