Chapter 2. Header Files
A header file is a file containing C declarations and macro definitions
(Chapter 3 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 you need to invoke system calls and libraries.
Your own header files contain declarations for interfaces between the
source files of your program. Each time you have a group of related
declarations and macro definitions all or most of which are needed in
several different source files, it is a good idea to create a header
file for them.
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, they
can be changed in one place, and programs that include the header file
will 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 find 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.
2.1. Include Syntax
Both user and system header files are included using the preprocessing
directive #include. It has two variants:
- #include <file>
This variant is used for system header files. It searches for a file
named file in a standard list of system directories. You can prepend
directories to this list with the -I option (Chapter 12 Invocation).
- #include "file"
This variant is used for header files of your own program. It searches
for a file named file first in the directory containing the
current file, then in the same directories used for <file>.
The argument of #include, whether delimited with quote marks or
angle brackets, behaves like a string constant in that comments are not
recognized, and macro names are not expanded. Thus, #include
<x/*y> specifies inclusion of a system header file named x/*y.
However, if backslashes occur within file, they are considered
ordinary text characters, not escape characters. None of the character
escape sequences appropriate to string constants in C are processed.
Thus, #include "x\n\\y" specifies a filename containing three
backslashes. (Some systems interpret \ as a pathname separator.
All of these also interpret / the same way. It is most portable
to use only /.)
It is an error if there is anything (other than comments) on the line
after the file name.