Include Files

To include any of the standard header files supplied with the C compilation system, use this format:

#include <stdio.h> 

The angle brackets (<>) cause the preprocessor to search for the header file in the standard place for header files on your system, usually the /usr/include directory.

The format is different for header files that you have stored in your own directories:

#include "header.h"

The quotation marks (" ") cause the preprocessor to search for header.h first in the directory of the file containing the #include line.

If your header file is not in the same directory as the source files that include it, specify the path of the directory in which it is stored with the -I option to cc. Suppose, for instance, that you have included both stdio.h and header.h in the source file mycode.c:

#include <stdio.h>
#include "header.h"

Suppose further that header.h is stored in the directory../defs. The command:

% cc -I../defs mycode.c

directs the preprocessor to search for header.h first in the directory containing mycode.c, then in the directory ../defs, and finally in the standard place. It also directs the preprocessor to search for stdio.h first in ../defs, then in the standard place. The difference is that the current directory is searched only for header files whose names you have enclosed in quotation marks.

You can specify the -I option more than once on the cc command-line. The preprocessor searches the specified directories in the order they appear. You can specify multiple options to cc on the same command-line:

% cc -o prog -I../defs mycode.c