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"
For statements of the form #include "foo.h" (where quotation marks are used), the compiler searches for include files in the following order:
The current directory (that is, the directory containing the “including” file)
The directories named with -I options, if any
The /usr/include directory
If your header file is not in the same directory as the source files that include it, use the -I compiler option to specify the path of the directory in which it is stored. For instance, suppose 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. You might then want to use this command:
% cc –I../defs mycode.c
It 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