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
The -I- option gives more control over the default search rules. Only the first -I- option on the command line works as described in this section.
include files of the form #include "foo.h", search the directories in the following order:
The directories named with -I options (both before and after -I-).
The directories for compiler-provided C++ header files, ANSI C header files, and special-purpose files.
The /usr/include directory.
include files of the form #include <foo.h>, search the directories in the following order:
The directories named in the –I options that appear after -I-.
The directories for compiler-provided C++ header files, ANSI C header files, and special-purpose files.
The /usr/include directory.
The following example shows the results of using -I- when compiling prog.c.
prog.c #include "a.h" #include <b.h> #include "c.h" c.h #ifndef _C_H_1 #define _C_H_1 int c1; #endif int/a.h #ifndef _A_H #define _A_H #include "c.h" int a; #endif int/b.h #ifndef _B_H #define _B_H #include <c.h> int b; #endif int/c.h #ifndef _C_H_2 #define _C_H_2 int c2; #endif
The following command shows the default behavior of searching the current directory (the directory of the including file) for include statements of the form #include "foo.h". When processing the #include "c.h" statement in inc/a.h, the preprocessor includes the c.h header file from the inc subdirectory. When processing the #include "c.h" statement in prog.c, the preprocessor includes the c.h file from the directory containing prog.c. Note that the -H option instructs the compiler to print the paths of the included files.
example% cc -c -Iinc -H prog.c inc/a.h inc/c.h inc/b.h inc/c.h c.h
The next command shows the effect of the -I- option. The preprocessor does not look in the including directory first when it processes statements of the form #include "foo.h". Instead, it searches the directories named by the -I options in the order that they appear in the command line. When processing the #include "c.h" statement in inc/a.h, the preprocessor includes the ./c.h header file instead of the inc/c.h header file.
example% cc -c -I. -I- -Iinc -H prog.c inc/a.h ./c.h inc/b.h inc/c.h ./c.h
Never specify the compiler installation area, /usr/include, /lib, or /usr/lib, as search directories.
For more information, see -I[-|dir].