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Until the early 1980s, large software development projects had a continual problem with the inclusion of headers. One group might have produced a graphics.h, for example, which started by including io.h. Another group might have produced keyboard.h, which also included io.h. If io.h cannot could not safely be included several times, arguments would break out about which header should include it. Sometimes an agreement was reached that each header should include no other headers, and as a result, some application programs started with dozens of #include lines, and sometimes they got the ordering wrong or forgot a required header.

Compliant Solution

All these complications disappeared with the discovery of a simple technique: each header should #define a symbol that means "I have already been included." The entire header is then enclosed in an inclusion include guard:

Code Block

bgColor	#ccccFF
lang	c

#ifndef HEADER_H
#define HEADER_H

/* ... contentsContents of <header.h> ... */

#endif /* HEADER_H */

Consequently, the first time that header.h is #include'd, all of its contents are included. If the header file is subsequently #include'd again, its contents are bypassed.

Because solutions such as this one make it possible to create a header file that can be included more than once, the C standard [ISO/IEC 9899:2011] Standard guarantees that the standard headers are safe for multiple inclusion.

Note that it is a common mistake to choose a reserved name (such as _HEADER_H_ or __HEADER_H__) for the name of the macro used in the inclusion include guard. See DCL37-C. Do not declare or define a reserved identifier for more information.