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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 include guard. See DCL37-C. Do not declare or define a reserved identifier for more information.

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 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.

Exceptions

PRE06-C-EX0: It is acceptable to include a header file containing an X-Macro [ Myers 2010 ] multiple times, and each time needs no inclusion guard.  Such header files should have an extension of .def.

Risk Assessment

Failure to include header files in an include guard can result in unexpected behavior.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

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