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 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.
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 include guard:
Consequently, the first time
#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 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__) 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.
Failure to include header files in an include guard can result in unexpected behavior.
|Axivion Bauhaus Suite
|LDRA tool suite
Use multiple include guards
|Polyspace Bug Finder
|CERT C: Rec. PRE06-C
|Checks for content of header file not guarded from multiple inclusions (rec. fully covered)
|SEI CERT C++ Coding Standard
|VOID PRE06-CPP. Enclose header files in an include guard
|Directive 4.10 (required)