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Wiki MarkupCare must be taken to make sure that included header file names are unique. According to C99 \[Make sure that included header file names are unique. According to the C Standard, subclause 6.10.2, paragraph 5 [ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\]:2011],

The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits (6.4.2.1) followed by a period (.) and a single nondigit. The first character shall not be a digit. The implementation may ignore distinctions of alphabetical case and restrict the mapping to eight significant characters before the period.

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Note that compliance with this recommendation does not require that short file names are used, only that the file names are unique.

Noncompliant Code Example

The following This noncompliant code example contains references to headers that may exist independently in various environments but can be ambiguously interpreted by a C99C-compliant compiler.:

#include "Library.h"
#include <stdio.h>
#include <stdlib.h>
#include "library.h"

#include "utilities_math.h"
#include "utilities_physics.h"

#include "my_library.h"

/* Rest of program ... */

Library.h and library.h may refer to the same file. Also, because only the first eight characters are guaranteed to be significant, it is unclear whether utilities_math.h and utilities_physics.h are parsed. Finally, if a file such as my_libraryOLD.h exists, it may inadvertently be included instead of my_library.h.

Compliant Solution

This compliant solution avoids the ambiguity by renaming the associated files to be unique under the above the preceding constraints.:

#include "Lib_main.h"
#include <stdio.h>
#include <stdlib.h>
#include "lib_2.h"

#include "util_math.h"
#include "util_physics.h"

#include "my_library.h"

/* Rest of program ... */

The only solution for mitigating ambiguity of a file, such as my_libraryOLD.h, is to rename old files with either a prefix (that would fall within the first eight characters) or add an extension (such as my_library.h.old).

Exceptions

PRE08-C-EX1: While C99 requires Although the C Standard requires only the first eight characters in the file name to be significant, most modern systems have long file names, and compilers on such systems can typically differentiate them. Consequently, long file names in headers may be used, provided that all the implementations to which the code is ported can distinguish between these file names.

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Failing to guarantee uniqueness of header files may result in the inclusion of an older version of a header file, which may include incorrect macro definitions or obsolete function prototypes or result in other errors that may or may not be detected by the compiler. Portability issues may also stem from the use of header names that are not guaranteed to be unique.

Automated Detection

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Tool	Version	Checker	Description
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-PRE08
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.PRE08	Fully implemented
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C5002

Related Vulnerabilities

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

Other Languages

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Related Guidelines

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Bibliography

References

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Image Added Image Added file inclusion" \[[MISRA 04|AA. Bibliography#MISRA 04]\] Rule 19.5Image Removed      01. Preprocessor (PRE)      Image Modified