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Care must be taken to make sure that included header file names are unique. According to C99:

The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits ( 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.

This means that:

  • Only the first eight characters in the filename are guaranteed to be significant.
  • The file only has one, nondigit character after the period in the filename.
  • The case of the characters in the filename is not guaranteed to be significant.

To guarantee header file names are unique, all included files should differ (in a case insensitive manner) in their first eight characters or in their (one character) file extension.

Non-Compliant Code Example

The following non-compliant code contains references to headers that may exist independently in various environments, but can be ambiguously interpreted by a C99-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, 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 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 to add an extension (such as my_library.h.old).

Risk Assessment

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, 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.




Remediation Cost




1 (low)

1 (unlikely)

1 (high)




[[ISO/IEC 9899-1999]] Section 6.10.2 "Source file inclusion"

PRE07-A. Avoid using repeated question marks       01. Preprocessor (PRE)      

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