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The C99 standard makes the following statements about parsing header files:

  • The first eight characters in the filename are significant
  • The file only has one character after the period in the filename
  • The case of the characters in the filename is not necessarily significant

To guarantee header filenames 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 on a specific architecture, 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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

PRE31-C

1 (low)

1 (unlikely)

1 (high)

P1

L3

References

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

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