Make sure that included header file names are unique. According to the C Standard, subclause 6.10.2, paragraph 5 [ISO/IEC 9899:2011],

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 file name are guaranteed to be significant.
  • The file has only one nondigit character after the period in the file name.
  • The case of the characters in the file name is not guaranteed to be significant.

To guarantee that 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.

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

This noncompliant code example contains references to headers that may exist independently in various environments but can be ambiguously interpreted by a C-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"

/* ... */

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 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"

/* ... */

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).


PRE08-C-EX1: 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.

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 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.




Remediation Cost









Automated Detection

Axivion Bauhaus Suite






Fully implemented

Helix QAC



Related Vulnerabilities

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

Related Guidelines


[ISO/IEC 9899:2011]Subclause 6.10.2, "Source File Inclusion"


  1. I wonder if there should be a discussion of the "subdir/header.h" notation, or the distinction between <header.h> and "header.h"?

    1. I added an exception as suggested above.

      What do you want to say about "subdir/header.h" notation, or the distinction between <header.h> and "header.h"?

      We are trying to omit "reference" material, unless we want to make a recommendation or rule concerning how it is to be used.

  2. 1. Would the following code violate this guideline?

    #include "a/b.h"
    #include "c/b.h"
    #include "b.h"

    2. How about:

    #include <d.h>
    #include "d.h"


    3. I have seen some compilers automatically append file extensions to header names mentioned in #include directives. Suppose a less experienced Programmer codes these lines:

    #include "e.h"
    #include "e"

    if the compiler finds a file called 'e.h' for the second directive, does the code then violate this guideline?

    1. My opinion would be:

      1. no.
      2. yes.
      3. yes.

      The intent of this guideline is to diagnose situations in which a developer may not be including the intended file.

      I tried to capture this idea under "False Positives" on the Tool Selection and Validation page. However, this is still a work in progress. I'm pondering what it means for a tool to conform with this standard, and what is a quality of implementation issue. Perhaps you can help.

      1. The intent of this guideline is to diagnose situations in which a developer may not be including the intended file.

        Agreed. I think perhaps we should focus more on this intent and less on including a duplicate file. I just saw an example C file which purposely includes the same .h file twice. The first include has a certain macro undefined, and the second include has it defined. Duplicate header inclusion is not necessarily a Bad Thing, the bad thing is when you include the wrong file.

        Also, since C99 leaves the details of what constitutes a unique header identifier as implementation-defined, the answers to Frank's queries vary from system to system, and you can create a system where all three questions can be answered 'no' and another system where all three questions can be answered 'yes'.