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The C Standard defines a set of predefined macros (see subclause 6.10.8) to help the user determine if the implementation being used is a conforming implementation, and if so, to which version of the C Standard it conforms. These macros can also help the user to determine which of the standard features are implemented.

The following tables list these macros and indicate in which version of the C Standard they were introduced. The following macros are required:

Macro NameC90C99C11

__STDC__

__STDC_HOSTED__


__STDC_VERSION__1


__DATE__

__FILE__

__LINE__

__TIME__

  1) __STDC_VERSION__ was introduced by an Amendment to C90, this version of the C Standard is commonly call C94 

 The following are optional environment macros:

Macro NameC90C99C11

__STDC_ISO_10646__


__STDC_MB_MIGHT_NEQ_WC__


__STDC_UTF_16__



__STDC_UTF_32__



  The following are optional feature macros:

Macro NameC90C99C11

__STDC_ANALYZABLE__



__STDC_IEC_559__


__STDC_IEC_559_COMPLEX__


__STDC_LIB_EXT1__ 



__STDC_NO_ATOMICS__



__STDC_NO_COMPLEX__



__STDC_NO_THREADS__



__STDC_NO_VLA__ 



 The following is optional and is defined by the user:

Macro NameC90C99C11
__STDC_WANT_LIB_EXT1__ 

 

Noncompliant Code Example (Checking Value of Predefined Macro)

C Standard predefined macros should never be tested for a value before the macro is tested for definition, as shown in this noncompliant code example:

#include <stdio.h>

int main(void) {
  #if (__STDC__ == 1)
    printf("Implementation is ISO-conforming.\n");
  #else
    printf("Implementation is not ISO-conforming.\n");
  #endif
  /* ... */

  return 0;
}

Compliant Solution (Testing for Definition of Macro)

In this compliant solution, the definition of the predefined macro __STDC__ is tested before the value of the macro is tested:

#include <stdio.h>

int main(void) {
  #if defined(__STDC__)
    #if (__STDC__ == 1)
      printf("Implementation is ISO-conforming.\n");
    #else
      printf("Implementation is not ISO-conforming.\n");
    #endif
  #else   /* !defined(__STDC__) */
    printf("__STDC__ is not defined.\n");
  #endif
  /* ... */
  return 0;
}

Compliant Solution (Test for Optional Feature)

This compliant solution tests to see if the C11 predefined macro __STDC_ANALYZABLE__ is defined and what value the implementation has given the macro:

#include <stdio.h>
 
int main(void) {
  #if defined (__STDC__)
    #if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)  /* C11 */
      #if defined(__STDC_ANALYZABLE__)
        #if (__STDC_ANALYZABLE__ == 1)
	      printf("Compiler conforms to Annex L (Analyzability).\n");
        #else
	      printf("Compiler does not support Annex L (Analyzability).\n");
        #endif
      #else
        printf("__STDC_ANALYZABLE__ is not defined.\n");
      #endif
    #else
      printf("Compiler not C11.\n");
    #endif
  #else
    printf("Compiler not Standard C.\n");
  #endif
 
  return 0;
}

Compliant Solution (Optional Language Features)

This compliant solution checks for the C11 optional language features in Annex K. If Annex K is supported by the implementation, the functions defined in Annex K are used; if Annex K is not supported, then the standard library functions are used. (See DCL09-C. Declare functions that return errno with a return type of errno_t.)

#if defined(__STDC_LIB_EXT1__)
  #if (__STDC_LIB_EXT1__ >= 201112L)
    #define USE_EXT1 1
    #define __STDC_WANT_LIB_EXT1__ 1 /* Want the ext1 functions */
  #endif
#endif
 
#include <string.h>
#include <stdlib.h>
 
int main(void) {
  char source_msg[] = "This is a test.";
  char *msg = malloc(sizeof(source_msg) + 1);
 
  if (msg != NULL) {
    #if defined(USE_EXT1)
      strcpy_s(msg, sizeof msg, source_msg);
    #else
      strcpy(msg, source_msg);
    #endif
  } 
  else {
    return EXIT_FAILURE;
  }
  return 0;
}

Compliant Solution (Optional Language Features)

The previous compliant solution comes close to violating PRE09-C. Do not replace secure functions with deprecated or obsolescent functions, and would if a function-like macro were defined which called either strcpy_s() or strcpy() depending on if USE_EXT1 were defined.  This compliant solution solves the problem by including a custom library that implements the optional language feature, which in this case is the Safe C Library available from SourceForge.

#if defined(__STDC_LIB_EXT1__)
  #if (__STDC_LIB_EXT1__ >= 201112L)
    #define USE_EXT1 1
    #define __STDC_WANT_LIB_EXT1__ 1 /* Want the ext1 functions */
  #endif
#endif
 
#include <string.h>
#include <stdlib.h>

#if !defined(USE_EXT1)
  #include "safe_str_lib.h"
#endif
  
int main(void) {
  char source_msg[] = "This is a test.";
  char *msg = malloc(sizeof(source_msg) + 1);
 
  if (msg != NULL) {
    strcpy_s(msg, sizeof msg, source_msg);
  } 
  else {
    return EXIT_FAILURE;
  }
  return 0;
}

Risk Assessment

Not testing for language features or the version of the implementation being used can lead to unexpected or undefined program behavior.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

PRE13-C

Low

Probable

Low

P6

L2

 

Automated Detection

 

Tool

Version

Checker

Description

Axivion Bauhaus Suite

6.9.0

CertC-PRE13
LDRA tool suite
9.7.1


Partially implemented

Related Vulnerabilities

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

Related Guidelines

ISO/IEC TR 24772:2013Pre-processor Directives [NMP]
ISO/IEC 9899:2011

6.10.8, "Predefined macro names"

K.3.7.1, "Copying functions"



  

7 Comments

  1. I'm not sure "a Standard conforming" is clear in this context... the CERT C Secure Coding Standard for the C Standard.  Should probably just say the C Standard.

     

    the phrase:

    "The tables below list these macros and which version of the C Standard they were introduced."

    doesn't seem to be correct, because the table appears to list all the versions of the standard in which the macros are defined.

    The (C94) is wierd.  I've never heard of this version of the standard, and don't know why it's in parenthesis.  A footnote might be necessary.

    In a related note, it is wierd to have empty columns in these tables.  Perhaps we should have the names of the standards in the column headers, and place check marks in the appropriate cells?  again, we might need a footnote for (C94)

     

     

     

    1. Ok, I will add headers and check marks - if I can figure out how to (smile) As for C94, C94 is C90 + Amend1. I think C94 is a common designation but will double check.

  2. Should __func__ be in the table?  Also, I agree that the tables could use some headers for clarity.  I kind of like Robert's suggestion of using checkmarks as the column values, and standard versions as the headers.

    1. No, _func_ is not a predefined macro, _func_ is a predefined identifier.

  3. The example with strcpy_s and strcpy violates the rule PRE09-C. Do not replace secure functions with deprecated or obsolescent functions. It could possible be changed so that the code defines strcpy_s itself if it's missing (obviously the actual reimplementation wouldn't be shown there'd just be ellipses).

    1. If it doesn't violate PRE09-C. Do not replace secure functions with deprecated or obsolescent functions, it comes pretty close.  I'm sure the next step would be to create a strcpy_s() macro to replace:

       

          #if defined(USE_EXT1)
            strcpy_s(msg, sizeof msg, source_msg);
          #else
            strcpy(msg, source_msg);
          #endif
      Which would definitely be a violation.