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:

  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:

  The following are optional feature macros:

 The following is optional and is defined by the user:

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.

Automated Detection

Tool	Version	Checker	Description
Axivion Bauhaus Suite		CertC-PRE13
LDRA tool suite			Partially implemented

Related Vulnerabilities

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

Related Guidelines