CERT C++: MSC39-C

Do not call va_arg() on a va_list that has an indeterminate value

Description

Rule Definition

Do not call va_arg() on a va_list that has an indeterminate value.¹

Polyspace Implementation

The rule checker checks for Use of indeterminate va_list values.

Examples

Use of indeterminate va_list values

Issue

This issue occurs when:

• You use a local va_list without initializing it first using va_start or va_copy.

  You might be using the local va_list in va_arg or a vprintf-like function (function that takes variable number of arguments).

• You use a va_list (variable argument list) from a function parameter directly instead of making a copy using va_copy and using the copy.

Note that the checker works on a per-function basis. If you initialize a va_list with va_start within a block, the checker considers the list as initialized beyond the block for the remainder of the function. Likewise, if you end the list with va_end within a block, the checker considers the list as ended beyond the block for the remainder of the function.

Risk

If you use a local va_list without initializing it first, the behavior is undefined.

If you pass a va_list to another function and use it there, the va_list has indeterminate values in the original calling function. Using the va_list in the calling function following the function call can produce unexpected results.

Fix

Initialize a local va_list with va_start or va_copy before using it.

Pass a va_list by reference. In the called function, make a copy of the passed va_list and use the copy. You can then continue to access the original va_list in the calling function.

Example – Direct Use of va_list From Another Function

#include <cstdarg>
#include <cstdio>
#include <climits>
  
int containsOutliers(size_t count, va_list ap) {
  for (size_t i = 1; i < count; ++i) {
    if (va_arg(ap, int) > INT_MAX) { //Noncompliant
      return 1;
    }
  }
  return 0;
}
 
int printList(size_t count, ...) {
  va_list ap; 
  va_start(ap, count);
 
  if (containsOutliers(count, ap)) {
    va_end(ap);
    return 1;
  }
 
  for (size_t i = 0; i < count; ++i) {
    printf("%d", va_arg(ap, int));
  }
 
  va_end(ap);
  return 0;
}

In this example, the checker flags the direct use of the va_list variable ap obtained as argument in the containsOutliers function.

Correction – Copy va_list Obtained from Another Function

To avoid the violation, pass the va_list by reference and make a copy of the variable in the containsOutliers function. Perform further operations on the copy.

#include <cstdarg>
#include <cstdio>
#include <climits>
  
int containsOutliers(size_t count, va_list* ap) {
  va_list copiedAp;
  va_copy (copiedAp, *ap);
  
  for (size_t i = 1; i < count; ++i) {
    if (va_arg(copiedAp, int) > INT_MAX) {
      return 1;
    }
  }
  return 0;
}
 
int printList(size_t count, ...) {
  va_list ap; 
  va_start(ap, count);
 
  if (containsOutliers(count, &ap)) {
    va_end(ap);
    return 1;
  }
 
  for (size_t i = 0; i < count; ++i) {
    printf("%d", va_arg(ap, int));
  }
 
  va_end(ap);
  return 0;
}

Check Information

Group: 49. Miscellaneous (MSC)

Version History

Introduced in R2019a