Page History

...

In the following non-compliant code example, cBlocks is multiplied by 16 and the result is stored in the unsigned long long int alloc. The result of this multiplication can overflow because it is a 32 bit operation and the resulting value stored in alloc is invalid.

 
void* AllocBlocks(size_t cBlocks) { 
  if (cBlocks == 0) return NULL; 
  unsigned long long alloc = cBlocks * 16; 
  return (alloc < UINT_MAX) 
     ? malloc(cBlocks * 16) 
     : NULL; 
} 

Compliant Solution 1

On architectures where {{unsigned long long int is guaranteed to have 2x the number of bits as size_tupcast the variable used in the multiplication to a 64-bit value.  This ensures the multiplication operation is performed

If size_t is implemented as a unsigned int type, the result of this multiplication can overflow because the multiplication is performed between two values of size int} and the resulting value stored in {{alloc is invalid.

If, on the other hand, size_t is implemented as an unsigned long long type, the result of the multiplication operation may not be representable as an unsigned long long value.

Compliant Solution 1

Upcasting only works when a size is represented using a smaller type. Because we recommend that all sizes be represented using size_t, upcasting is never a good idea. Instead, check to make sure the multiplication does not overflow using standard methods:

void *

 
  void* AllocBlocks(size_t cBlocks) {
  size_t alloc; 

  if (cBlocks == 0) return NULL; 
  unsigned long longsize_t alloc = (unsigned long long)multsize_t(cBlocks *, 16); 
  returnif (alloc < UINT_MAX) 
  ? malloc(cBlocks * 16) 
  : NULL; 
  } 
  

The assumption concerning the relationship of unsigned long long int and size_t must be document in the header for each file that depends upon this assumption for correct execution.

errno) {
    return NULL;
  }
  else {
    return malloc(alloc);
  }
  
} /* end AllocBlocks */

References

• ISO/IEC 9899-

...

• 1999 Section 7.20.3 Memory Management Functions
• Seacord 05 Chapter 4 Dynamic Memory Management