Page History

...

Depending on the compiler and platform, this structure could be any of a variety of sizes. Assuming 32-bit integers and 64-bit doubles, the size might be 12 or 16 bytes, depending on alignment rules.

Non-Compliant Coding Example

This non-compliant example demonstrates the incorrect way to declare a triangular array of integers. On a platform with 64-bit integers, the loop will access memory outside the allocated memory section.

/* assuming 32-bit pointer, 32-bit integer */
size_t i;
int ** triarray = calloc(100, 4);
if (triarray == NULL) {
  /* handle error */
}

for (i = 0; i < 100; i++) {
   triarray[i] = calloc(i, 4);
   if (triarray[i] == NULL) {
     /* handle error */
   }
}

Compliant Solution

The above example can be fixed by replacing the hard-coded value 4 with the size of the type using sizeof.

size_t i;
int **triarray = calloc(100, sizeof(int *));

if (!triarray) {
   /* handle error */ 
}

for (i = 0; i < 100; i++) {
   triarray[i] = calloc(i, sizeof(int));
   if (!triarray[i]) {
      /* handle error */
   }
}

Risk Assessment

If non-compliant code is ported to a different platform, it could introduce a buffer or stack overflow vulnerability.

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.6, "Representations of types," and Section 6.5.3.4, "The sizeof operator"