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The result of calling malloc(0) or calloc() to allocate 0 bytes (calloc(1,0), calloc(0,0), or calloc(0,1)) is undefined. From a practical standpoint, allocating 0 bytes with calloc() and malloc() can lead to programming errors with critical security implications, such as buffer overflows. This occurs because the result of allocating 0 bytes with calloc() and malloc() may not considered an error, thus the pointer returned may not be NULL. Instead, the pointer may reference a block of memory on the heap of size zero. If memory is fetched from, or stored in this a location serious error could occur.

Non-compliant Code Example 1

In this example, a dynamic array of integers is allocated to store s elements. However, if s is zero, the call to malloc(s) will return a reference to a block of memory of size 0. When data is copied to this location, a heap-buffer overflow will occur.

list = (int*)malloc(s);
if (i_list == NULL) {
  /* Handle Allocation Error */
}
/* Continue Processing list */

Compliant Code Example 1

To assure that zero is never passed as a size argument to malloc(), a check must be made on s to assure it is not zero.

if (s== 0) {
  /* Handle Error */
}
list = (int*)malloc(s);
if (i_list == NULL) {
  /* Handle Allocation Error */
}
/* Continue Processing list */

References

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