The result of calling malloc(0), or calling 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.
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In this example, the user defined function calc_size() (not shown) is used to calculate the size of the string other_srting. The result of calc_size() is returned to str_size and used as the size parameter in a call to malloc(). However, if calc_size returned zero, then when the strncpy() is executed, a heap buffer overflow will occur.
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To assure that zero is never passed as a size argument to malloc(), a check must be made on the size parameter.
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size_t str_size = calc_size(other_string);
if (str_size > 0) {
char *str_copy = malloc(str_size);
if (str_copy == NULL) {
/* handle error */
}
strcpy(str_copy, other_string);
}
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