incompleteArrayFill

Message: Array 'a[5]' might be filled incompletely. Did you forget to multiply the size given to 'memset()' with 'sizeof(*a)'?
Category: Correctness
Severity: Warning/Portability
Language: C/C++

Description

memset()/memcpy()/memmove() is given a byte count equal to the number of elements in an array, rather than their combined size in bytes - unless each element happens to be exactly one byte, this only fills part of the array. When the array's element type is specifically bool, this is reported at portability severity instead: sizeof(bool) happens to be 1 on most common platforms (so the code "works" there), but the C/C++ standard doesn't guarantee it, so the exact same code is a real, silent bug on a platform where bool is wider.

Motivation

These functions take their size argument in bytes, not element count. Passing the element count directly is an easy mistake - it compiles fine, and on typical platforms with 1-byte types it can even happen to be correct, but for any array element wider than one byte it silently leaves most of the array untouched.

How to fix

Multiply the element count by sizeof(*array) (or the element type's size) to get the correct byte count.

Before:

#include <cstring>
void f() {
    int a[5];
    memset(a, 123, 5); // <- fills 5 bytes, not 5 ints
}

After:

#include <cstring>
void f() {
    int a[5];
    memset(a, 123, 5 * sizeof(*a));
}