memset.c

Go to the documentation of this file.

// MUSL memset implementation:
// https://github.com/esmil/musl/blob/master/src/string/memset.c

#include <stdint.h>
#include <string.h>

void* __attribute__((weak)) memset(void* dest, int c, size_t n)
{
    unsigned char* s = dest;
    size_t k;

    /* Fill head and tail with minimal branching. Each
     * conditional ensures that all the subsequently used
     * offsets are well-defined and in the dest region. */

    if(!n)
    {
        return dest;
    }
    s[0] = s[n - 1] = (unsigned char)c;
    if(n <= 2)
    {
        return dest;
    }
    s[1] = s[n - 2] = (unsigned char)c;
    s[2] = s[n - 3] = (unsigned char)c;
    if(n <= 6)
    {
        return dest;
    }
    s[3] = s[n - 4] = (unsigned char)c;
    if(n <= 8)
    {
        return dest;
    }

    /* Advance pointer to align it at a 4-byte boundary,
     * and truncate n to a multiple of 4. The previous code
     * already took care of any head/tail that get cut off
     * by the alignment. */

    k = -(uintptr_t)s & 3;
    s += k;
    n -= k;
    n &= (unsigned long)-4;
    n /= 4;

    // Cast to void first to prevent alignment warning
    uint32_t* ws = (uint32_t*)(void*)s;
    uint32_t wc = c & 0xFF;
    wc |= ((wc << 8) | (wc << 16) | (wc << 24));

    /* Pure C fallback with no aliasing violations. */
    for(; n; n--, ws++)
    {
        {
            *ws = wc;
        }
    }

    return dest;
}