In bitmap.h, why does bitmap_zero need memset?

Question

In include/linux/bitmap.h, in the bitmap_zero(), why use memset?

static inline void bitmap_zero(unsigned long *dst, int nbits)
{
        if (small_const_nbits(nbits))
                *dst = 0UL;
        else {
                int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
                memset(dst, 0, len);
        }
}

IS the *det = OUL not enough?

Answer 1

The definition of small_const_nbits is:

#define small_const_nbits(nbits) \
    (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG)

BITS_PER_LONG is normally 32 or 64, depending on what machine you're on.

So, if you're trying to clear fewer than that many bits, you can certainly do it in a single operation - that's the first half of the if statement. If it's longer than 32 or 64 bits, you need to set multiple words, and that's done by the memset call.