How to flip any bit in a memory block?

Question

How can I flip any bit I want in a chunk of memory:

int size = 4000;
void* block = malloc(size);
bzero(block, size);
// flip bit #100 in block

Thanks

Answer 1

void flip_bit (void *block, int bit)
{
    unsigned char *b = (unsigned char *) block;
    b[bit/8] ^= 1 << (bit % 8);
}