在 glibc malloc.c for calloc() (准确地说,public_cALLOc())实现中,当它尝试以两种方式将它正在执行的内存归零时,如果字节数大于 36,则立即调用 memset()否则,它会专门按字节放置 0,如下所示:
glibc-2.13/malloc/malloc.c
void * public_cALLOc()
{
.....
int_malloc();
...
...
/* Unroll clear of <= 36 bytes (72 if 8byte sizes). We know that
contents have an odd number of INTERNAL_SIZE_T-sized words;
minimally 3. */
...
if (nclears > 9)
MALLOC_ZERO(d, clearsize); /* this is nothing but memset(d,0,clearsize) */
else {
*(d+0) = 0;
*(d+1) = 0;
if (nclears > 4) {
*(d+2) = 0;
*(d+3) = 0;
if (nclears > 6) {
*(d+4) = 0;
*(d+5) = 0;
if (nclears > 8) {
*(d+6) = 0;
*(d+7) = 0;
*(d+8) = 0;
}
}
}
---------------------------------
问题是,为什么我们不直接对所有人做memset(),这个区别有什么需要。
谢谢,卡皮尔