为了磨练我的C技能,我下载了eglibc源代码,发现了strncpy函数。我不明白为什么他要区分n≤4的情况并进行4次测试。
也许与内存布局有关,我不确定,请指教。
int
STRNCMP (const char *s1, const char *s2, size_t n)
{
unsigned char c1 = '\0';
unsigned char c2 = '\0';
if (n >= 4)
{
size_t n4 = n >> 2;
do
{
c1 = (unsigned char) *s1++;
c2 = (unsigned char) *s2++;
if (c1 == '\0' || c1 != c2)
return c1 - c2;
c1 = (unsigned char) *s1++;
c2 = (unsigned char) *s2++;
if (c1 == '\0' || c1 != c2)
return c1 - c2;
c1 = (unsigned char) *s1++;
c2 = (unsigned char) *s2++;
if (c1 == '\0' || c1 != c2)
return c1 - c2;
c1 = (unsigned char) *s1++;
c2 = (unsigned char) *s2++;
if (c1 == '\0' || c1 != c2)
return c1 - c2;
} while (--n4 > 0);
n &= 3;
}
while (n > 0)
{
c1 = (unsigned char) *s1++;
c2 = (unsigned char) *s2++;
if (c1 == '\0' || c1 != c2)
return c1 - c2;
n--;
}
return c1 - c2;
}
也许与内存布局有关,我不确定,请指教。