c - strncmp的实现

为了磨练我的 C 技能，我下载了 eglibc 源代码并遇到了 strncpy。我不明白他为什么区分 n

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;
}

可能与我不知道的内存布局有关，请赐教。

最佳答案

这是一个 unrolled loop 。以让二进制文件更大一点为代价，它通过为每 4 个要比较的字节消除 3 个递减、3 个分支和 3 个条件来加速字符串比较。

通过使用与 Duff's device 相同的技术，优化甚至可以更进一步，尽管不清楚这实际上会更快。从链接页面，

关于c - strncmp的实现，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/37034869/