linux内核字符串函数的实现

公司笔试和面试中常要你编写strcpy/strcmp/memcpy/memcmp/strstr/atoi 等等这样的基础函数的实现。
我想在这儿总结下:(如下代码都是来自最新Linux kernel的库函数)
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//strcpy in linux kernel  linux-3.3/linux/lib/string.c
char *strcpy(char *dest, const char *src) {
         char *tmp = dest; //很多时候可以在这儿对传入参数进行有效检查。内核里面没有这样做。
         while ((*dest++ = *src++) != '\0')
             /* nothing */;
    return tmp;
 }

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int strcmp(const char *cs, const char *ct)
 {
         unsigned char c1, c2;

         while (1) {
                 c1 = *cs++;
                 c2 = *ct++;
                 if (c1 != c2)
                         return c1 < c2 ? -1 : 1;
                 if (!c1)
                         break;
         }
         return 0;
 }

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void *memcpy(void *dest, const void *src, size_t count)
 {
         char *tmp = dest;
         const char *s = src;

         while (count--)
                 *tmp++ = *s++;
         return dest;
 }

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int memcmp(const void *cs, const void *ct, size_t count) {
          const unsigned char *su1, *su2;
          int res = 0;

          for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
                 if ((res = *su1 - *su2) != 0)
                     break;
          return res;
}

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char *strstr(const char *s1, const char *s2)
 {
         size_t l1, l2;

         l2 = strlen(s2);
         if (!l2)
                 return (char *)s1;
         l1 = strlen(s1);
         while (l1 >= l2) {
                 l1--;
                 if (!memcmp(s1, s2, l2))
                         return (char *)s1;
                 s1++;
         }
         return NULL;
 }

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