linux/lib/string.c

 /**
  * strlen - Find the length of a string
  * @s: The string to be sized
  */
 size_t strlen(const char *s)
 {
     const char *sc;

     for (sc = s; *sc != '\0'; ++sc)
         /* nothing */;
     return sc - s;
 }

 /**
  * strnlen - Find the length of a length-limited string
  * @s: The string to be sized
  * @count: The maximum number of bytes to search
  */
 size_t strnlen(const char *s, size_t count)
 {
     const char *sc;

     for (sc = s; count-- && *sc != '\0'; ++sc)
         /* nothing */;
     return sc - s;
 }

 /**
  * strcpy - Copy a %NUL terminated string
  * @dest: Where to copy the string to
  * @src: Where to copy the string from
  */
 char *strcpy(char *dest, const char *src)
 {
     char *tmp = dest;

     while ((*dest++ = *src++) != '\0')
         /* nothing */;
     return tmp;
 }

 /**
  * strncpy - Copy a length-limited, %NUL-terminated string
  * @dest: Where to copy the string to
  * @src: Where to copy the string from
  * @count: The maximum number of bytes to copy
  *
  * The result is not %NUL-terminated if the source exceeds
  * @count bytes.
  *
  * In the case where the length of @src is less than  that  of
  * count, the remainder of @dest will be padded with %NUL.
  *
  */
 char *strncpy(char *dest, const char *src, size_t count)
 {
     char *tmp = dest;

     while (count) {
         if ((*tmp = *src) != ) /* src到尽头后不再自加使得dest后面填'\0'了 */
             src++;
         tmp++;
         count--;
     }
     return dest;
 }

 /**
  * strcat - Append one %NUL-terminated string to another
  * @dest: The string to be appended to
  * @src: The string to append to it
  */
 char *strcat(char *dest, const char *src)
 {
     char *tmp = dest;

     while (*dest)
         dest++;
     while ((*dest++ = *src++) != '\0')
         ;
     return tmp;
 }

 /**
  * strncat - Append a length-limited, %NUL-terminated string to another
  * @dest: The string to be appended to
  * @src: The string to append to it
  * @count: The maximum numbers of bytes to copy
  *
  * Note that in contrast to strncpy(), strncat() ensures the result is
  * terminated.
  */
 char *strncat(char *dest, const char *src, size_t count)
 {
     char *tmp = dest;

     if (count) {
         while (*dest)
             dest++;
         while ((*dest++ = *src++) != ) {
             if (--count == ) {
                 *dest = '\0';
                 break;
             }
         }
     }
     return tmp;
 }

 /**
  * strcmp - Compare two strings
  * @cs: One string
  * @ct: Another string
  */
 int strcmp(const char *cs, const char *ct)
 {
     unsigned char c1, c2;

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

 /**
  * strncmp - Compare two length-limited strings
  * @cs: One string
  * @ct: Another string
  * @count: The maximum number of bytes to compare
  */
 int strncmp(const char *cs, const char *ct, size_t count)
 {
     unsigned char c1, c2;

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

 /**
  * strchr - Find the first occurrence of a character in a string
  * @s: The string to be searched
  * @c: The character to search for
  */
 char *strchr(const char *s, int c)
 {
     for (; *s != (char)c; ++s)
         if (*s == '\0')
             return NULL;
     return (char *)s;
 }

 /**
  * strnchr - Find a character in a length limited string
  * @s: The string to be searched
  * @count: The number of characters to be searched
  * @c: The character to search for
  */
 char *strnchr(const char *s, size_t count, int c)
 {
     for (; count-- && *s != '\0'; ++s)
         if (*s == (char)c)
             return (char *)s;
     return NULL;
 }

 /**
  * memcmp - Compare two areas of memory
  * @cs: One area of memory
  * @ct: Another area of memory
  * @count: The size of the area.
  */
 int memcmp(const void *cs, const void *ct, size_t count)
 {
     const unsigned char *su1, *su2;
     int res = ;

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

 /**
  * memcpy - Copy one area of memory to another
  * @dest: Where to copy to
  * @src: Where to copy from
  * @count: The size of the area.
  *
  * You should not use this function to access IO space, use memcpy_toio()
  * or memcpy_fromio() instead.
  */
 void *memcpy(void *dest, const void *src, size_t count)
 {
     char *tmp = dest;
     const char *s = src;

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

 /* Not static to avoid a conflict with the prototype in the Linux headers. */
 void *memmove(void *dest, const void *src, size_t size)
 {
     uint8_t *d = dest;
     const uint8_t *s = src;
     size_t i;
     /* 判断地址高低, 可出现重叠区域 */
     if (d < s) {
         for (i = ; i < size; ++i)
             d[i] = s[i];
     } else if (d > s) {
         i = size;
         while (i-- > )
             d[i] = s[i];
     }

     return dest;
 }

 /**
  * strstr - Find the first substring in a %NUL terminated string
  * @s1: The string to be searched
  * @s2: The string to search for
  */
 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;
 }

 /**
  * strnstr - Find the first substring in a length-limited string
  * @s1: The string to be searched
  * @s2: The string to search for
  * @len: the maximum number of characters to search
  */
 char *strnstr(const char *s1, const char *s2, size_t len)
 {
     size_t l2;

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