一些常用字符串操作函数的内部实现

    刚学C语言的时候经常做这样的题，比如实现strcpy  strcat库函数的内部实现，有的公司笔试也会考这些，我整理了一下常用的字符串库函数的内部实现，截自linux内核中的lib/string.c文件，绝对标准的程序，供大家参考。

    

memset:

/*
 * memset - Fill a region of memory with the given value
 * @s: Pointer to the start of the area.
 * @c: The byte to fill the area with
 * @count: The size of the area.
 */
void *memset(void *s, int c, size_t count)
{
    char *xs = s;

    while (count--)
        *xs++ = c;
    return s;
}

memcpy:

/*
* memcpy - Copy one area of memory to another
* @dest: Where to copy to
* @src: Where to copy from
* @count: The size of the area.
*/
void *memcpy(void *dest, const void *src, size_t count)
{
char *tmp = dest;
const char *s = src;

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

memmove:

/*
 * memmove - Copy one area of memory to another
 * @dest: Where to copy to
 * @src: Where to copy from
 * @count: The size of the area.
 * Unlike memcpy(), memmove() copes with overlapping areas.
 */
void *memmove(void *dest, const void *src, size_t count)
{
    char *tmp;
    const char *s;

    if (dest <= src) {
        tmp = dest;
        s = src;
        while (count--)
            *tmp++ = *s++;
    } else {
        tmp = dest;
        tmp += count;
        s = src;
        s += count;
        while (count--)
            *--tmp = *--s;
    }
    return dest;
}

memcmp:

/*
 * 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 = 0;

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

strcpy:

/*
 * 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');

    return tmp;
}

strncpy:

/*
 * 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) != 0)
            src++;
        tmp++;
        count--;
    }

    return dest;
}

strcat:

/*
 * 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:

/*
 * 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++) != 0) {
            if (--count == 0) {
                *dest = '\0';
                break;
            }
        }
    }

    return tmp;
}

strcmp:

/*
 * strcmp - Compare two strings
 * @cs: One string
 * @ct: Another string
 */
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;
}

strncmp:

/*
 * 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 ? -1 : 1;
        if (!c1)
            break;
        count--;
    }

    return 0;
}

strchr:

/*
 * 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;
}

strlen:

/*
 * 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);

    return sc - s;
}

strnlen:

/*
 * 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);

    return sc - s;
}

strsep:

/*
 * strsep - Split a string into tokens
 * @s: The string to be searched
 * @ct: The characters to search for
 *
 * strsep() updates @s to point after the token, ready for the next call.
 */
char *strsep(char **s, const char *ct)
{
    char *sbegin = *s;
    char *end;

    if (sbegin == NULL)
        return NULL;

    end = strpbrk(sbegin, ct);
    if (end)
        *end++ = '\0';
    *s = end;

    return sbegin;
}

strstr:

/*
 * 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)
{
    int 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;
}