解读linux对string.h函数的实现

/*
file:string.h
#ifndef _LINUX_STRING_H_
#define _LINUX_STRING_H_

/* We don't want strings.h stuff being user by user stuff by accident */

#ifdef __KERNEL__

#include   /* for size_t */
#include  /* for NULL */
#include    /* for inline ((always_inline)) */

#ifdef __cplusplus
extern "C" {
#endif

extern char * ___strtok;
extern char * strpbrk(const char *,const char *);
extern char * strtok(char *,const char *);
extern char * strsep(char **,const char *);
extern __kernel_size_t strspn(const char *,const char *);


/*
 * Include machine specific inline routines
 */
#include

#ifndef __HAVE_ARCH_STRCPY
extern char * strcpy(char *,const char *);
#endif
#ifndef __HAVE_ARCH_STRNCPY
extern char * strncpy(char *,const char *, __kernel_size_t);
#endif
#ifndef __HAVE_ARCH_STRCAT
extern char * strcat(char *, const char *);
#endif
#ifndef __HAVE_ARCH_STRNCAT
extern char * strncat(char *, const char *, __kernel_size_t);
#endif
#ifndef __HAVE_ARCH_STRCMP
extern int strcmp(const char *,const char *);
#endif
#ifndef __HAVE_ARCH_STRNCMP
extern int strncmp(const char *,const char *,__kernel_size_t);
#endif
#ifndef __HAVE_ARCH_STRNICMP
extern int strnicmp(const char *, const char *, __kernel_size_t);
#endif
#ifndef __HAVE_ARCH_STRCHR
extern char * strchr(const char *,int);
#endif
#ifndef __HAVE_ARCH_STRRCHR
extern char * strrchr(const char *,int);
#endif
#ifndef __HAVE_ARCH_STRSTR
extern char * strstr(const char *,const char *);
#endif
#ifndef __HAVE_ARCH_STRLEN
extern __kernel_size_t strlen(const char *);
#endif
#ifndef __HAVE_ARCH_STRNLEN
extern __kernel_size_t strnlen(const char *,__kernel_size_t);
#endif

#ifndef __HAVE_ARCH_MEMSET
extern void * memset(void *,int,__kernel_size_t);
#endif
#ifndef __HAVE_ARCH_MEMCPY
extern void * memcpy(void *,const void *,__kernel_size_t);
#endif
#ifndef __HAVE_ARCH_MEMMOVE
extern void * memmove(void *,const void *,__kernel_size_t);
#endif
#ifndef __HAVE_ARCH_MEMSCAN
extern void * memscan(void *,int,__kernel_size_t);
#endif
#ifndef __HAVE_ARCH_MEMCMP
extern int memcmp(const void *,const void *,__kernel_size_t);
#endif
#ifndef __HAVE_ARCH_MEMCHR
extern void * memchr(const void *,int,__kernel_size_t);
#endif

#ifdef __cplusplus
}
#endif

#endif
#endif /* _LINUX_STRING_H_ */
*/

实现文件string.c

C代码

/*
 *  linux/lib/string.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 * stupid library routines.. The optimized versions should generally be found
 * as inline code in
 *
 * These are buggy as well..
 *
 * * Fri Jun 25 1999, Ingo Oeser
 * -  Added strsep() which will replace strtok() soon (because strsep() is
 *    reentrant and should be faster). Use only strsep() in new code, please.
 */

#include
#include
#include

#ifndef __HAVE_ARCH_STRNICMP
/**
 * strnicmp - Case insensitive(忽视大小写), length-limited string comparison
 * @s1: One string
 * @s2: The other string
 * @len: the maximum number of characters to compare
 */
int strnicmp(const char *s1, const char *s2, size_t len)
{
    /* Yes, Virginia, it had better be unsigned */
    unsigned char c1, c2;

    c1 = 0; c2 = 0;
    if (len) {
        do {
            c1 = *s1; c2 = *s2;
            s1++; s2++;
                          //是否已到字符串的末尾或两字符串是否有空串,如果到了末尾或有空串,则比较完毕
            if (!c1)
                break;
            if (!c2)
                break;
                          //如果没有,且字符串相等,则继续比较下个字符
            if (c1 == c2)
                continue;
                          //如果不相同,则同时转换为小写字符再进行比较
            c1 = tolower(c1);
            c2 = tolower(c2);
                          //如果不相同,则比较完毕,否则继续
            if (c1 != c2)
                break;
        } while (--len);
    }
    return (int)c1 - (int)c2;
}
#endif

char * ___strtok;

#ifndef __HAVE_ARCH_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')
        /* nothing */;
    return tmp;
}
#endif

#ifndef __HAVE_ARCH_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
 *
 * Note that unlike userspace strncpy, this does not %NUL-pad the buffer.
 * However, the result is not %NUL-terminated if the source exceeds
 * @count bytes.
 */
char * strncpy(char * dest,const char *src,size_t count)
{
    char *tmp = dest;

    while (count-- && (*dest++ = *src++) != '\0')
        /* nothing */;

    return tmp;
}
#endif

#ifndef __HAVE_ARCH_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;
}
#endif

#ifndef __HAVE_ARCH_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) { //如果追加的字符数不为0
        while (*dest) //达到字符串的末尾
            dest++;
        while ((*dest++ = *src++)) { //如果没有到源字符串末尾
            if (--count == 0) { //如果已经追加了count字符
                *dest = '\0';//末尾置null字符
                break;//结束循环
            }
        }
    }

    return tmp;
}
#endif

#ifndef __HAVE_ARCH_STRCMP
/**
 * strcmp - Compare two strings
 * @cs: One string
 * @ct: Another string
 */
int strcmp(const char * cs,const char * ct)
{
    register signed char __res;

    while (1) {
        if ((__res = *cs - *ct++) != 0 || !*cs++)
            break;
    }

    return __res;
}
#endif

#ifndef __HAVE_ARCH_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)
{
    register signed char __res = 0;

    while (count) {
        if ((__res = *cs - *ct++) != 0 || !*cs++)
            break;
        count--;
    }

    return __res;
}
#endif

#ifndef __HAVE_ARCH_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;
}
#endif

#ifndef __HAVE_ARCH_STRRCHR
/**
 * strrchr - Find the last occurrence of a character in a string
 * @s: The string to be searched
 * @c: The character to search for
 */
char * strrchr(const char * s, int c)
{
       const char *p = s + strlen(s);
       do {
           if (*p == (char)c)
               return (char *)p;
       } while (--p >= s);
       return NULL;
}
#endif

#ifndef __HAVE_ARCH_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)
        /* nothing */;
    return sc - s;
}
#endif

#ifndef __HAVE_ARCH_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)
        /* nothing */;
    return sc - s;
}
#endif

#ifndef __HAVE_ARCH_STRSPN
/**
 * strspn - Calculate the length of the initial substring of @s which only
 *  contain letters in @accept
 * @s: The string to be searched
 * @accept: The string to search for
 */
size_t strspn(const char *s, const char *accept)
{
    const char *p;
    const char *a;
    size_t count = 0;

    for (p = s; *p != '\0'; ++p) {
        for (a = accept; *a != '\0'; ++a) {
            if (*p == *a)
                break;
        }
        if (*a == '\0')
            return count;
        ++count;
    }

    return count;
}
#endif

#ifndef __HAVE_ARCH_STRPBRK
/**
 * strpbrk - Find the first occurrence of a set of characters
 * @cs: The string to be searched
 * @ct: The characters to search for
 */
char * strpbrk(const char * cs,const char * ct)
{
    const char *sc1,*sc2;

    for( sc1 = cs; *sc1 != '\0'; ++sc1) {
        for( sc2 = ct; *sc2 != '\0'; ++sc2) {
            if (*sc1 == *sc2)
                return (char *) sc1;
        }
    }
    return NULL;
}
#endif

#ifndef __HAVE_ARCH_STRTOK
/**
 * strtok - Split a string into tokens
 * @s: The string to be searched
 * @ct: The characters to search for
 *
 * WARNING: strtok is deprecated, use strsep instead.
 */
char * strtok(char * s,const char * ct)
{
    char *sbegin, *send;

    sbegin  = s ? s : ___strtok;
    if (!sbegin) {
        return NULL;
    }
    sbegin += strspn(sbegin,ct);
    if (*sbegin == '\0') {
        ___strtok = NULL;
        return( NULL );
    }
    send = strpbrk( sbegin, ct);
    if (send && *send != '\0')
        *send++ = '\0';
    ___strtok = send;
    return (sbegin);
}
#endif

#ifndef __HAVE_ARCH_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.
 *
 * It returns empty tokens, too, behaving exactly like the libc function
 * of that name. In fact, it was stolen from glibc2 and de-fancy-fied.
 * Same semantics, slimmer shape. ;)
 */
char * strsep(char **s, const char *ct)
{
    char *sbegin = *s, *end;

    if (sbegin == NULL)
        return NULL;

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

    return sbegin;
}
#endif

#ifndef __HAVE_ARCH_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.
 *
 * Do not use memset() to access IO space, use memset_io() instead.
 */
void * memset(void * s,int c,size_t count)
{
    char *xs = (char *) s;

    while (count--)
        *xs++ = c;

    return s;
}
#endif

#ifndef __HAVE_ARCH_BCOPY
/**
 * bcopy - Copy one area of memory to another
 * @src: Where to copy from
 * @dest: Where to copy to
 * @count: The size of the area.
 *
 * Note that this is the same as memcpy(), with the arguments reversed.
 * memcpy() is the standard, bcopy() is a legacy BSD function.
 *
 * You should not use this function to access IO space, use memcpy_toio()
 * or memcpy_fromio() instead.
 */
void bcopy(const void * srcp, void * destp, size_t count)
{
    const char *src = srcp;
    char *dest = destp;

    while (count--)
        *dest++ = *src++;
}
#endif

#ifndef __HAVE_ARCH_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.
 *
 * 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 = (char *) dest, *s = (char *) src;

    while (count--)
        *tmp++ = *s++;

    return dest;
}
#endif

#ifndef __HAVE_ARCH_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, *s;

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

    return dest;
}
#endif

#ifndef __HAVE_ARCH_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;
}
#endif

#ifndef __HAVE_ARCH_MEMSCAN
/**
 * memscan - Find a character in an area of memory.
 * @addr: The memory area
 * @c: The byte to search for
 * @size: The size of the area.
 *
 * returns the address of the first occurrence of @c, or 1 byte past
 * the area if @c is not found
 */
void * memscan(void * addr, int c, size_t size)
{
    unsigned char * p = (unsigned char *) addr;

    while (size) {
        if (*p == c)
            return (void *) p;
        p++;
        size--;
    }
    return (void *) p;
}
#endif

#ifndef __HAVE_ARCH_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;
}
#endif

#ifndef __HAVE_ARCH_MEMCHR
/**
 * memchr - Find a character in an area of memory.
 * @s: The memory area
 * @c: The byte to search for
 * @n: The size of the area.
 *
 * returns the address of the first occurrence of @c, or %NULL
 * if @c is not found
 */
void *memchr(const void *s, int c, size_t n)
{
    const unsigned char *p = s;
    while (n-- != 0) {
            if ((unsigned char)c == *p++) {
            return (void *)(p-1);
        }
    }
    return NULL;
}

#endif

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