strtok

function

char * strtok ( char * str, const char * delimiters );

Split string into tokens

A sequence of calls to this function split str into tokens, which are sequences of contiguous characters separated by any of the characters that are part of delimiters.

On a first call, the function expects a C string as argument for str, whose first character is used as the starting location to scan for tokens. In subsequent calls, the function expects a null pointer and uses the position right after the end of last token as the new starting location for scanning.

第一次调用时第一个参数使用的是string的指针
第二次及后续的调用使用的是null作为参数


strtok实现过程中，使用某个静态变量来记录分析的str的位置。
陷阱：strtok 对参数进行了修改，在delimeter的位置换成了 \0 。
           忽略了这一点，调了半天的bug.

char * __cdecl strtok(char *s1, const char *delimit)
{
    static char *lastToken = NULL; /* UNSAFE SHARED  */
    char *tmp;

    /* Skip leading delimiters if new string. */
    if ( s1 == NULL ) {
        s1 = lastToken;
        if (s1 == NULL)         /* End of story? */
            return NULL;
    } else {
        s1 += strspn(s1, delimit);
    }

    /* Find end of segment */
    tmp = strpbrk(s1, delimit);
    if (tmp) {
        /* Found another delimiter, split string and save state. */
        *tmp = '\0';
        lastToken = tmp + 1;
    } else {
        /* Last segment, remember that. */
        lastToken = NULL;
    }

    return s1;
}

#include <sys/cdefs.h>
#include <string.h>

/*
 * Span the string s2 (skip characters that are in s2).
 */
size_t
strspn(s1, s2)
    const char *s1;
    register const char *s2;
{
    register const char *p = s1, *spanp;
    register char c, sc;

    /*
     * Skip any characters in s2, excluding the terminating \0.
     */
cont:
    c = *p++;
    for (spanp = s2; (sc = *spanp++) != 0;)
        if (sc == c)
            goto cont;
    return (p - 1 - s1);
}

#include <sys/cdefs.h>
#include <string.h>

/*
 * Find the first occurrence in s1 of a character in s2 (excluding NUL).
 */
char *
strpbrk(s1, s2)
    register const char *s1, *s2;
{
    register const char *scanp;
    register int c, sc;

    while ((c = *s1++) != 0) {
        for (scanp = s2; (sc = *scanp++) != 0;)
            if (sc == c)
                return ((char *)(s1 - 1));
    }
    return (NULL);
}