source: buchla-68k/libsm/strtok.c@ c3aee8a

/*
   =============================================================================
        strtok.c -- tokenize a string
        Version 1 -- 1987-06-12

        Uses strpbrk and strspn to break 'string' into tokens on
        sequentially subsequent calls.  Returns NULL when no
        non-separator characters remain.
        'subsequent' calls are calls with first argument == NULL.
   =============================================================================
*/

#define NULL    (int8_t *)0

extern int16_t strspn(int8_t *string, int8_t *charset);
extern int8_t *strpbrk(int8_t *string, int8_t *brkset);

int8_t *strtok(int8_t *string, int8_t *sepset)
{
        register int8_t *p, *q, *r;
        static int8_t *savept;

        /* first or subsequent call ? */

        p = (string == NULL) ? savept : string;

        if (p == 0)              /* return if no tokens remain */
                return(NULL);

        q = p + strspn(p, sepset);      /* skip leading separators */

        if (*q == '\0')          /* return if no tokens remain */
                return(NULL);

        if ((r = strpbrk(q, sepset)) == NULL)    /* move past token */
                savept = 0;     /* indicate this is last token */
        else {
                *r = '\0';
                savept = ++r;
        }

        return(q);
}