标识符未定义怎么弄啊？

strsep是linux下的函数，windows下没有，添加一个自定义函数，如下：

char *strsep(char **stringp, const char *delim)
{
    char *s;
    const char *spanp;
    int c, sc;
    char *tok;
    if ((s = *stringp)== NULL)
        return (NULL);
    for (tok = s;;) {
        c = *s++;
        spanp = delim;
        do {
            if ((sc =*spanp++) == c) {
                if (c == 0)
                    s = NULL;
                else
                    s[-1] = 0;
                *stringp = s;
                return (tok);
            }
        } while (sc != 0);
    }
    /* NOTREACHED */
}