非阻塞读和写：str_cli函数（修订版）

void str_cli(FILE *fp, int sockfd)
{
    int        maxfdp1, val, stdineof;
    ssize_t    n, nwritten;
    fd_set     rset, wset;
    char       to[MAXLINE], fr[MAXLINE];
    char       *toiptr, *tooptr, *friptr, *froptr;

    val = fcntl(sockfd, F_GETFL, 0);
    fcntl(sockfd, F_SETFL, val | O_NONBLOCK);

    val = fcntl(STDIN_FILENO, F_GETFL, 0);
    fcntl(STDIN_FILENO, F_SETFL, val | O_NONBLOCK);

    val = fcntl(STDOUT_FILENO, F_GETFL, 0);
    fcntl(STDOUT_FILENO, F_SETFL, val | O_NONBLOCK);

    toiptr = tooptr = to;    /* initialize buffer pointers */
    friptr = froptr = fr;
    stdineof = 0;

    maxfdp1 = max(max(STDIN_FILENO, STDOUT_FILENO), sockfd) + 1;
    for ( ; ; ) {
        FD_ZERO(&rset);
        FD_ZERO(&wset);
        if (stdineof == 0 && toiptr < &to[MAXLINE]) 
            FD_SET(STDIN_FILENO, &rset);    /* read from stdin */
        if (friptr < &fr[MAXLINE])
            FD_SET(sockfd, &rset);          /* read from socket */
        if (tooptr != toiptr)
            FD_SET(sockfd, &wset);          /* data to write to socket */
        if (froptr != friptr)
            FD_SET(STDOUT_FILENO, &wset);   /* data to write to stdout */

        select(maxfdp1, &rset, &wset, NULL, NULL);

        if (FD_ISSET(STDIN_FILENO, &rset)) {
            if ( (n = read(STDIN_FILENO, toiptr, &to[MAXLINE] - toiptr)) < 0) {
                if (errno != EWOULDBLOCK)
                    err_sys("read error on stdin");
            } else if (n == 0) {
                fprintf(stderr, "%s: EOF on stdin\n", gf_time());
                stdineof = 1;                  /* all done with stdin */
                if (tooptr == toiptr)
                    shutdown(sockfd, SHUT_WR); /* send FIN */
　　　　　　　} else {
                fprintf(stderr, "%s: read %d bytes from stdin\n", gf_time(), n);
                toiptr += n;                   /* # just read */
                FD_SET(sockfd, &wset);         /* try and write to socket below */
            }
        }

        if (FD_ISSET(sockfd, &rset)) {
            if ( (n = read(sockfd, friptr, &fr[MAXLINE] - friptr)) < 0) {
                if (errno != EWOULDBLOCK)
                    err_sys("read error on socket");
　　　　　　　} else if (n == 0) {
                fprintf(stderr, "%s: EOF on socket\n", gf_time());
                if (stdineof)
                    return;                    /* normal termination */
                else
                    err_quit("str_cli: server terminated prematurely");
            } else {
                fprintf(stderr, "%s: read %d bytes from socket\n", gf_time(), n);
                friptr += n;                   /* # just read */
                FD_SET(STDOUT_FILENO, &wset);  /* try and write below */
            }
        }

        if (FD_ISSET(STDOUT_FILENO, &wset) && ( (n = friptr - froptr) > 0)) {
            if ( (nwritten = write(STDOUT_FILENO, froptr, n)) < 0) {
                if (errno != EWOULDBLOCK)
                    err_sys("write error to stdout");
　　　　　　  } else {
                fprintf(stderr, "%s: wrote %d bytes to stdout\n", gf_time(), nwritten);
                froptr += nwritten;            /* # just written */
                if (froptr == friptr)
                    froptr = friptr = fr;      /* back to beginning of buffer */
            }
        }

        if (FD_ISSET(sockfd, &wset) && ( (n = toiptr - tooptr) > 0)) {
            if ( (nwritten = write(sockfd, tooptr, n)) < 0) {
                if (errno != EWOULDBLOCK)
                    err_sys("write error to socket");
            } else {
                fprintf(stderr, "%s: wrote %d bytes to socket\n", gf_time(), nwritten);
                tooptr += nwritten;    　　　　      /* # just written */
                if (tooptr == toiptr) {
                    toiptr = tooptr = to;           /* back to beginning of buffer */
                    if (stdineof)
                        shutdown(sockfd, SHUT_WR);  /* send FIN */
                }
            }
        }
    }
}

char  *gf_time(void)
{
    struct timeval tv;
    time_t         t;
    static char    str[30];
    char           *ptr;

    if (gettimeofday(&tv, NULL) < 0)
        err_sys("gettimeofday error");

    t = tv.tv_sec;    /* POSIX says tv.tv_sec is time_t; some BSDs don‘t agree. */
    ptr = ctime(&t);
    strcpy(str, &ptr[11]);
    /* Fri Sep 13 00:00:00 1986\n\0 */
    /* 0123456789012345678901234 5  */
    snprintf(str+8, sizeof(str)-8, ".%06ld", tv.tv_usec);

    return(str);
}