mjpg—stream的readline函数

 其实也就是在实现行缓存读函数fgets 

/* read just a single line or timeout */
int _readline(int fd, iobuffer *iobuf, void *buffer, size_t len, int timeout)
{  /*函数原理：从iobuf.buf中一个字节一个字节的将数据读取存放到buffer中去，直到遇见换行符"\n",或者长度达到了*/
    char c = '\0', *out = buffer;
    int i;

    memset(buffer, 0, len);

    for(i = 0; i < len && c != '\n'; i++) {
        if(_read(fd, iobuf, &c, 1, timeout) <= 0) {
            /* timeout or error occured */
            return -1;
        }
        *out++ = c;
    }

    return i;
}

int _read(int fd, iobuffer *iobuf, void *buffer, size_t len, int timeout)
{
    int copied = 0, rc, i;/*copied 表示已经读了多少字节*/
    fd_set fds;
    struct timeval tv;

    memset(buffer, 0, len);

    while((copied < len)) {
        i = MIN(iobuf->level, len - copied);/*第一次 i=0，因为level初始化为0，又因为len为1且copied=0*/
        memcpy(buffer + copied, iobuf->buffer + IO_BUFFER - iobuf->level, i);

        iobuf->level -= i;
        copied += i;
        if(copied >= len) /*第二次读的时候copied=0，buffer还没有清空*/
            return copied;

        /* select will return in case of timeout or new data arrived 
		 * 当客户端发有数据或者超时的时候，select函数就返回
		 */
        tv.tv_sec = timeout; /*每次进入都重新计时*/
        tv.tv_usec = 0;
        FD_ZERO(&fds);
        FD_SET(fd, &fds);
        if((rc = select(fd + 1, &fds, NULL, NULL, &tv)) <= 0) {
            if(rc < 0)
                exit(EXIT_FAILURE);

            /* this must be a timeout，第二次读的时候，靠上面的返回不行了，只能依靠select超时返回 */
            return copied;
        }

        init_iobuffer(iobuf);

        /*
         * there should be at least one byte, because select signalled it.
         * But: It may happen (very seldomly), that the socket gets closed remotly between
         * the select() and the following read. That is the reason for not relying
         * on reading at least one byte.
         */
        if((iobuf->level = read(fd, &iobuf->buffer, IO_BUFFER)) <= 0) {
            /* an error occured */
            return -1;
        }

        /* align data to the end of the buffer if less than IO_BUFFER bytes were read */
        memmove(iobuf->buffer + (IO_BUFFER - iobuf->level), iobuf->buffer, iobuf->level);
    }

    return 0;
}