UNP_2_Read Line Function

UNP Code

ssize_t Readline(int fd, void *ptr, size_t maxlen)
{
    ssize_t n;

    if ( (n = readline(fd, ptr, maxlen)) < 0)
        err_sys("readline error");  //  套头函数，多了错误信息

    return(n);
}

ssize_t readline(int fd, void *vptr, size_t maxlen)
{
    int  n, rc;
    char c, *ptr;

    ptr = vptr;
    for (n = 1; n < maxlen; n++)
    {
        if ( (rc = my_read(fd, &c)) == 1)   //  my_read相当于一个数据池的作用
        {                                   //  readline只专注于行的逻辑关系
            *ptr++ = c;
            if (c == '\n')  break;          /* newline is stored, like fgets() */
        }
        else 
        {
            if (rc == 0)     
            {
                if (n == 1) return(0);  /* EOF, no data read */
                else    break;          /* EOF, some data was read */

            } else return(-1);          /* error, errno set by read() */
        }
    }

    *ptr = 0;    /* null terminate like fgets() */
    return(n);
}

//  static function
static ssize_t my_read(int fd, char *ptr)
{
    static int     read_cnt = 0;
    static char    *read_ptr;
    static char    read_buf[MAXLINE];   //  缓存池

    if (read_cnt <= 0) 
    {
again:
        //  sizeof(read_buf) = 1 => read_cnt = 1，这里可以做缓存机制
        if ( (read_cnt = read(fd, read_buf, sizeof(read_buf))) < 0) 
        {
            if (errno == EINTR) //  Interrupted system call
                goto again;
            return(-1); //  其它错误
        }

        if (read_cnt == 0)
            return(0);

        read_ptr = read_buf;
    }

    read_cnt--;
    *ptr = *read_ptr++;
    return(1);
}