Read Back

• to read data
• read method will be blocked when there is no data to read until one of four situations has happened:
  • there is data
  • connection is closed, return 0
  • connection is reset, return error
  • received a signal of interruption, return error (errno = EINTR)
• use read_all() to receive whole data

1. read()

method

/* read method */
int read(int fd, char* buf, int len)

/* read_all method */
int read_all(int fd, void* buf, int n)
{    
    int nleft = n, nbytes;    
    char* ptr = buf;    
    for(; nleft > 0; )
    {
        nbytes = read(fd, ptr, nleft);        
        if(nbytes < 0)
        {
            if (errno == EINTR) 
                nbytes = 0;
            else 
                return -1;
        }        
        else if (nbytes == 0)
            break;        
        nleft -= nbytes;        
        ptr += nbytes;    
    }    
    return (n - nleft);
}

parameters

• fd: socket描述符
• buf: 接受緩衝區
• len: 要讀取數據的大小

return value

• >=0: success
  • len: data of buffer is greater than len
  • actual_len: data of buffer is less than len
• -1: failure
• errno: wrong code

2. recv()

method

int recv(int sockfd, void buf, int len, int flags)

parameters

• fd: socket描述符
• buf: 接受緩衝區
• len: 接受數據長度
• flags: 接受數據的控制參數
  • 0: read()
  • MSG_OOB: 發送帶外數據
  • MSG_PEEK: 接收數據後, 不從緩衝區移走數據
  • MSG_WAITALL: 數據量不夠時, 讀操作等待, 並不返回; 但在收到信號鏈接端口時, 仍然會結束

return value

• >=0: success
• -1: failure
• errno: wrong code

3. readv()

• 讀多個非連續緩衝區, 稱為散佈讀

4. recvmsg()

• 類似recv(), 但功能更強大