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
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