How to read exactly one line?

问题

I have a Linux file descriptor (from socket), and I want to read one line. How to do it in C++?

回答1:

I you are reading from a TCP socket you can't assume when the end of line will be reached. Therfore you'll need something like that:

std::string line;
char buf[1024];
int n = 0;
while(n = read(fd, buf, 1024))
{
   const int pos = std::find(buf, buf + n, '\n')
   if(pos != std::string::npos)
   {
       if (pos < 1024-1 && buf[pos + 1] == '\n')
          break;
   }
   line += buf;
}

line += buf;

Assuming you are using "\n\n" as a delimiter. (I didn't test that code snippet ;-) )

On a UDP socket, that is another story. The emiter may send a paquet containing a whole line. The receiver is garanted to receive the paquet as a single unit .. If it receives it , as UDP is not as reliable as TCP of course.

回答2:

Pseudocode:

char newline = '\n';
file fd;
initialize(fd);
string line;
char c;
while( newline != (c = readchar(fd)) ) {
 line.append(c);
}

Something like that.

回答3:

Here is a tested, quite efficient code:

bool ReadLine (int fd, string* line) {
  // We read-ahead, so we store in static buffer 
  // what we already read, but not yet returned by ReadLine.
  static string buffer; 

  // Do the real reading from fd until buffer has '\n'.
  string::iterator pos;
  while ((pos = find (buffer.begin(), buffer.end(), '\n')) == buffer.end ()) {
    char buf [1025];
    int n = read (fd, buf, 1024);
    if (n == -1) {    // handle errors
      *line = buffer;
      buffer = "";
      return false;
    }
    buf [n] = 0;
    buffer += buf;
  }

  // Split the buffer around '\n' found and return first part.
  *line = string (buffer.begin(), pos);
  buffer = string (pos + 1, buffer.end());
  return true;
}

It's also useful to setup signal SIGPIPE ignoring in reading and writing (and handle errors as shown above):

signal (SIGPIPE, SIG_IGN);

回答4:

Using C++ sockets library:

class LineSocket : public TcpSocket
{
public:
  LineSocket(ISocketHandler& h) : TcpSocket(h) {
    SetLineProtocol(); // enable OnLine callback
  }
  void OnLine(const std::string& line) {
    std::cout << "Received line: " << line << std::endl;
    // send reply here
    {
      Send( "Reply\n" );
    }
  }
};

And using the above class:

int main()
{
  try
  {
    SocketHandler h;
    LineSocket sock(h);
    sock.Open( "remote.host.com", port );
    h.Add(&sock);
    while (h.GetCount())
    {
      h.Select();
    }
  }
  catch (const Exception& e)
  {
    std::cerr << e.ToString() << std::endl;
  }
}

The library takes care of all error handling.

Find the library using google or use this direct link: http://www.alhem.net/Sockets/

来源：https://stackoverflow.com/questions/1583353/how-to-read-exactly-one-line

标签

c++

sockets

file-descriptor