How to read exactly one line? - c++

Question

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

Answer 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.

Answer 2

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

Answer 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);

Answer 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/