Sockets

Brandon9000 · 04-11-2013, 09:50 AM

I have written a socket client program in C++. At some point, I have to send data, and this is how I do it:

lenSent = write(serverSocket,sendBuf, len);

I can use send() instead of write(). It makes no difference to this question. I have set it to be non-blocking, because the thread must not get stuck. Unfortunately, it is connecting to a server which exits if I do a partial write to the socket.

How can I prevent a non-blocking socket from doing a partial write? Is there some way to poll it and make sure that it can handle the number of bytes I wish to send?

Thanks.

Brandon

manu-tm · 04-11-2013, 11:20 AM

For that, you can use 'select'. You will find useful info in manpages:

Code:

man send

Also:
http://beej.us/guide/bgnet/output/ht...age/index.html
http://beej.us/guide/bgnet/output/ht...ed.html#select

Brandon9000 · 04-11-2013, 11:57 AM

The man page for send says:

"The select(2) call may be used to determine when it is possible to send more data."

This isn't really enough for me to know how to do it. I have never used select before.

manu-tm · 04-11-2013, 01:57 PM

Have you read this: http://beej.us/guide/bgnet/output/ht...d.html#sendall ?

Here is an example:

Code:

/*
 * send_full() - Socket must be opened in non-blocking mode
 */

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/select.h>

#define SEND_RECV_TIMEOUT_SEC	1
#define SEND_RECV_TIMEOUT_USEC	0

enum {
	SELECT_ERROR,
	SELECT_TIMED_OUT,
	SELECT_TRUE,
	SELECT_FALSE
};

int writable_data_is_available_on_socket(int sck)
{
	fd_set		write_set;
	struct timeval	timeout;
	int		i;

	FD_ZERO(&write_set);
	FD_SET(sck, &write_set);
	timeout.tv_sec = SEND_RECV_TIMEOUT_SEC;
	timeout.tv_usec = SEND_RECV_TIMEOUT_USEC;
	if ((i = select(sck + 1, NULL, &write_set, NULL, &timeout)) == -1) {
		fprintf(stderr, "select() error: %s\n", strerror(errno));
		fflush(stderr);
		return SELECT_ERROR;
	} else if (i == 0) {
		return SELECT_TIMED_OUT;
	} else {
		if (FD_ISSET(sck, &write_set))
			return SELECT_TRUE;
		else
			return SELECT_FALSE;
	}
}

/*
 * Return n bytes sent or -1 if error (connection closed by server or ?)
 */
int send_full(int sck, const char *str)
{
	int	len = strlen(str), i, j = 0;

	while (writable_data_is_available_on_socket(sck) == SELECT_TRUE) {
		if ((i = send(sck, str + j, len, 0)) != -1) {
			if (i > 0) {
				j += i;
				len -= i;
				if (len == 0)
					break;
			}
		} else {
			j = -1;
			if (errno == EPIPE) {
				fprintf(stderr, "Connection closed by server\n");
				fflush(stderr);
			} else {
				fprintf(stderr, "send() error: %s\n", strerror(errno));
				fflush(stderr);
			}
			break;
		}
	}
	return j;
}

Brandon9000 · 04-11-2013, 02:01 PM

Thanks! Does select do the write itself?

manu-tm · 04-11-2013, 02:17 PM

Quote:

Originally Posted by Brandon9000

Thanks! Does select do the write itself?

No, you should use send_full() instead of write() inside your code.

Brandon9000 · 04-11-2013, 02:36 PM

Thanks again. I am also experimenting with checking to see how many unsent bytes are in the socket's internal buffer and waiting until it reaches zero with:

int error, value;
for (.....value > 0.....)
error = ioctl(serverSocket, SIOCOUTQ, &value);

Brandon