LinuxQuestions.org - Sockets - Prevent Partial Writes

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Sockets - Prevent Partial Writes

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

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

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.

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;

}

Thanks! Does select do the write itself?

Quote:

Originally Posted by Brandon9000 (Post 4929907)

Thanks! Does select do the write itself?

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

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