CODEWITHSUNDEEP
csocketstcp
Poliquin
Poliquin

Reputation: 2987

Using recv to receive incoming char array

Am back with C sockets question.

I am trying to receive a char array from a client via TCP. I have the following recv code:

char buffer[2048];
int recvBytes = 0;

while (1) {

    if ((clntSockfd = accept(servSockfd, (struct sockaddr *) &clntAddr, &clntSockAddrLen)) == -1) {
        printf("Unable to accept client connection\n");
    exit(1);
}
    else { 
        printf("Client connection accepted!\n");
    }

    /* clear buffer */
    memset(buffer, 0, sizeof(buffer));

    recvBytes = recv (clntSockfd, buffer, sizeof(buffer), 0);
    printf("Bytes received: %i\n", recvBytes);
    printf("Char array received: %s\n", buffer);
}

I noted that upon execution, recvBytes shows zero in the first iteration. It's only after the second iteration that it picks up the incoming char array.

I would like to find out will recv block until I receive data from the incoming socket? How should I discard the unwanted chars in the buffer - i.e. drop the chars received from the first iteration.

Many thanks!

-- edit ---

Hi all, I have appended the console output from eclipse.

**... <- initialization stuff
Binding socket to port
Socket binded
Start listening on port 8081.
Client connection accepted!                  *while loop - first iteration
Bytes received: 0                            *while loop - first iteration
Char array received:                         *while loop - first iteration
Client connection accepted!                  *while loop - second iteration
Bytes received: 22                           *while loop - second iteration
Char array received: 1~Message~HowAreYou?    *while loop - second iteration**

Hope the above console output illustrates my questions clearer, regarding the first and second iterations. From the above, the first iteration is not receiving any bytes but recv does not block. I thought recv was supposed to block until it receives bytes?

Upvotes: 2

Views: 5127

Answers (3)

Jay
Jay

Reputation: 24915

If your socket is a blocking socket, then recv function will block till you get some data. recv function will return 0 only when the other side closes the connection on most systems.

I suggest you recheck your code and execution and let us know the exact output.

Refer here and here for more details.

Upvotes: 1

Ben Voigt
Ben Voigt

Reputation: 283961

I believe the recv only returns 0 if the remote end closes the connection (for stream sockets such as TCP) or if a packet with no payload is received (datagram sockets such as UDP).

If the socket has been set to non-blocking mode (default is blocking), and there's no data yet, you'd get an error code, either EAGAIN or EWOULDBLOCK depending on the OS.

In any case, each time recv returns, the new data will be stored at the beginning of the buffer. It won't append to previously received data unless you calculate a pointer designed to store new data after the old.

Upvotes: 1

ckv
ckv

Reputation: 10838

If you know what to expect of the data that you receive you can drop the first set of characters. For example if you know what will the length of the packet be or the source of the message or the header information then you can validate and ignore the first set of characters.

Upvotes: 0

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