Using recv to receive incoming char array

Question

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?

recv does block until there are characters so there are no 'unwanted' chars in the buffer - whatever the value returned (recvBytes) is the number of chars in the buffer — KevinDTimm
@KevinDTimm: Unless setsockopt is used to put the socket into non-blocking mode. — Ben Voigt
If the OP would post more information, we could give better answers (IOW, the quality of the answer is equal to the quality of the question) — KevinDTimm
Hi KevinDTimm, Ben Voight, thanks for the heads up on recv. Also, I'm not running on non-blocking. — Poliquin
Your code and output do not match. Where is "Client connection accepted!" coming from? Post actual code with actual results. Also recv() returns 0 if the connection is closed, otherwise it blocks until at least 1 character is available in blocking mode. — Mark Tolonen

Ben Voigt Ben Voigt · Accepted Answer · 2010-12-30T04:34:42

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.

Using recv to receive incoming char array

3 Answers