Question

I am working on network programming using epoll and I have this code…

int read = read(socket, buf, bufsize);

I have a huge buffer size and I assumed it will receive everything clients sent.
However, I started facing problems like packet segmentation.

One example is that if a client sent 500 bytes but it somehow got into two 250 bytes packets then there is no way to handle this situation.

I looked up online and found this code

int handle_read(client *cli, struct epoll_event *ev) {
    size_t len = 4096;
    char *p;
    ssize_t received;

    cli->state = 1;
    if (cli->buffer != NULL) {
        //free(cli->buffer);
        //printf("Buff not null %s\n", cli->buffer);
    }

    //allocate space for data
    cli->buffer = (char*)malloc( (size_t)(sizeof(char) * 4096) );
    p = cli->buffer;

    do { //read until loop conditions
        received = recv(ev->data.fd, p, len, 0);
        if (received < 0 && errno != EAGAIN && errno != EWOULDBLOCK) {
            //if error, remove from epoll and close socket
            printf("Handle error!!!\nClient disconnected!\n");
            epoll_ctl(epollfd, EPOLL_CTL_DEL, ev->data.fd, ev);
            close(ev->data.fd);
        }
        p = &cli->buffer[received];
    } while (received >= len && errno != EAGAIN && errno != EWOULDBLOCK);

    return received;
}

Do you guys think it handles all the exceptions might happen while receiving? Also could you please provide me tutorials or examples that handles socket exceptions? Sample codes online don’t cover details.. Thanks in advance

Answer 1

recv can return any of three things, and your code needs to handle each one correctly:

1) Positive number. This means it read some bytes.

2) Negative number. This means an “error” occurred.

3) Zero. This means the other end of the connection performed a successful shutdown() (or close()) on the socket. (In general, a return of 0 from read() or recv() means EOF.)

The “error” case further breaks down into “EAGAIN or EWOULDBLOCK” and “everything else”. The first two just means it is a non-blocking socket and there was no data to give you at this time. You probably want to go back and call poll() (or select() or epoll()) again to avoid busy waiting…

“Everything else” means a real error. You need to handle those too; see the POSIX spec for recv() for a complete list.

Given all this, I would say your sample code is bad for several reasons. It does not handle 0 (closed connection) properly. It does not handle any errors. It does a busy-loop when the recv() returns EAGAIN/EWOULDBLOCK.

Oh, and it uses sizeof(char), which is a sure sign it was written by somebody who is not familiar with the C or C++ programming languages.