How to read/send packets correctly?

Question

I am trying to send packets via UDP, and I am not allowed to use SOCK_RAW (school project). The packet I am sending has a header struct ip and a string data part. I put them into one char array (the packet itself is configured correctly).

Here is how I send:

sendPacket(packet);

where packet is a char[] and

where sendPacket is defined as:

void IPNode::sendPacket(char* packet){
        //define socket, destSockAddr
        int success = sendto(socket, packet, sizeof(packet), 0,
                (struct sockaddr *) &destSockAddr, sizeof(destSockAddr));
    }
}

The packet seems to be correct. However, this is how I read it in.

while (true) {
    struct sockaddr_in remoteAddr;
    socklen_t remoteAddrLen = sizeof(remoteAddr);
    char buffer[BUF_SIZE];
    int recvlen = recvfrom(myListenSocket, buffer, BUF_SIZE, 0, 0, 0);
    onReceive(buffer);
// other stuff

}

where onReceive is:

void onReceive(char* packet) {
    ip* ptr = (ip*)packet; //the struct ip is the beginning of the packet
    struct ip ipCpy = *ptr;
    struct in_addr inAddrCpy = ipCpy.ip_src;
    char* ipString = inet_ntoa(inAddrCpy);
    cout << ipString << endl;
    return;
}

However, the ipString that is printed is different from the ipString that was in the packet before being sent. Am I accessing the received packets wrongly?

You'll need to pass both the buffer and the length to onReceive(). — user207421
– user207421, Commented Mar 4, 2015 at 22:53

ScottMcP-MVP · Accepted Answer · 2015-03-04 22:12:00Z

1

You are sending sizeof(packet) bytes. But sizeof(packet) is 4, because packet is a pointer. You need a better way to keep track of the actual size you want to send.

answered Mar 4, 2015 at 22:12

ScottMcP-MVP

10.5k2 gold badges17 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

dhke Over a year ago

A pointer is not necessarily 4 bytes, but yes sizeof(char *) is the size of the pointer not the length of the data contained.

user3517454 Over a year ago

Hm I changed that to be the actual size, and now it's not receiving 0.0.0.0 but it's still the wrong address.

Collectives™ on Stack Overflow

How to read/send packets correctly?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related