Multiple recv() until timeout? - c

Question

So I'm trying to do the following:
I have two participants (let's call them A and B) communicating via TCP socket (send() and recv()). A is sending a counter and a random Nonce, B is just responding with that same message it gets. A then checks if the response matches the sent packet and if yes, it increments the counter and repeats.
This is a code snippet illustrating what A does at the moment:
send(sock, payload, strlen(payload), 0);
struct timeval t_out;
t_out.tv_sec = 0;
t_out.tv_usec = 5000;
setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO,&t_out,sizeof(t_out)) <0)
int len = recv(sock, rx_buffer, sizeof(rx_buffer) - 1, 0);
if (len < 0)
{
print("Timeout reached, recv failed: errno %d", errno);
}
else
{
rx_buffer[len] = 0;
if(strncmp(rx_buffer, payload, payload_len) == 0)
{
pack_nr++;
}
}
Now I'm encountering one problem.
Let's say B, for some reason, has a delay in responding. This causes something like that:
A sends something like "1xyz"
B has a delay ......
A times out and resends something like "1abc"
B's first response ("1xyz") reaches A, A decides that this is the wrong payload
B's second response ("1abc") reaches A too, but A is only executing one recv() and it's unseen for now
A resends something like "1uvw"
A reads "1abc" from recv() and again decides that this is the wrong payload
B's third response ("1uvw") reaches A, and so on and on
So what I'd like to do is to put a recv() in a loop, so that in step 5, A would first look for another response from B until the timeout is reached.
So is there clever way to do this? I was thinking about something like
send(sock, payload, strlen(payload), 0);
int flag = 0;
gettimeofday(&start_time, NULL);
while((tx_time < start_time + timeout) && flag = 0)
{
gettimeofday(&tx_time, NULL);
recv(sock, rx_buffer, sizeof(rx_buffer) - 1, 0);
if(rx_buffer is okay)
{
flag = 1;
}
wait_a_bit();
}
if(flag == 1) pack_nr++;

Answer 1

"... B is just responding with that same message it gets. A then checks if the response matches the sent packet ..."
You have a code problem and a terminology problem.
First, the terminology problem: Don't say "matches the sent packet". The data can be sent in one packet or ten packets, TCP doesn't care. You don't receive packets, you receive data that may be split or combined across packets as TCP wishes. It really helps (trust me) to be very precise in your use of words. If you mean a message, say "message". If you mean data, say "data". If you mean a datagram, say "datagram".
Unfortunately, your code problem is enormous. You want B to respond to A with the same message it received. That means you need a protocol that sends and receives messages. TCP is not a message protocol. So you need to implement a message protocol and write code that actually sends and receives messages.
If A write "foobar", B might receive "foobar" or it might first receive "foo" and then later "bar". If A writes "foo" then "bar", B might receive "foobar" or "f" and then "oobar". That's TCP. If you need a message protocol, you need to implement one.

Answer 2

First off, you are not checking for a timeout correctly. recv() could fail for any number of reasons. You need to check errno (or WSAGetLastError() on Windows) to find out WHY it failed. But even if it did actually fail due to timeout, TCP is a byte stream, the delayed data may still show up (especially since 5000 microseconds (0.005 seconds) is way too short a timeout to reliably use for TCP network traffic), but your sender would have moved on. The only sensible thing to do if a timeout occurs in TCP is to close the connection, since you don't know the state of the stream anymore.
In your situation, you are basically implementing an ECHO protocol. Whatever the sender sends just gets echoed back as-is. As such, if you send 4 bytes (which you are not verifying, BTW), then you should keep reading until 4 bytes are received, THEN compare them. If any failure occurs in that process, immediately close the connection.
int sendAll(int sock, void *data, int len)
{
char *ptr = (char*) data;
while (len > 0) {
int sent = send(sock, ptr, len, 0);
if (sent < 0) {
if (errno != EINTR)
return -1;
}
else {
ptr += sent;
len -= sent;
}
}
return 0;
}
int recvAll(int sock, void *data, int len)
{
char *ptr = (char*) data;
while (len > 0) {
int recvd = recv(sock, ptr, len, 0);
if (recvd < 0) {
if (errno != EINTR)
return -1;
}
else if (recvd == 0) {
return 0;
}
else {
ptr += recvd;
len -= recvd;
}
}
return 1;
}
...
int payload_len = strlen(payload);
if (sendAll(sock, payload, payload_len) < 0)
{
// error handling
close(sock);
}
else
{
struct timeval t_out;
t_out.tv_sec = 5;
t_out.tv_usec = 0;
if (setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, &t_out, sizeof(t_out)) < 0)
{
// error handling
close(sock);
}
else
{
int res = recvAll(sock, rx_buffer, payload_len);
if (res < 0)
{
if (errno == EAGAIN || errno == EWOULDBLOCK)
print("Timeout reached");
else
print("recv failed: errno %d", errno);
close(sock);
}
else if (res == 0)
{
print("disconnected");
close(sock);
}
else
{
if (memcmp(rx_buffer, payload, payload_len) == 0)
{
print("data matches");
pack_nr++;
}
else
print("data mismatch!");
}
}
}