Question

My goal is create an app client server, written in C++.
When the server read an input from the client, should process the string and give an output.
Basically, I have a simply echo server that send the same message.
But if the user types a special string (like “quit”), the program have to do something else.
My problem is that this one dont happend, because the comparison between strings is not working… I dunno why!
Here a simple code:

  while(1) {
            int num = recv(client,buffer,BUFSIZE,0);
            if (num < 1) break;
            send(client, ">> ", 3, 0);
            send(client, buffer, num, 0);

            char hello[6] ="hello";
            if(strcmp(hello,buffer)==0) {
                send(client, "hello dude! ", 12, 0);
            }

            buffer[num] = '\0';
            if (buffer[num-1] == '\n')
                buffer[num-1] = '\0';
            std::cout << buffer;
            strcpy(buffer, "");
        }

Why the comparison is not working?
I have tried many solutions…but all failed 🙁

Answer 1

Your data in buf may not be NULL-terminated, because buf contains random data if not initialized. You only know the content of the first num bytes. Therefore you also have to check how much data you’ve received before comparing the strings:

const char hello[6] ="hello";
size_t hello_sz = sizeof hello - 1;
if(num == hello_sz && memcmp(hello, buffer, hello_sz) == 0) { ...

As a side note, this protocol will be fragile unless you delimit your messages, so in the event of fragmented reads (receive “hel” on first read, “lo” on the second) you can tell where one message starts and another one ends.