Question

I been working in a project for my Data Networks class and they ask me to concatenate a header like this:

struct ip
{
    unsigned long a;
    unsigned long b;
    unsigned int l;
} IP;

And a message which it’s a char* let’s say “Hello”.

So, I use this method to concatenate those two in a single char*:

memcpy(sendBuf, (void*)&sendHeader, sizeof(sendHeader));
memcpy(&sendBuf[sizeof(sendHeader)], readMessage, lengthMessage);

With lengthMessage being the number of characters of the message +1 which is the null termination character.

So, sendBuf it’s defined like this:

char sendBuf[BUFLEN + 1]   // BUF_LEN = 128

And then I put this char* in a queue defined like this:

concurrency::concurrent_queue<char*> IP_in_queue;

So, I want to check if the information it’s correct, so I just check everything:

char* s;
IP_in_queue.try_pop(s);
numbytes = sizeof(s);
// Copy from buf to the header
memcpy( (void*)&readHeader, s, sizeof( IP_PACKET_HEADER_T));
// Copy message part
memcpy( sendedString, &s[sizeof(IP_PACKET_HEADER_T)], numbytes - sizeof(IP_PACKET_HEADER_T));
// Append \0 to the end
sendedString[numbytes - sizeof(IP_PACKET_HEADER_T)] = '\0';

So, before I queue my char*, we know that the size of sendBuf is 129, but when I check the number of bytes after I dequeue it’s way too different, the value of number of bytes it’s 4, but even in that way I get the information right, so I don’t understand that, maybe I missing some important things, but the variable numbytes shouldn’t be at least more?

I hope I made myself clear and maybe someone can explain me this a little bit better.

Thanks

Answer 1

sizeof does not tell you the length of the string – it tells you the size of the type char*, which on a 32-bit machine is 4 bytes. You want strlen, the number of bytes in the string.

But even that isn’t what you want. Your structure IP can have nulls in it, so even strlen won’t give you a correct answer. If you put your string first instead of last you can fix that, but I’d consider that kind of a hack. You should add the size explicitly to the start of your message.