Using recv

**jcafaro10** · 04-07-2009

I'm learning some socket programming in C and I've noticed something interesting with recv. If on my client code I send a message that's really large, say 2048 bytes, the first time I send it, it only receives 1024 bytes like I would expect. Then, the second time I send the 2048 byte request, it receives 1024 bytes but strlen(input) is output as being 1026 bytes. Where do these extra two bytes come from?

Code:

while(1)
{
		int acceptsock = accept(sock, NULL, NULL);
		char* input = (char*) malloc(1024*sizeof (char));
		printf("receive call got %d\n",recv(acceptsock, input, 1024, 0));
		printf("Server recieved: %s\n",input);
		printf("Size of received message: %d\n",strlen(input));
		//do stuff with input
		free(input);
               //more code
}

**quzah** · 04-07-2009

If I had to guess, newline and carriage return.

Quzah.

**jcafaro10** · 04-07-2009

Why? The same thing is being done twice, shouldn't the results be the same? Where does a newline and carriage return come from?

Thanks

**quzah** · 04-07-2009

How are you getting your input? Do you hit enter? That's what I was guessing. If it's not a string, why are you using strlen on it?

Quzah.

**jcafaro10** · 04-07-2009

I just send a really long string

Code:

   char message[2048];
   memset(message,'\0',2049);
   int i = strlen(message);
   while(strlen(message)<2048)
   {
   	message[i]='!';
 	i++;
 	printf("message has length %d\n",strlen(message));
   }
   printf("message to send is %d\n",strlen(message));

**quzah** · 04-07-2009

Why are you trying to memset more room than your array holds? My guess is you have a bunch of crazy stuff like that scattered around your code.

Quzah.

**jcafaro10** · 04-07-2009

If I suppose I did that to make sure the length is 2048

**quzah** · 04-07-2009

You can't write past the end of your array and expect your program to work right.

Quzah.

**jcafaro10** · 04-07-2009

I guess you're right. So if I wanted a 2048 long string, I'd need a 2049 sized buffer?

**matsp** · 04-08-2009

Yes. Anything going beyond 2048 in your current code will be a buffer overflow - I thought you were studying that so that you could avoid it? Obviously didn't work.

Second, why not do:

Code:

char message[2049];
message[2048] = 0;
memset(message, '!', 2048);
// Optionally printf strlen(message).

Also, unless you also put a zero at the end of the receiving buffer and have enough space to receive ALL of the message, strlen() on the receiving side will continue until it finds a zero. Consider this:

Code:

char message[20000];
memset(message, '$', sizeof(message)-1);
message[sizeof(message)-1] = 0;
recv(acceptsock, message, 1024, 0);
printf("Message len = %d\n", strlen(message));

If you now recieve 1024 '!', then the message will contain 18975 '$', then a zero - so strlen is 20000. In your case, if you transmit 1024 or more bytes, strlen() will go outside the buffer.

--
Mats

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