Server/Client UDP

FaresH · 11-28-2008, 09:57 AM

Hi,

We have a exercise to hand over in the few days to come. I tried to do it, I done most of the job but I am kind of stuck.

ftclient <host> <port> <filename> <save as>
ftserver <port> <size of each package> <delay between sent package>

The client sends a filename to the server.
If the server has the file, it sends 4 bytes to the client, which are size of the file (Using the stat() function on the server side). Else it sends one byte of 0xff, and the client closes the socket and unlink the file.
If the file exist, the client gets ready to receive the packages.
If in 10 seconds no pack arrives (I used select() ), the client aborts.
If a package arrives, it has 1 byte of success, 4 bytes of offset, and the rest is the content of the message.

What I am not able to understand is how the client receives the message in a buffer full of bytes and translate it. I know I must use the ntohl (network to host long) function. Both sides use the same architectures.

These are my question:

What is the size I should use for the buffer ?
How to copy the content of the message into another char * [] ?
How to translate from the buffer full of bytes into something I can use ?
How do I use htonl and ntohl ? I need to convert network bytes and write them to a file, how ?

Any code example would more then appreciated...

This is what I wrote in the core so far:

Loop in main

Code:

// Get ready to receive the content of the file
while(received < fileSize ){
	// Wait 10 seconds for the next package from the server
	if( select(sd+1, &readfd, NULL, NULL, &timeout) != 0 ){
		if( read(sd, &buffer, sizeof(buffer)) < 0 ){
			perror("Error in reading the package\n")
			exit(1);
		}
		// If first byte is -1 abort
		if( atoi(ntohl(buffer[0]) != -1 ){
			perror("Error sent from server\n");
			exit(1);
		}
		offset = getOffset(buffer);
		lseek(fd, offset, SEEK_SET);
		copyMsg(buffer, &temp);
		write(fd, temp, sizeof(temp), 0);
	 }else{
	        perror("Timeout\n");
	        exit(1);
        }
}

getOffset function

Code:

int getOffset( char * buffer[] ){
	static int place=1, i=1, size=0 ;
	if( i == 4 ){
		return(place);
	}
	if( (size = atoi(ntohl(buffer[i]))) > 0 ){
		place *=  size ;
	}
	i++;
	return(getOffset(buffer));
}

Mara · 11-28-2008, 03:46 PM

Your ntohl() call is incorrect. You pass one byte to it (char), when there should be uint32_t. In fact, you should pass the complete 4 byte value, not byte by byte (ntohl changes the order of bytes, not bits).

Example:

Code:

uint32_t orig = 0;
uint32_t result = 0;
unsigned char byte0 = (unsigned char) buffer[0];
unsigned char byte1 = (unsigned char) buffer[1];
unsigned char byte2 = (unsigned char) buffer[2];
unsigned char byte3 = (unsigned char) buffer[3];
orig = (byte0 << 24) | (byte1 << 16) | (byte2 << 8) | byte3;
result = ntohl(orig);

This snipped is rather long to show how it works. You may want to print the intermediate results to see the details.

The other side should perform a similar code.

Edit: I'm assuming the data is binary, not as a string!

FaresH · 11-30-2008, 05:21 AM

I got the client and server to communicate.

An integer is 4 bytes, a char is 1 byte.
I need to know how to convert the integer ( which represents the file size ) into char array of 4 cells...

chrism01 · 12-02-2008, 05:52 PM

http://irc.essex.ac.uk/www.iota-six....ntf_sscanf.asp