Thread: Trouble with sending binary data

Hi all. Thanks ahead for reading this, heres the background
I've been trying to make a simple peer-to-peer and I'm stuck at the very last part of this. Now don't get me wrong, I'm in 3rd year univ, and we've been working on this for 3 days, about 8 hrs each day (Just the part I'm about to explain now) and the teacher assitant can't help either.

::: (coding in linux)

Code:

void processFile(char *file,int peerfd){
	char buf[1];
	int fd;

	printf("Filename is %s\n",file);
	
	if ((fd=open(file,O_RDONLY)) ==-1){
		fprintf(stdout,"Usage Error: File does not exist\n");
		exit(1);
	}
	else{
		while(read(fd,buf,sizeof(buf))!=0){
			write(peerfd,buf,sizeof(buf));
			printf("r");
		}
	close(fd);	
	}

Where peerfd is the file descripter that is made , connected to the other peer. The problem is, once we run it and send a file (file exists etc) , hit hangs, so we hit cntrl-c to terminate it, and only then it displays on the peer window (file received). So we fix this by typing close(peerfd), but doing this makes pictures send fine, but txt files only get sent 1byte (notice the size of buff)

Edit:
I talked to the TA, it seems that the peer is expecting some symbol, or some flag that says "OK the file is finished sending". See I'm testing this against a client that I know works, but we're not aloud to look at it . So when I close the file descriptor myself, by saying close (fdpeer), it gives me
read from peer: Connection reset by peer
Which means the peer is still expecting data (even though I finished sending) and its what I would call a brutal disconnection.

Thanks!!!

Originally Posted by itsme86

Since sizeof(buf) is 1, the file will always be an exact mutliple of sizeof(buf).

That's only true until someone changes it. And I guarantee that the person who changes it will do so late at night, while drunk, three hours before they fly to another continent for 3 weeks with no way of contacting them :-)

Thanks for the fast replies!!


So the problem is not the extra garbage, because of the buffer size that I specified.

I can do the error checking, >0 and whatnot, and if -1 an error occurs. But say an error DID occur. I somehow have to tell the peer that I'm finished sending. Ok, fair enough. Lets say an error does occur i force the file descriptor to close, nothing I can do about it.

But I am sending it one byte at a time, so therefore, I am writing one byte at a time. And it's not sending that very last byte that is supposed to let the other side terminate the connection.

Thanks again!

That's only true until someone changes it. And I guarantee that the person who changes it will do so late at night, while drunk, three hours before they fly to another continent for 3 weeks with no way of contacting them :-)

Assume no one will be touching my code after I get it to run... thanks!

Originally Posted by Mcmaxx

But I am sending it one byte at a time, so therefore, I am writing one byte at a time. And it's not sending that very last byte that is supposed to let the other side terminate the connection.

Well, how could it send it? You haven't written that part! Surely the assigment explains what this termination symbol is, right? So send it when the transmission is finished, either normally OR because of an error.

Originally Posted by dwks

Never make assumptions.

I appreciate the concern but for something that has been causing us over 24 hours of problems, I will deal with that after.

I edited the code

Code:

if ((fd=open(file,O_RDONLY)) ==-1){
		fprintf(stdout,"Usage Error: File does not exist\n");
		exit(1);
	}
	else{
		while((i=read(fd,buf,sizeof(buf)))!=0){
			write(peerfd,buf,sizeof(buf));
			printf("r");
			if (i<0){
				fprintf(stderr,"Error occured reading file");
				close(peerfd);
			}	

		}
	close(fd);	
	}

For the simple error checking.

If any suggestions arise, please tell me.

One thing I found out is I'm reading until =0, and then I stop reading, but what if because of that , I miss out the final byte that lets the FD know its terminated?
Thanks

Originally Posted by brewbuck

Well, how could it send it? You haven't written that part! Surely the assigment explains what this termination symbol is, right? So send it when the transmission is finished, either normally OR because of an error.

Thanks for the reply

Code:

write(peerfd,buf,sizeof(buf));

I did write the code, it's writing back to the file descriptor of the other client. The client is reading the data , successfully(since it sends all of it except that very last byte)

I have an idea, but I think its cheating::


Since I'm testing this with the profs client, maybe the prof has some statement in his I/O loop that says "if this byte == /n, stop reading, terminate connection".. and maybe THIS is why I can't figure out what the termination byte is.


This again is cheating, and he didn't provide any help on this matter. We've emailed him and posted it on his forums, but he seems to be the type to not give 2nd thoughts to the problem.

Originally Posted by dwks

Code:

while((i=read(fd,buf,sizeof(buf)))!=0){

I implemented it in the revised code

Code:

if (i<0){
fprintf(stderr,"Error occured reading file");
close(peerfd);
break;
}

I just added the break; now, but the FD connected to the peer is terminated, the FD from reading the file is terminated outside the while loop, and it breaks the while loop (break

Thanks for the reply!! I'm so relieved at least I'm getting some progress

Hey guys


I'm thinking forget this complicated guessing and checking, I'll leave that for my real job

I'm just going to read and send until I have no more data, and then receive and write until I get no more data.

Screw the prof and his bs