File Transfer Using UDP/IP

I'll start by saying this is an assignment for a class - so I'm not looking for a complete solution - just some guidance on a few questions. I am a C novice at best.

My assignment is to write a program using UDP/IP to send a file from one machine to another within our network. I've got the mechanics of sending a char message from one machine to the next so I'm working on figuring out how to read, parse, add the header and CRC information prior to sending.

What I have so far....

File: send.c

Code:

---

#include <sys/types.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>

#define MSG "This is a test message!"
#define BUFMAX 100

//Command Arguments:
// [0] - send
// [1] - destination machine
// [2] - destination port number
// [3] - file to be sent

int main(int argc, char* argv[]){
    int sk;
    char buf[BUFMAX];

    FILE *fp;
    fp = fopen(argv[3],"r");

    fread(buf, BUFMAX, 1, fp);

    // My question is in this area...how do I get binary values?

    int k;
    for(k=0;k<BUFMAX;k++){
      printf("The buffer contains %d\n",(char *)buf[k]);
    }

    struct sockaddr_in remote;
    struct hostent *hp;

    sk = socket(AF_INET, SOCK_DGRAM, 0);

    remote.sin_family = AF_INET;


    hp = gethostbyname(argv[1]);

    if(hp==NULL){
        printf("Can't find hostname. %s\n", argv[1]);
        exit(1);
    }

    bcopy(hp->h_addr, &remote.sin_addr.s_addr, hp->h_length);

    remote.sin_port = ntohs(atoi(argv[2]));

    dlsendto(sk, MSG, strlen(MSG)+1, 0, &remote,
    sizeof(remote),0);
    read(sk,buf,BUFMAX);
    printf("%s\n", buf);
    close(sk);
}

---

What I'm trying to figure out is this....I need to read in the file, parse it into a 100 bit message, add a header and CRC-16 codes to create a frame, then send it through the socket to be validated, assembled and saved on the other side.

I can read the file, but when I output the values I'm getting characters instead of binary. Is there another way to display the data so I can validate what's being passed? I've spent a great deal of time looking through articles on Binary files, text to binary, and can't seem to find an example that explains what I'm doing wrong.

Any guidance would be great.

If you stick to sending just text files, it's very easy to see whether you received what you sent.

There's no difference when it comes to sending binary data, so long as you're using fread/fwrite to read/write the file.

> I need to read in the file, parse it into a 100 bit message
Do you mean 100 bits?
It's like 12.5 bytes.
It's not necessarily a problem, but it does need a bit of extra care when dealing with the buffers.

Ok - so perhaps I'm not explaining this properly.

The text file contains a message. For the sake of making this easy - lets assume myFile.txt contains the message "Hello World".

I need take that file and pass it from one server to another using UDP/IP. So the characters become:

H = 0100 1000
e = 0110 0101
etc.

I need to take 100 bits at a time, if the message contains more than 100 bits, then I need break it into 100 bit sections. I need to do a CRC calculation on it and write an 8-bit header (format provided by the professor. So my final message will be:

Header (8bits) + Message (100bits) + CRC(8 bits)

This frame is sent to the receiving machine, the CRC is calculated to ensure no errors and re-assembled into the txt file. There's more - i.e. ACK response, resending, etc - but I'm not that far yet.

Hoping this helps make sense of what I'm saying.

Are you sure it's 100 bits and not 100 bytes? The latter would make more sense because 1) you're putting more data into a packet, and 2) you don't have to worry about dealing with partial bytes.

So - I'm re-reading the assignment. It says in one of the last paragraphs that "the maximum frame size is 128 bytes long". So perhaps your correct.

I am slightly confused however. How does this change anything? I was assuming that the letters would convert somehow to bytes.

I'm afraid I don't understand how to go from Text to Bytes any more than I know to go from Text to Bits.

Thanks for the thoughts.

So then, the message I need to send is 1 byte header + 100 byte chunks of the message + CRC code (I think). How can I take a text file and parse it into 100 byte chunks?

Code:

---

char buff[128];
while ( (n=fread(&buff[28],1,100,fp) > 0 ) {
  // do something with n bytes stored in buff
  // add 28 bytes of whatever to the start in buff[0] to buff[27]
}

---

There is no difference between characters and bytes - not at this level of code anyway.

Ok - thanks Salem & Clairvoyant1332.

I spent so much time focused on searching for bit conversion it never occurred to me that each character was a byte.