Writing hex nums from console input to string

This is a discussion on Writing hex nums from console input to string within the C Programming forums, part of the General Programming Boards category; Hey there Well, basically I have a server-client connection and want to send a packet trough console input stream. I ...

  1. #1
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    Writing hex nums from console input to string

    Hey there

    Well, basically I have a server-client connection and want to send a packet trough console input stream. I want to type in hex what to send, so if I wanted to send a hello world to the server I'd write:
    Code:
    48656C6C6F20576F726C64 <- this into the console
    H e l l o   W o r l d  <- instead of that
    But how can I fill a memory location with 0x48 when I type "48", instead of 0x34 (4) and 0x38 (8)?

    And also, a bit offtopic, it's long time ago I programmed with sockets (in windows), when I use winsocks send(socket, buf, length, flag), will zero bytes within <length> bytes also be sent or will the packet truncated if it finds a zero termination?

    TIA, Hawk


    Edit: Main problem is, that I am unable to format many following hex-numbers with scanf, I want to send variable packet sizes.
    Last edited by Hawkin; 10-16-2007 at 01:41 PM.

  2. #2
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Hawkin View Post
    But how can I fill a memory location with 0x48 when I type "48", instead of 0x34 (4) and 0x38 (8)?
    You want to break the string into little groups of two digits each, and convert from hex representation. Try this:

    Code:
    unsigned int val;
    int scanned;
    do
    {
        scanned = sscanf(string, "%2x", &val);
        if(scanned == 1)
        {
            /* Go to the next pair of digits */
            string += 2;
            /* Successfully read a value -- do something with it */
        }
    } while(scanned == 1);
    And also, a bit offtopic, it's long time ago I programmed with sockets (in windows), when I use winsocks send(socket, buf, length, flag), will zero bytes within <length> bytes also be sent or will the packet truncated if it finds a zero termination?
    The zero byte is ONLY special for C strings and C string processing functions. You don't have to worry about it with send().

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    Thank you, that was exactly what I needed

  4. #4
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Hawkin View Post
    Thank you, that was exactly what I needed
    Beware that the code I posted is NOT robust against errors. If you have whitespace in your string, it will break. If the string is not an even number of characters, it will break. If there is an invalid hexadecimal digit in the string, it will stop processing when it hits it.

    But hopefully it gives you the idea.

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    Yes I know (and experienced) that you can easily cause errors. But I only needed it and will need it for testing and debugging purposes. The original client has hardcoded packets and will change the content with normal string input and does check for several possible errors.

  6. #6
    Frequently Quite Prolix dwks's Avatar
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    One way to make it a little more robust would be to use something like this:
    Code:
    int number, next = 0;
    const char *buffer = "48656C6C6F20576F726C64";
    
    while(sscanf(buffer, "&#37;2x%p", &number, &next) == 2) {
        /* process number */
        buffer += next;
    }
    That should handle whitespace properly.
    dwk

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