need help understanding this function

This is a discussion on need help understanding this function within the C Programming forums, part of the General Programming Boards category; Hi all I'm trying to figure out how this function works, pointers give me a hard time still especially a ...

  1. #1
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    need help understanding this function

    Hi all I'm trying to figure out how this function works, pointers give me a hard time still especially a function with several pointers as parameters.

    I understand the function takes a string and if it has a % or + it removes those, I don't understand the nibble part of the function is where I'm confused. Long story short guess I just wonder if anyone could comment it so I can understand what's happening.

    Code:
    /* Unencode a urlencoded string*/
    /*----------------------------------------------------------------------------*/
    static void unencode(char *dst,char *src,size_t len)
    {	char c;
    	while (c = *src++,len--)
    	{	if (c == '+') c = ' ';
    		else if (c == '%')
    		{	c = *src++;
    			c = (nibble(c) << 4) | nibble(*src++);
    			len -= 2;
    		}
    		*dst++ = c;
    	}
    	*dst = '\0';
    }

  2. #2
    and the hat of wrongness Salem's Avatar
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    Well a nibble is just 4 bits of a byte, and can be represented as a single character 0-9A-F.

    The &#37; in URL encoding is followed by 2 hex digits, say %41 would be a long winded way of saying 'A'
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    Registered User manofsteel972's Avatar
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    Doing a bit of research on urlencodeing and decoding you have a string with a single char being represented by a &#37; and a 2 digit hex code for the actual char in string format so "%22" would be equal to " and "%28" would equal to (

    Since the hex digits are in string format you have to convert the ascii char '2' into the actual hex number. A nibble is just a single hex digit. A high and low nibble represent a single char.

    So it is just a string to hex conversion.


    [edit]
    That isn't very portable is it? I mean wouldn't there be endian issues?
    Last edited by manofsteel972; 08-04-2007 at 12:40 AM.
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  4. #4
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    Quote Originally Posted by manofsteel972 View Post
    Doing a bit of research on urlencodeing and decoding you have a string with a single char being represented by a % and a 2 digit hex code for the actual char in string format so "%22" would be equal to " and "%28" would equal to (

    Since the hex digits are in string format you have to convert the ascii char '2' into the actual hex number. A nibble is just a single hex digit. A high and low nibble represent a single char.

    So it is just a string to hex conversion.


    [edit]
    That isn't very portable is it? I mean wouldn't there be endian issues?
    Endian issues only arise if you have "words" longer than a byte. Since all (that I know of) encodings used for URL's are single-byte, it shouldn't be a problem - I know of no processor that stores NIBBLES "the wrong way around".

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  5. #5
    Registered User manofsteel972's Avatar
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    Ok. So a single byte char is the only data type not affected by endian issues. And wide char or multibyte unicode isn't affected either since it is still handled as bytes right?

    [edit] Answered my own question. A wide char is affected by endian issues.
    Last edited by manofsteel972; 08-04-2007 at 10:46 AM.
    "Knowledge is proud that she knows so much; Wisdom is humble that she knows no more."
    -- Cowper

    Operating Systems=Slackware Linux 9.1,Windows 98/Xp
    Compilers=gcc 3.2.3, Visual C++ 6.0, DevC++(Mingw)

    You may teach a person from now until doom's day, but that person will only know what he learns himself.

    Now I know what doesn't work.

    A problem is understood by solving it, not by pondering it.

    For a bit of humor check out xkcd web comic http://xkcd.com/235/

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