48 byte binary to 6 byte binary

Question

I read a 17 byte hex string from command line "13:22:45:33:99:cd" and want to convert it into binary value of length 6 bytes.

For this, I read a 17 byte hex string from command line "13:22:45:33:99:cd" and converted it into binary digits as 0001 0011 0010 0010 0100 0101 0011 0011 1001 1001 1100 1101. However, since I am using a char array, it is of length 48 bytes. I want the resulting binary to be of length 6 bytes i.e, I want 0001 0011 0010 0010 0100 0101 0011 0011 1001 1001 1100 1101 to be of 6 bytes (48 bits) and not 48 bytes. Please suggest how to accomplish this.

auselen · Answer 1 · 2012-10-12T09:09:49.207

3

#include <stdio.h>

const char *input = "13:22:45:33:99:cd";
int output[6];
unsigned char result[6];
if (6 == sscanf(input, "%02x:%02x:%02x:%02x:%02x:%02x",
                &output[0], &output[1], &output[2],
                &output[3], &output[4], &output[5])) {
    for (int i = 0; i < 6; i++)
        result[i] = output[i];
} else {
    whine("something's wrong with the input!");
}

edited Oct 12 '12 at 09:09

answered Oct 12 '12 at 08:29

auselen

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-1 Actually. This isn't right. First, `%x` is for `ints`, not `chars`. Second, `char` may be a signed type and therefore a value such as 0xFF will be outside of the range of `char`, causing signed overflow, which, in turn is undefined behavior. – Alexey Frunze Oct 12 '12 at 08:36
@AlexeyFrunze noticed that. fixed. – auselen Oct 12 '12 at 08:40
@MichaelKrelin-hacker why 6? why not 17? – auselen Oct 12 '12 at 08:44
@auselen, `scanf`-family functions return the number of items assigned, no? – Michael Krelin - hacker Oct 12 '12 at 09:02
@MichaelKrelin-hacker feel free to edit the post, I didn't get what you mean. – auselen Oct 12 '12 at 09:04
why is 'output' integer array? Pls help me understand. – user1727270 Oct 12 '12 at 09:06
btw I'm sure this is not what OP is asking in the first place, see http://stackoverflow.com/questions/12772685/how-to-convert-mac-string-to-a-byte-address-in-c – auselen Oct 12 '12 at 09:13
@user1727270 because scanf with %x expects an integer pointer. if you pass byte/char it will overwrite something else. – auselen Oct 12 '12 at 11:25

score 3 · Answer 2 · answered Oct 12 '12 at 08:29

3

Split the string on the ':' character, and each substring is now a string you can pass to e.g. strtoul and then put in a unsigned char array of size 6.

answered Oct 12 '12 at 08:29

Some programmer dude

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score 1 · Answer 3 · answered Oct 12 '12 at 08:28

1

You pack them together. Like high nibble with low nibble: (hi<<4)|lo (assuming both are 4-bits).

Although you may prefer to convert them byte by byte, not digit by digit in the first place.

answered Oct 12 '12 at 08:28

Michael Krelin - hacker

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48 byte binary to 6 byte binary

3 Answers3