How do I convert a 24-bit integer into a 3-byte array?

Question

Hey Im totally out of my depth and my brain is starting to hurt.. :(

I need to covert an integer so that it will fit in a 3 byte array.(is that a 24bit int?) and then back again to send/receive this number from a byte stream through a socket

I have:

NSMutableData* data = [NSMutableData data];

 int msg = 125;

 const void *bytes[3];

 bytes[0] = msg;
 bytes[1] = msg >> 8;
 bytes[2] = msg >> 16;

 [data appendBytes:bytes length:3];

 NSLog(@"rtn: %d", [[[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding] intValue]);

 //log brings back 0

I guess my main problem is that I do not know how to check that I have indeed converted my int correctly which is the converting back that I need to do as well for sending the data.

Any help greatly appreciated!

Answer 1

You could use a union:

union convert {
    int i;
    unsigned char c[3];
};

to convert from int to bytes:

union convert cvt;
cvt.i = ...
// now you can use cvt.c[0], cvt.c[1] & cvt.c[2]

to convert from bytes to int:

union convert cvt;
cvt.i = 0; // to clear the high byte
cvt.c[0] = ...
cvt.c[1] = ...
cvt.c[2] = ...
// now you can use cvt.i

Note: using unions in this manner relies on processor byte-order. The example I gave will work on a little-endian system (like x86).