Break an int into 2 char's

gualala · Apr 30, 2008

Hi,
I'm writing some codes in C18. I have an unsigned int that I wish to break into two chars. I saw something on union struct for a byte, but I don't know how to change it into int. Could anyone help? Thanks.

I wish I can have a variable, say "Value",
Value = 0x8FFF;
Value.HighByte // gives 0x8F
Value.LowByte // gives 0xFF
or any other way that works?

The code I saw for char, so that we can use something like Flags = 0; to set all to zeros or Flags.Timeout = 1; to set a specific value.

Code:

union
{
  struct
  {
    unsigned Timeout:1;
    unsigned None:7;
  } Bit;
  unsigned char Byte;
} Flags;

eng1 · Apr 30, 2008

gualala said:
Hi,
I'm writing some codes in C18. I have an unsigned int that I wish to break into two chars. I saw something on union struct for a byte, but I don't know how to change it into int. Could anyone help? Thanks.

I wish I can have a variable, say "Value",
Value = 0x8FFF;
Value.HighByte // gives 0x8F
Value.LowByte // gives 0xFF
or any other way that works?

I would do a right-shift and use casts.

Code:

unsigned char low_byte = Value;    // cast
unsigned int tmp = Value>>8;       // right-shift
unsigned char high_byte = tmp;     // cast

kchriste · Apr 30, 2008

You can do it eng1's way with shifts or this way with unions:

Code:

union {
	unsigned int Integer;
	struct{
		unsigned char LowByte;
		unsigned char HighByte;
		};
	} Value;

	Value.Integer = 0x1234;
	// Value.HighByte == 0x12;
	// Value.LowByte == 0x34;

The advantage of the union is you can operate directly on the Bytes of the Integer.

Torben · Apr 30, 2008

Or you could do it with a little pointer arithmetic if you like:

Code:

#include <stdio.h>

int main(void)
{
    unsigned int value = 0x8FFF;
    unsigned char *low_byte = (unsigned char *) &value,
        *high_byte = low_byte + 1;

    printf("0x%04x => Low: 0x%02x; High: 0x%02x\n", value, *low_byte, *high_byte);

    return 0;
}

This also allows you access to the individual bytes, but just remember this: https://xkcd.com/371/

Torben

Pommie · Apr 30, 2008

Torben said:
Or you could do it with a little pointer arithmetic if you like:

Torben

Are we sure that C18 stores ints in little endian format.

Mike.

Torben · Apr 30, 2008

Pommie said:
Are we sure that C18 stores ints in little endian format.

Mike.

Nope! Just going with what the OP posted, figuring s/he'd just swap the labels if not. Checking the C18 User's Guide confirms it though (page 12).

Torben

ikalogic · May 2, 2008

Torben said:
This also allows you access to the individual bytes, but just remember this: https://xkcd.com/371/
Torben

That's orriginal! loooooooool

Break an int into 2 char's

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