using bits as flags

[Scott Kelley]

Wanted to create a 'flag byte' to use for a number of things. The following
seemed a reasonable approach:

#define connected = 0x01
unsigned char FlagByte;

why does this work:
FlagByte &= ~0x01; // set connected flag = 0

& this doesn't
FlagByte &= ~connected; // set connected flag = 0

Thanks,
Scott Kelley

 

[Martin Dickopp]

Wanted to create a 'flag byte' to use for a number of things. The following
seemed a reasonable approach:

#define connected = 0x01

This means that every occurrence of `connected' is replaced by `= 0x01'.

unsigned char FlagByte;

why does this work:
FlagByte &= ~0x01; // set connected flag = 0

& this doesn't
FlagByte &= ~connected; // set connected flag = 0

Think about what this becomes when `connected' is replaced (given what I
have written above), and you'll surely see what the mistake is.

Martin

 

[Malcolm]

#define connected = 0x01

you mean either

#define connected 0x01

or

int connected = 0x01;

 

[Martin Ambuhl]

Wanted to create a 'flag byte' to use for a number of things. The following
seemed a reasonable approach:

#define connected = 0x01

^^^

lose the bogus '='

 

[Peter Nilsson]

Wanted to create a 'flag byte' to use for a number of things. The following
seemed a reasonable approach:

#define connected = 0x01

#define connected 0x01u

Note: bit manipulations with signed (and hence possibly negative) quantities
are potentially problematic in ISO C.

 

[Scott Kelley]

Thanks all

I was so focused on another area that I didn't see the obvious.

SK