Using "Bitflags" in C

Bitflags are a method of storing multiple values, which are not mutucally exclusive, in one variable. You've probably seen them before. Each flag is a bit position which can be set on or off. You then have a bunch of bitmasks #defined for each bit position so you can easily manipulate it:

 
#define LOG_ERRORS            1  // 2^0, bit 0
#define LOG_WARNINGS          2  // 2^1, bit 1
#define LOG_NOTICES           4  // 2^2, bit 2
#define LOG_INCOMING          8  // 2^3, bit 3
#define LOG_OUTGOING         16  // 2^4, bit 4
#define LOG_LOOPBACK         32  // and so on...

// Only 6 flags/bits used, so a char is fine
unsigned char flags;

// initialising the flags
// note that assignming a value will clobber any other flags, so you
// should generally only use the = operator when initialising vars.
flags = LOG_ERRORS;
// sets to 1 i.e. bit 0

//initialising to multiple values with OR (|)
flags = LOG_ERRORS | LOG_WARNINGS | LOG_INCOMING;
// sets to 1 + 2 + 8 i.e. bits 0, 1 and 3

// setting one flag on, leaving the rest untouched
// OR bitmask with the current value
flags |= LOG_INCOMING;

// testing for a flag
// AND with the bitmask before testing with ==
if ((flags & LOG_WARNINGS) == LOG_WARNINGS)
   ...

// testing for multiple flags
// as above, OR the bitmasks
if ((flags & (LOG_INCOMING | LOG_OUTGOING))
         == (LOG_INCOMING | LOG_OUTGOING))
   ...

// removing a flag, leaving the rest untouched
// AND with the inverse (NOT) of the bitmask
flags &= ~LOG_OUTGOING;

// toggling a flag, leaving the rest untouched
flags ^= LOG_LOOPBACK;

 

WARNING: DO NOT use the equality operator (i.e. bitflags == bitmask) for testing if a flag is set - that expression will only be true if that flag is set and al others are unset. To test for a single flag

 
if (flags == LOG_WARNINGS) //DON'T DO THIS
   ...