Unsigned masks and packed flags

The idea

Unsigned integers are useful for sets of boolean flags. A mask identifies selected positions: OR sets them, AND with an inverted mask clears them, and XOR toggles them. Testing flags & mask asks whether any masked bit is set; comparing that result with mask asks whether every required bit is set. These are different contracts when the mask contains several bits.

Use unsigned operands for bit manipulation and ensure shift counts are smaller than the promoted operand’s width. A count outside that range is not a portable way to obtain zero. Bitwise operators compute on individual bits, while logical operators convert operands to truth values and may short-circuit. Parenthesize comparisons around masks so operator precedence cannot change the intended test.

#include <stdio.h>
int main(void) {
  unsigned flags = 0u;
  const unsigned READ = 1u, WRITE = 2u, EXECUTE = 4u;
  flags |= READ | WRITE;
  flags &= ~WRITE;
  printf("%u %d\n", flags, (flags & EXECUTE) != 0);
}

1 0

Clearing WRITE leaves READ set, and EXECUTE remains absent.