$\newcommand{\N}{\mathbb N} \newcommand{\Z}{\mathbb Z} \newcommand{\Q}{\mathbb Q} \newcommand{\R}{\mathbb R} \newcommand{\lt}{<} \newcommand{\gt}{>} \newcommand{\amp}{&} \definecolor{fillinmathshade}{gray}{0.9} \newcommand{\fillinmath}[1]{\mathchoice{\colorbox{fillinmathshade}{$\displaystyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\textstyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\scriptstyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\scriptscriptstyle\phantom{\,#1\,}$}}} $

Section 32.3 Combining Bitwise Operators and Enums

Bitwise operators and enums pair especially well together when each enum constant represents a single bit— a technique often used to represent a set of independent on/off options (called flags) packed into a single integer. For example:

Compile and run the code below in your own C environment.

#include <stdio.h>

int main(void) {
	enum filePermissions { 
		READ = 1,   //0001
		WRITE = 2,  //0010
		EXECUTE = 4 //0l00
	};
	int myPermission = READ | WRITE;

	if (myPermission & EXECUTE) {
		printf("You can run this file.\n");
	}
	else{
		printf("You cannot run this file.\n");
	}


	return 0;
}

Since READ, WRITE, and EXECUTE each occupy a different bit position ($0001\text{,}$ $0010\text{,}$ $0100$), we can combine several permissions into one integer using bitwise OR (|), and later test whether a specific permission is set using bitwise AND (&)— all without needing a separate variable for each flag.

Prev Top Next