How do I print bits in C?
(For the byte-by-byte version, see
“How do I print bytes in C?”
.)
What do the individual bits of values in C look like? We can see the bits by printing them out with this fun program:
#include <stdio.h>
#include <stdint.h>
#include <limits.h>
#include <arpa/inet.h>
#include <errno.h>
void print_byte_as_bits(char val) {
for (int i = 7; 0 <= i; i--) {
printf("%c", (val & (1 << i)) ? '1' : '0');
void print_bits(char * ty, char * val, unsigned char * bytes, size_t num_bytes) {
printf("(%*s) %*s = [ ", 15, ty, 16, val);
for (size_t i = 0; i < num_bytes; i++) {
print_byte_as_bits(bytes[i]);
printf(" ");
printf("]\n");
#define SHOW(T,V) do { T x = V; print_bits(#T, #V, (unsigned char*) &x, sizeof(x)); } while(0)
int main() {
SHOW(int, 0);
SHOW(int, 1);
SHOW(int, 17);
SHOW(int, -17);
SHOW(int, 256);
SHOW(int, INT_MAX);
SHOW(int, ~INT_MAX);
SHOW(unsigned int, 17);
SHOW(unsigned int, -17);
SHOW(unsigned int, UINT_MAX);
SHOW(unsigned int, UINT_MAX+1);
SHOW(unsigned char, 255);
SHOW(long, 17);
SHOW(short, 17);
SHOW(uint32_t, 17);
SHOW(uint32_t, htonl(17));
SHOW(uint16_t, 17*256+18);
SHOW(uint16_t, htons(17*256+18));
SHOW(void*, &errno);
SHOW(unsigned int, 1 << 1);
SHOW(unsigned int, 1 << 2);
SHOW(unsigned int, 1 << 4);
SHOW(unsigned int, 1 << 8);
SHOW(unsigned int, 1 << 16);
return 0;
Here are the results:
$ ./a.out
( int) 0 = [ 00000000 00000000 00000000 00000000 ]
( int) 1 = [ 00000001 00000000 00000000 00000000 ]
( int) 17 = [ 00010001 00000000 00000000 00000000 ]
( int) -17 = [ 11101111 11111111 11111111 11111111 ]
( int) 256 = [ 00000000 00000001 00000000 00000000 ]
( int) INT_MAX = [ 11111111 11111111 11111111 01111111 ]
( int) ~INT_MAX = [ 00000000 00000000 00000000 10000000 ]
( unsigned int) 17 = [ 00010001 00000000 00000000 00000000 ]
( unsigned int) -17 = [ 11101111 11111111 11111111 11111111 ]
( unsigned int) UINT_MAX = [ 11111111 11111111 11111111 11111111 ]
( unsigned int) UINT_MAX+1 = [ 00000000 00000000 00000000 00000000 ]
( unsigned char) 255 = [ 11111111 ]
( long) 17 = [ 00010001 00000000 00000000 00000000 00000000 00000000 00000000 00000000 ]
( short) 17 = [ 00010001 00000000 ]
( uint32_t) 17 = [ 00010001 00000000 00000000 00000000 ]
( uint32_t) htonl(17) = [ 00000000 00000000 00000000 00010001 ]
( uint16_t) 17*256+18 = [ 00010010 00010001 ]
( uint16_t) htons(17*256+18) = [ 00010001 00010010 ]
( void*) &errno = [ 00001000 11000100 11100110 11100110 11111111 01111111 00000000 00000000 ]
( unsigned int) 1 << 1 = [ 00000010 00000000 00000000 00000000 ]
( unsigned int) 1 << 2 = [ 00000100 00000000 00000000 00000000 ]
( unsigned int) 1 << 4 = [ 00010000 00000000 00000000 00000000 ]
( unsigned int) 1 << 8 = [ 00000000 00000001 00000000 00000000 ]
( unsigned int) 1 << 16 = [ 00000000 00000000 00000001 00000000 ]
Observations:
My machine puts most-significant bytes in lower memory addresses
Zero is always represented with every byte set to 0
The maximum value for unsigned types is represented with every byte set to 255
Signed types use a twos complement representation
