Bit fiddling in C

bits.c

#include "stdio.h"
#include "stdint.h"

/**
 * Print the binary representation of an integer `n` with length `len`.
 * Any unsigned integer can be safely cast to uintmax_t.
 *
 * Shift right then mask to the last bit.
 */
void printb2(uintmax_t n, size_t len)
{
	size_t w = len - 1;
	for (size_t i = 0; i < len; i++)
		printf("%ju ", (n >> (w - i)) & 0x01);
}

/**
 * Print the binary representation of an integer `n` with length `len`.
 * Any unsigned integer can be safely cast to uintmax_t.
 *
 * Start with a bit mask where the MSB is set then shift right until the LSB.
 */
void printb3(uintmax_t n, size_t len)
{	
	/* Note the 1U is important because otherwise 1 will be treated as a
	 * signed value, resulting in all the bits to the left being set. */
	for (uintmax_t i = 1U << (len - 1); i > 0; i >>= 1)
		printf("%i ", n & i ? 1 : 0);
}

/**
 * Print the binary representation of an integer `n` with length `len`.
 * Any unsigned integer can be safely cast to uintmax_t.
 *
 * Same as printb3, using `while` instead of `for`.
 */
void printb4(uintmax_t n, size_t len)
{
	while (--len < SIZE_MAX)
		printf("%i ", n & (1 << len) ? 1 : 0);
}

/**
 * Print the binary representation of an 8-bit wide integer.
 */
void printb(uint8_t i)
{
	printf("%x ", (i >> 7) & 0x01);
	printf("%x ", (i >> 6) & 0x01);
	printf("%x ", (i >> 5) & 0x01);
	printf("%x ", (i >> 4) & 0x01);
	printf("%x ", (i >> 3) & 0x01);
	printf("%x ", (i >> 2) & 0x01);
	printf("%x ", (i >> 1) & 0x01);
	printf("%x",  (i >> 0) & 0x01);
}

/*
Swap the second and fourth bytes in an integer sequence
http://stackoverflow.com/questions/17409874
See also: http://commandcenter.blogspot.com/2012/04

int someInt = 0x12345678;

int byte2 = someInt & 0x00FF0000;
int byte4 = someInt & 0x000000FF;
int swapd = (someInt & 0xFF00FF00) | (byte2 >> 16) | (byte4 << 16);

               byte2 = 0x00340000
               byte4 = 0x00000078
someInt & 0xFF00FF00 = 0x12005600
         byte2 >> 16 = 0x00000034
         byte4 << 16 = 0x00780000

   0x12005600 
 | 0x00000034 
 | 0x00780000
 = 0x12785634
 */

enum byte_order {
	BIG_ENDIAN,
	LITTLE_ENDIAN
};

enum byte_order get_byte_order() {
	
}

int main()
{
	uint8_t  i8  = 42;
	uint16_t i16 = 42;
	uint32_t i32 = 42;

	printf("b:  "), printb(i8),       printf("\n");
	printf("b2: "), printb2(i8, 8),   printf("\n");
	printf("b3: "), printb3(i8, 8),   printf("\n");
	printf("b4: "), printb4(i8, 8),   printf("\n\n");
	printf("b2: "), printb2(i16, 16), printf("\n");
	printf("b3: "), printb3(i16, 16), printf("\n");
	printf("b4: "), printb4(i16, 16), printf("\n\n");	
	printf("b2: "), printb2(i32, 32), printf("\n");
        printf("b3: "), printb3(i32, 32), printf("\n");
        printf("b4: "), printb4(i32, 32), printf("\n");

	return 0;
}