Swap alternate bytes in a integer - c

Question

Problem: swap alternate bytes as below:
Input: uint8_t buf[4] = {0xab,0xcd,0xef,0xba};
Output: 0xcdababef
I have the below code for doing that but I am wondering if there is any better way to shorten the code.
#include <stdint.h>
#define SWAP_16(buf) (((buf & 0xFF00) >> 8) | ((buf & 0x00FF) << 8))
int main()
{
unsigned int value;
int i, j=0;
uint8_t buf[4] = {0,4,0,0};
unsigned int mask = 0xFFFF;
unsigned int tmp_value;
unsigned int size = 4;
for (i = size - 1 ;i >= 0; i--) {
tmp_value |= (buf[j] << 8*i);
j++;
}
value = SWAP_16((tmp_value & (mask << 16)) >> 16) << 16 |
SWAP_16(tmp_value & mask);
return 0;
}

Answer 1

Assuming unsigned int is 32-bits, you can simply use:
value = ((value & 0xff00ff00) >> 8) | ((value & 0x00ff00ff) << 8);
to swap the bytes in each pair of bytes in value. It's similar to your SWAP_16() macro except that it does both halves of the value at once.

Answer 2

unsigned int forward = 0x12345678;
unsigned int reverse;
unsigned char *f = &forward;
unsigned char *r = &reverse;
r[0]=f[3];
r[1]=f[2];
r[2]=f[1];
r[3]=f[0];
now reverse will be 0x78563412

Answer 3

Here is one way:
#include <stdio.h>
#include <stdint.h>
int main(void)
{
uint8_t buf[4] = {0xab,0xcd,0xef,0xba};
unsigned int out = buf[1] * 0x1000000u + buf[0] * 0x10000u + buf[3] * 0x100u + buf[2];
printf("%x\n", out);
}

Answer 4

It's not immediately clear from your question if it's not an option, but you could merely just swap the bytes in the array if you know the size won't change:
#include <stdio.h>
#include <stdint.h>
#define SWAPPED(b) { b[1], b[0], b[3], b[2] }
#define PRINT(b) printf("0x0%x\n", *((uint32_t*)b));
int main()
{
uint8_t buf[4] = {8,4,6,1};
uint8_t swapped[4] = SWAPPED(buf);
PRINT(buf);
PRINT(swapped);
return 0;
}
The output for this on my machine is:
0x01060408
0x06010804
This is because of endian-ness and printing an array casted to an integer type, but the bytes are swapped as you ask in your question.
Hope that helps.

Answer 5

Use a union
#include <stdint.h>
#define SWAP_VAR(T, v1, v2) do { \
T v = (v1); \
(v1) = (v2); \
(v2) = v; \
} while (0);
union U32
{
uint32_t u;
unsigned char a[4];
};
uint32_t swap32(uint32_t u)
{
union U32 u32 = {u};
SWAP_VAR(unsigned char, u32.a[0], u32.a[1]);
SWAP_VAR(unsigned char, u32.a[2], u32.a[3]);
return u32.u;
}
Use it like this:
#include <stdint.h>
uint32_t swap32(uint32_t u);
int main(void)
{
uint32_t u = 0x12345678;
u = swap32(u);
}

Answer 6

unsigned int n = ((unsigned int)buf[0] << 16) |
((unsigned int)buf[1] << 24) |
((unsigned int)buf[2] << 0) |
((unsigned int)buf[3] << 8);