convert big endian to mid little endian

Question

I have the following big endian in C:

int32_t num = 0x01234567;

I'd like to convert it to mid little endian like this : 0x45670123

How do I use the bitwise operator to do that in C.

Answer 1

A very simple approach would be:

• Read a byte from num with the AND operator.
• Shift the read byte to the position you want in the output number.
• OR the shifted byte with your output number.
• Repeat until done.

Example:

uint32_t num = 0x01234567;
uint32_t output = 0;

uint32_t firstByte = num & 0xff000000; // firstByte is now 0x01000000
// Where do we want to have 0x01 in the output number?
// 0x45670123
//       ^^ here
// Where is 0x01 currently?
// 0x01000000
//   ^^ here
// So to go from 0x01000000 to 0x00000100 we need to right shift the byte by 16 (4 positions * 4 bits)
uint32_t adjByte = firstByte >> 16; // adjByte is now 0x0100
// OR with output
output |= adjByte;

AND, Shift & OR operator on wikipedia.