Question

I’m building some code to read a RIFF wav file and I’ve bumped into something odd.

The first 4 bytes of the file header are the word RIFF in big-endian ascii coding:

   0x5249 0x4646

I read this first element using:

char *fileID = new char[4];
filestream.read(fileID,4);

When I write this to screen the results are as expected:

std::cout << fileID << std::endl;
>>  RIFF

Now, the next 4 bytes give the size of the file, but crucially they’re little-endian.

So, I write a little function to flip the bytes, based on a union:

int flip4bytes(char* input){

   union flip {int flip_int; char flip_char[4];};

   flip.flip_char[0] = input[3];
   flip.flip_char[1] = input[2];
   flip.flip_char[2] = input[1];
   flip.flip_char[3] = input[0];

   return flip.flip_int;

 }

This looks good to me, except when I call it, the value returned is totally wrong. Interestingly, the following code (where the bytes are not reversed!) works correctly:

int flip4bytes(char* input){

   union flip {int flip_int; char flip_char[4];};

   flip.flip_char[0] = input[0];
   flip.flip_char[1] = input[1];
   flip.flip_char[2] = input[2];
   flip.flip_char[3] = input[3];

   return flip.flip_int;

 }

This has thoroughly confused me. Is the union somehow reversing the bytes for me?! If not, how are the bytes being converted to int correctly without being reversed?

I think there’s some facet of endian-ness here that I’m ignorant to..

Answer 1

You are simply on a little-endian machine, and the “RIFF” string is just a string and thus neither little- nor big-endian, but just a sequence of chars. You don’t need to reverse the bytes on a little-endian machine, but you need to when operating on a big-endian.