如何在C ++中的Big Endian中移位位

Question

Here is code for little endian bit shift, i want to convert it in big endian bit shift. please help me out. actually this is LZW decompression code using little endian shift. but i want big endian code

unsigned int input_code(FILE *input)
{
unsigned int val;
static int bitcount=0;
static unsigned long inbitbuf=0L;

  while (bitcount <= 24)
  {
      inbitbuf |=(unsigned long) getc(input) << (24-bitcount);
      bitcount += 8;
  }

  val=inbitbuf >> (32-BITS);
  inbitbuf <<= BITS;
  bitcount -= BITS;

  return(val);
}


void output_code(FILE *output,unsigned int code)
{
static int output_bit_count=0;
static unsigned long output_bit_buffer=0L;

output_bit_buffer |= (unsigned long) code << (32-BITS-output_bit_count);
output_bit_count += BITS;
while (output_bit_count >= 8)
{
    putc(output_bit_buffer >> 24,output);
    output_bit_buffer <<= 8;
    output_bit_count -= 8;
}
}

Answer 1

You probably want something like.

unsigned char raw[4];
unsigned int val;
if (4 != fread(raw, 1, 4, input)) {
  // error condition, return early or throw or something
}
val = static_cast<unsigned int>(data[3])
    | static_cast<unsigned int>(data[2]) << 8 
    | static_cast<unsigned int>(data[1]) << 16 
    | static_cast<unsigned int>(data[0]) << 24; 

if you were doing little endian, reverse the indexes and everything stays the same.

A good rant on endianness and the code that people seem to write, if you want more.

Answer 2

Its a good idea to mask (perform a bitwise OR against) the bytes one at a time before shifting them . Obviously if you are shifting a 16 bit integer the unmasked bits will just be pushed off either end into oblivion. But for integers larger that 16 bits (I actually had to use 24 bit integers once) it's best to mask each byte before shifting and recombining (perform a bitwise OR on) them.