//----- Include files ---------------------------------------------------------
#include <stdio.h>                  // Needed for printf()
#include <stdlib.h>                 // Needed for rand()
#include <string.h>

//----- Type defines ----------------------------------------------------------
typedef unsigned char      byte;    // Byte is a char
typedef unsigned short int word16;  // 16-bit word is a short int
typedef unsigned int       word32;  // 32-bit word is an int

//----- Defines ---------------------------------------------------------------
#define POLYNOMIAL 0x04c11db7L      // Standard CRC-32 ppolynomial
#define BUFFER_LEN       4096      // Length of buffer

//----- Gloabl variables ------------------------------------------------------
static word32 crc_table[256];       // Table of 8-bit remainders

//----- Prototypes ------------------------------------------------------------
void gen_crc_table(void);
word32 update_crc(word32 crc_accum, byte *data_blk_ptr, word32 data_blk_size);


//===== Main program ==========================================================
int main()
{
  byte        *buff; // Buffer of packet bytes
  word32      crc32;            // 32-bit CRC value
  word16      i;                // Loop counter (16 bit)
  word32      j;                // Loop counter (32 bit)
  char        *crc_hex_string, *crc_bin_string;

  crc_hex_string = malloc(sizeof(char)*1024);
 buff = malloc(sizeof(char)*BUFFER_LEN);

  // Initialize the CRC table
  gen_crc_table();

  // Load buffer with BUFFER_LEN random bytes
  for (i=0; i<BUFFER_LEN; i++)
    buff[i] = (byte) rand();

  // Compute and output CRC
  printf("size of orig buffer = %d \n",strlen(buff));
  crc32 = update_crc(-1, buff, BUFFER_LEN);
  sprintf(crc_hex_string,"%08X",crc32);
  printf("CRC = %08X string = %s\n", crc32,crc_hex_string);

  strcat(buff, crc_hex_string);

  printf("size of new buffer = %d, size of crc buffer =%d \n",strlen(buff),sizeof(crc_hex_string));

  crc32 = update_crc(-1, buff, BUFFER_LEN + strlen(crc_hex_string));
  printf("CRC = %08X \n", crc32);

  return 0;
}

//=============================================================================
//=  CRC32 table initialization                                               =
//=============================================================================
void gen_crc_table(void)
{
  register word16 i, j;
  register word32 crc_accum;

  for (i=0;  i<256;  i++)
  {
    crc_accum = ( (word32) i << 24 );
    for ( j = 0;  j < 8;  j++ )
    {
      if ( crc_accum & 0x80000000L )
        crc_accum = (crc_accum << 1) ^ POLYNOMIAL;
    else
        crc_accum = (crc_accum << 1);
    }
    crc_table[i] = crc_accum;
  }
}

//=============================================================================
//=  CRC32 generation                                                         =
//=============================================================================
word32 update_crc(word32 crc_accum, byte *data_blk_ptr, word32 data_blk_size)
{
   register word32 i, j;

   for (j=0; j<data_blk_size; j++)
   {
     i = ((int) (crc_accum >> 24) ^ *data_blk_ptr++) & 0xFF;
     crc_accum = (crc_accum << 8) ^ crc_table[i];
   }
   crc_accum = ~crc_accum;

   return crc_accum;
}

the checksum is then appended to the message and the result
transmitted. In this case the transmission would be: 11010110111110.

At the other end, the receiver can do one of two things:

   a. Separate the message and checksum. Calculate the checksum for
      the message (after appending W zeros) and compare the two
      checksums.

   b. Checksum the whole lot (without appending zeros) and see if it
      comes out as zero!