CRC Generator Program

The following C functions create and use a CRC lookup table. The output of this program is text for a statically initialized array that you may include in your program. The test cases verify that the lookup function works.

\#include \<stdio.h\>

static unsigned table[256];
void PrintTable(void);
void BuildXMODEM(void);
unsigned XMODEMcrc(unsigned char byt, unsigned crc);
unsigned crc_buf(unsigned char *buf, unsigned length, unsigned crc);

void main(void)
{
  unsigned crc;

  printf("/* XMODEM crc table */\n");
  BuildXMODEM();
  /* Do this only once.*/
  /* No need to do this at all if the array is statically initialized. */
  PrintTable();

  crc = 0;/* It is important to initialize the CRC! */
  crc = XMODEMcrc('M', crc);
  if (crc != 0x9969)
    printf("Test case 1 failed. crc = %04X.\n", crc);
  crc = 0;
  crc = XMODEMcrc('T', crc);
  if (crc != 0x1A71)
    printf("Test case 2 failed. crc = %04X.\n", crc);
  crc = 0;
  crc = XMODEMcrc('T', crc);
  crc = XMODEMcrc('H', crc);
  crc = XMODEMcrc('E', crc);
  if (crc != 0x1E0A)
    printf("Test case 3 failed. crc = %04X.\n", crc);
  crc = 0;
  crc = crc_buf("1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ", 36, crc);
  crc = crc_buf("abcdefghijklmnopqrstuvwxyz!@#$%^&*()", 36, crc);
  if (crc != 0xA18A)
    printf("Test case 4 failed. crc = %04X.\\n", crc);
}

void PrintTable(void)
/* Print the table for including in a C program. */
{
int i;

  printf("unsigned table[256] = \n{\n");
  for (i=0; i < 256; I++)
  {
    if ((i % 8) == 0)
      putchar('\t');
    printf("0x%04X, ", table[i]);
    if (((i+1) % 8) == 0)
      putchar('\n');
  }
  printf("};\n\n");
}

void BuildXMODEM(void)
/* Initialize the table at run-time */
{
  int i;
  unsigned char z;

  for (i=0; i < 256; I++)
  {
    z = i ^ (i >> 4);
    table[i] = z ^ ((unsigned)z << 5) ^ ((unsigned)z << 12);
  }
}

unsigned XMODEMcrc(unsigned char byt, unsigned crc)
/* Update the CRC with a new byte */
{
  crc = (crc << 8) ^ table[byt ^ (crc >> 8)];
  return crc;
}

unsigned crc_buf(unsigned char *buf, unsigned length, unsigned crc)
/* Calculate the crc of a buffer full of characters */
{
  while (length--)
  {
    crc = XMODEMcrc(*buf, crc);
    buf++;
  }
  return crc;
}