[ b a c k ]

/********************************************************************
 * cs.c (check sum) is a program that computes a 16 bit checksum over
 * a file that is specified by the user.  A 32 bit integer is used
 * to accumulate the sum and the carry is added immediately back into
 * the sum.
 *
 * Author:		William Martin - boneshakerbike@hotmail.com
 * Completed:		Monday, February 05, 2001
 * Dependencies:	None - small test program!
 * Input:		Keyboard - file name (must be in current
 *                      directory and < 30 chars.
 * Output:	        The bytes read and the checksum value in Int.
 *******************************************************************/


#include <stdio.h>
#include <stdlib.h> //for size_t, a type def.

/********************************************************************
 * checksum is a function that computes the checksum of a buffer
 * (of a file) and prints the total to the screen.
 *
 * Completed:		Monday, February 05, 2001
 * Dependencies:	none.
 * Input:		pointer to a buffer containing the file
 *			characters (must be < 50000), size of the 
 *			read(file), and pointer to file name.
 * Output:		The checksum value in int printed to screen.
 * Sample Call:		checksum(bufer, length_of_read, file_name);
 *******************************************************************/
void checksum(void *buffer, size_t length_of_read, void *file){
      unsigned char *bufer = (unsigned char *)buffer;
      int check_sum_array[2];// holds 16bit(2 x 8) sum
      unsigned int carry = 0;
      size_t i;
      for (i = 0; i < length_of_read; i = i + 2){
	if (i == length_of_read + 1){// for odd bit left at EOF
		check_sum_array[1] += (unsigned int)(*bufer++);
	}else{// normal operations
		check_sum_array[0] += (unsigned int)(*bufer++);
		check_sum_array[1] += (unsigned int)(*bufer++);
	}
        do{ //like grade school math, add from right to left.
		 if(check_sum_array[1] > 255){
	  		carry = check_sum_array[1] / 256;
	 		check_sum_array[1] = check_sum_array[1] %256;
			check_sum_array[0] += carry;// add carry back
  			carry = 0;// reset
 		 }
		 if(check_sum_array[0] > 255){
         		carry = check_sum_array[0] / 256;
		  	check_sum_array[0] = check_sum_array[0] % 256;
		 	check_sum_array[1] += carry;// add carry back
			carry = 0;// reset
	  	 }
       }while ((check_sum_array[1] > 255) &&
		(check_sum_array[0] > 255));
       }
       printf("The checksum of %s is %d%d\n",
      	     file, check_sum_array[0], check_sum_array[1]);
}

/********************************************************************
 * In the main program we get the name of the file, open it, read
 * it into a buffer, do a checksum, then close the file.
 *
 * Completed:		Monday, February 05, 2001
 * Dependencies:	checksum(buffer, size, file_name).
 * Input:		keyboard, the name of file.
 * Output:		The bytes read then checksum() is called.
 *******************************************************************/
main(){
      FILE *file_pointer;	// file handle name
      size_t length_of_read;	// size of file
      char bufer[50000];	// bufer to hold file contents
      char file_name[30];	// file name

      //get filename and open, error will be printed
      printf("Enter file name to do checksum on ==> ");
      scanf("%s", file_name);
      if (NULL == (file_pointer = fopen(file_name, "rb")))
      {
         printf("Unable to open %s for reading\n", file_name);
         return -1;
      }

      // read file and save length.
      length_of_read = fread(bufer, sizeof(char), sizeof(bufer), file_pointer);

      // report file size.
      printf("%d bytes read.\n", length_of_read);

      // call checksum function, close file, and end program!
      checksum(bufer, length_of_read, file_name);
      fclose(file_pointer);
      return 0;
}

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