checksum sender vs reciever?

Have the written this one program & unsure of how to reverse it so that it will have a sum check for the reciever & then sum check for sender and compare the 2 sum that will return zero.

Code:

---

/*        Demonstrate the calculation of a checksum using one's
        complement arithmetic.
          Written by:
          Date:
*/
#include <stdio.h>
#include <string.h>
#include <pstdint.h>

int main (void)
{
//        Local Declarations
        uint32_t sum      = 0x00000000;
        uint16_t checksum = 0x0000;

        char*    str      = "ABCDEFGHI";
        int      len;
        int i = 0;

//        Statements
        len = strlen (str);
        if (len % 2  == 1)
          // Make the number of characters even
          len++;
        for (i = 0; i < len; i += 2)
            sum = (sum + str[i] * 256 + str[i + 1]);
       
        // Add carries into lower 16 bits
        while (sum >> 16)
          sum = (sum & 0xffff) + (sum >> 16);

        // Complement
        checksum = ~sum;

        printf ("str:      %s\n",  str);
        printf ("checksum: %#06X\n", checksum);
        system("Pause");

        return 0;
}        // main

/*        Results:
str:      ABCDEFGHI
checksum: 0XA5EA

---

Please help.
thank you.

If you want a comparison that will return 0 when the two numbers are equal, just use subtraction. I don't know what you're asking with the first part: presumably the sender will calculate the checksum and send it, the receiver will calculate the checksum of what was received and will compare that with the received checksum.