How do i write this hash array to a text file?

Question

I have spent the last 8 hours writing this:

#include <stdio.h>
#include "md5.h"

#define BUFFER_SIZE 1024

void PrintHash (BYTE byHash[]);

int main (int iArgC, char *apszArgV[])
{
   FILE *f;
   MD5_CTX ctx;          
   BYTE byHash[16];            
   BYTE byBuffer[BUFFER_SIZE]; 
   int iReadBytes;             

   if (iArgC < 2) {
      printf ("Usage: print_md5 <file name>\n");
      return 1;
   }

   f = fopen (apszArgV[1], "rb+");
   if (f != NULL) {

      md5_init(&ctx);   

      while (!feof(f)) {
         iReadBytes = fread(byBuffer, sizeof(BYTE), BUFFER_SIZE, f);
         md5_update(&ctx, byBuffer, iReadBytes);  
         if (iReadBytes < BUFFER_SIZE) break;
      }

      md5_final(&ctx, byHash);


      printf("File:%s Md5:", apszArgV[1]);
      PrintHash (byHash);
      printf ("\n");

      fclose (f);
   }
}

void PrintHash (BYTE byHash[])
{
   for (int i = 0; i < 15; i++) {
      printf ("%02X", byHash[i]);
   }
}

Now all i want to do, is to write the hash to the last line of the same file i have calculated the hash from.

I have tried putting this at the same line where i call the PrintHash function, does not seem to work as it generates a new hash for the same file every time i run it.

f = fopen(apszArgV[1],"a");

         for(i=0; i<16 ;i++) {
            fprintf(fp, "%02X", byHash[i]);

         }
         fclose(fp);

May someone point out what i am doing wrong?

new to c btw