Synchronizing Parent & Child process

I want to synchronize the parent and child process to alternatively print 1 to 10 into a file. And output which process printed the number. The following code prints alternatively, but the same number. Please help me out!

#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 #define MSGSIZE 16
 int parent= 1;
 int n=-1;
 main ()
 {
   int i;
   char *msg = "How are you?";
   char inbuff[MSGSIZE];
   int p=0;
   FILE *fp1,*fp2;
   pid_t ret;
 
   ret = fork ();
   if (ret > 0)
     {
       i = 0;
       while (i < 10)
     {
 
       n++;
      // sprintf(msg, "%d", n);
      // fp=fopen("abc.txt","a");
      // write (p[1], itoa(n,msg,MSGSIZE), MSGSIZE);
      // sleep (2);
     // fclose(fp);
       //read (p[0], inbuff, MSGSIZE);
 
     fp1=fopen("abc.txt","r");
     fscanf(fp1,"%d",&n);
     fclose(fp1);
     fp1=fopen("abc.txt","w");
      fprintf(fp1,"%d",n); 
     printf("Parent: %d\n", n);
       i++;
     fclose(fp1);
     sleep(2);
 
     }
     exit(1);
     }
   else
     {
       i = 0;
       while (i < 10)
     {
 
       n++;
       //sleep (1);
      // read (p[0], inbuff, MSGSIZE);
 
      fp2=fopen("abc.txt","r");
      fscanf(fp2,"%d",&n);
      fclose(fp2);
      fp2=fopen("abc.txt","w");
 
      fprintf(fp2,"%d",n);        
       printf("Child: %d\n", n);
            i++;
 
      sleep(2);
 
     }
     }
   exit (0);
 }
 

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