updating number in a file through C program

 #include <stdio.h>

int main(){

int num;

FILE * ptr;

ptr=fopen("updatefile.txt","r");

fscanf(ptr,"%d",&num);

printf("The value of num is %d",num);

fclose(ptr);

ptr=fopen("updatefile.txt","w");

fprintf(ptr,"%d",2*num);

 return 0;

}