The C program below will create a file , open the same file, and write in it.

/**
C program to create a file and write to same file
C program to open a file by Codebind.com
*/
#include <stdio.h>
int main()
{
  int num;
  FILE *fptr;
  fptr = fopen("program.txt","w");

  if(fptr == NULL)
    {
      printf("Error!");
      return 1;
    }

  printf("Enter a number to save it in a file: ");
  scanf("%d",&num);

  fprintf(fptr,"%d",num);
  fclose(fptr);

  return 0;
}

/*
OUTPUT
Enter a number to save it in a file: 5555555555555555555555555555555555555555555

*/