I m trying to convert C code to C++. Facing some difficulties help me out!

//This is struct

typedef struct index
{
  int id;
  char word[20];
  int  count;
}indexs;

// Function prototype

void fileScanner(FILE * ifp,int i);
void fileRead(FILE *ofp);
int  findString(char s[],char u[]);
int searchEntry(indexs [],char key[],int n);
void StoreIndexInFile(FILE *);  // <---is this correct?
void fileDisplay(FILE *ifp,FILE *ofp);

// Function to be called

void StoreIndexInFile(FILE *ifp,indexs ind[],int n)
{
   indexs *i;
   fprintf(ifp,"\nid word(root)   frequency\n\n");
   for(i=ind;i<ind+n;i++)
   {
    fprintf(ifp,"%d %-20s %d \n",i->id,i->word,i->count);
   }
}

How can I call this function(with parameters)?

I think you'll need to clarify things a bit before anyone can give you a helpful answer. Is your question about how you would call this function in general, or specifically about the difference between calling it in C and calling it in C++? I ask because, in most cases, there wouldn't be a significant difference - though the function itself would probably be defined differently, using an ofstream rather than a FILE, and you could redefine the index structure (or more likely, class) using string objects, but that's a different question altogether.

If you need help understanding how functions are called in general, this post should help explain the relationship between functions, function prototypes, and function calls.