Writing the random-access file

// Writing data randomly to a random-access file
#include <stdio.h>
#include <stdlib.h>

// clientData structure definition
struct clientData {
  unsigned int acctNum; // account number
  char lastName[15];    // account last name
  char firstName[10];   // account first name
  double balance;       // account balance
};                      // end structure clientData

int main(int argc, char *argv[]) {
  FILE *cfPtr; // credit.dat file pointer

  // create clientData with default information
  struct clientData client = {0, "", "", 0.0};

  // fopen opens the file; exits if file cannot be opened
  if ((cfPtr = fopen("credit.dat", "rb+")) == NULL) {
    printf("%s: File could not be opened.\n", argv[0]);
    exit(-1);
  } // end if
  // require user to specify account number
  printf("%s", "Enter account number"
               "(1 to 100, 0 to end input)\n?");
  scanf("%d", &client.acctNum);

  // user enters information, which is copied into file
  while (client.acctNum != 0) {
    // user enters last name, first name and balance
    printf("%s", "Enter lastname, firstname, balance\n? ");

    // set record lastName, firstName and balance value
    fscanf(stdin, "%14s%9s%lf", client.lastName, client.firstName,
           &client.balance);

    // seek position in file to user-specified record
    fseek(cfPtr, (client.acctNum - 1) * sizeof(struct clientData), SEEK_SET);

    // write user-specified information in file
    fwrite(&client, sizeof(struct clientData), 1, cfPtr);

    // enable user to input another account number
    printf("%s", "Enter account number\n? ");
    scanf("%d", &client.acctNum);
  } // end while

  fclose(cfPtr); // fclose closes the file
} // end main