how to dynamically create an array of structure and store the data in file.

Hi everyone.
I am writing a program for address book. There are insert, display and delete options. In insertion, it takes input data and stores them to a file. whenever I add new contact it adds them to the file.
After saving the data to file, Can i dynamically allocate an array of struct addressbook to store each contact details. So that if I want to display or delete a particular contact it will be easy other than opening a file, comparing each element in the file.
Depending upon the number of contacts saved into the file, we have to dynamically allocate array for struct addressbook and store the details.

Code:

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#define FIRST_NAME_LENGTH  15
#define LAST_NAME_LENGTH  15
#define NUMBER_LENGTH      15
#define ADDRESS_LENGTH    15
/* Structure defining a name */
struct Name
{
  char lastname[LAST_NAME_LENGTH];
  char firstname[FIRST_NAME_LENGTH];
};

/* Structure defining a phone record */
struct addressbook
{
  char answer;
  struct Name name;
  char address[ADDRESS_LENGTH];
  char phonenumber[NUMBER_LENGTH];
 
};
struct addressbook a;
void add_record()
{
  printf("enter details\n");
  printf("enter lastname of person :\n");
  scanf("%s", a.name.lastname);
  printf("enter firstname of person :\n");
  scanf("%s", a.name.firstname);
  printf("enter address of person :\n");
  scanf("%s", a.address);
  printf("enter phone number of person :\n");
  scanf("%s", a.phonenumber);

  if((fp = fopen(filename,"a+")) == NULL){
    printf("Error opening %s for writing. Program terminated.\n", filename);
    abort();
  }
  fwrite(&a, sizeof(a), 1, fp);                  /* Write to the file */

/* depending on the number of times we insert data to file, can we dynamically declare an array for stuct addressbook to store the file data. */

  fclose(fp);                                    /* close file */
  printf("New record added\n");
}

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My original program is very long, so I posted the code for structure for addressbook and insert function in my program.

See Lesson 15: Singly Linked Lists and File Operations. I would suggest using a binary file and setting up a Random Access File system

I edited my code. My original program is very long, so I posted only the main part of my program where I am having doubt. Thank you.

Yep! except for the very first time the address book is created. After that every time the program is run, it reads the file and stores it into an in-core linked list of nodes and it'd boost performance as memory is faster than disk.

What seems to be the problem?

First of all, linked lists are a massively silly way of building inventory programs (like an address book). You want a random access file system so you can go straight to the record you want, change it and replace it in place, without rewriting the entire file.

Study the following code... look at how it accesses the disk file it creates... In particular notice that it only uses one struct for everything... you should be able to work with the same concepts in your own code...

Code:

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//random access file demo
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <ctype.h>
#include <string.h>

#define FNAME "random.dat"

// test data struct
struct t_Record
  { int randnum;
    char word[16]; }
  Record;



///////////////////////////////////////////////////////
// Random Access File Handlers
//

// open or create the file
FILE *FileOpen(char* Filename)
  { FILE* pFile;
    pFile = fopen(Filename,"rb+");
    if (!pFile)
      pFile = fopen(Filename,"wb+");
    return pFile; }


// Write a record to the file
int WriteRecord(FILE *File, int RecNum)
  { if( fseek(File, RecNum * sizeof(Record), SEEK_SET) == 0 )
      if ( fwrite(&Record,sizeof(Record),1,File) )
        return 1;
    return 0; }


// read a record from the file
int ReadRecord(FILE *File, int RecNum)
  { if( fseek(File, RecNum * sizeof(Record), SEEK_SET) == 0 )
      if ( fread(&Record,sizeof(Record),1,File) )
        return 1;
    return 0; }



////////////////////////////////////////////////////////
// this is for demonstration purposes only
// you would not do this in a real program
void InitFile(FILE* File)
 { int x, y;
  memset(&Record,sizeof(Record),0);
  for (x = 0; x < 1000; x++)
      { Record.randnum = rand();
        for (y = 0; y < ((Record.randnum % 15) + 1); y++)
          Record.word[y] = (rand() % 26) + 'a';
        Record.word[y] = 0;
        if (! WriteRecord(File,x))
          printf("Oh drat!");  } }
 

 
//////////////////////////////////////////////////////////
// program mains
//
int main (void)
  { FILE *File;
    int Quit = 0;
    int Rec = 0; // record number

    srand(time(NULL));

    File = FileOpen(FNAME);
    if (!File)
      { printf("Curses foiled again!\n\n");
        exit(-1); }

    // write out 1000 test records 
    printf("Create a new file? (y/n)  ");
    if ( toupper( getchar() ) == 'Y')
      InitFile(File);
 
    // lets peek
    do
      { printf("Enter a record number (0 - 999) to load : ");
        if (! scanf("%d",&Rec) )
          Quit = 1;  // enter any letter to quit
        else
          { // read and display the record
          if (! ReadRecord(File,Rec) )
              printf("Could not read record %d\n",Rec);
            else
              { printf("-----\n");
                printf("Record Number : %d\n",Rec);
                printf("Number Value  : %d\n",Record.randnum);
                printf("Record Name  : %s\n",Record.word);
                printf("-----\n");
                getchar();
                printf("Do you want to rename this record (y/n)? ");
                if ( toupper( getchar() ) == 'Y')
                  { printf("Enter a new name : ");
                    if ( scanf("%s",Record.word) )
                      WriteRecord(File,Rec);  } } }
      } while (! Quit);


    fclose(File);
    return 0; }

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