how to store and read data from files

Question

I am new to C in linux. I am trying to store data to file and read them back. Is this the correct way. When I try to compile this i am getting errors. Can anyone help me please. thanks in advance.

 #include<stdio.h>
       typedef struct
    {
      int select;
      char lastname[25];
      char firstname[25];
      char address[25];
      char phonenumber[25];
    } addressbook;

    addressbook a[5];
     FILE *fp;
    int main()
    {
      int i;

     for( i=0; i<5 ; i++)
    {
     printf("enter details\n");
       printf("enter lastname:\n");
       scanf("%s", a[i].lastname);
       printf("enter firstname:\n");
       scanf("%s", a[i].firstname);
       printf("enter address:\n");
       scanf("%s", a[i].address);
       printf("enter phone number:\n");
       scanf("%s", a[i].phonenumber);
      fp = fopen("addressbook.dat","a+");
      fwrite(&a, sizeof(a), 1, fp);
      fclose(fp);
    }

    for(i=0; i<5; i++)
    {
      fopen("addressbook.dat", "r");
      fread(&a, sizeof(a), 1, fp );
      printf("lastname:%s\n", a[i].lastname);
      printf("firstname:%s\n", a[i].firstname);
      printf("address:%s\n", a[i].address);
      printf("phonenumber:%s\n", a[i].phonenumber);
      fclose(fp);
    }
    return 0;
    }

i am not getting any output. it was blank.

Answer 1

Check out this code, and let me explain you what all was wrong in your code.

    #include<stdio.h>

    typedef struct
    {
          int select;
          char lastname[25];
          char firstname[25];
          char address[25];
          char phonenumber[25];
    } addressbook;

    #define ARRAYLEN 2

    addressbook a[ARRAYLEN];
    FILE *fp;

    int main()
    {
         int i;

         fp = fopen("addressbook.dat","a+");

         for( i=0; i<ARRAYLEN ; i++)
         {
           printf("enter details\n");
           printf("enter lastname:\n");
           scanf("%s", a[i].lastname);
           printf("enter firstname:\n");
           scanf("%s", a[i].firstname);
           printf("enter address:\n");
           scanf("%s", a[i].address);
           printf("enter phone number:\n");
           scanf("%s", a[i].phonenumber);
           fwrite(&a[i], sizeof(a), 1, fp); /* notice, array indexed */
        }
        fclose(fp);

        fopen("addressbook.dat", "r");
        for(i=0; i<ARRAYLEN; i++)
        {
          fread(&a[i], sizeof(a), 1, fp );
          printf("lastname:%s\n", a[i].lastname);
          printf("firstname:%s\n", a[i].firstname);
          printf("address:%s\n", a[i].address);
          printf("phonenumber:%s\n", a[i].phonenumber);
        }
        fclose(fp);

        return 0;
    }

Actually, your code as-is (apart from the edit you've already done) isn't so incorrect, but it had some small yet crucial flaws.

1. The only real change is this :-

   fwrite(&a[i],...
   

   and,

   fread(&a[i],...
   

   i.e. pass the address of the particular array-element that you want to write, not the entire array. Also, even though you were passing address of the entire array, the no. of byte/characters you were asking library to write, was just sizeof(thestructure), so essentially the remaining was truncated. Without that, what you were writing into the file was something like...

   A          <-- file contents after, first iteration
   AAB        <-- file contents after, second iteration
   AABABC     <-- file contents after, third iteration
   AABABCABCD <-- file contents after, fourth iteration
   ....
   

   I think you'd figure out from that, what was wrong. Also the contents of your addressbook.dat was text, so a simple "cat addressbook.dat" (on Linux) would have told you what was wrong :-)

2. You are opening and closing file in every iteration. Now this is not an error, but just a sub-optimal thing, and quite likely to be something you do not want to do. File operations are costly, and opening/closing those cost quite a few CPU cycles. You are better off, opening file once for all writes, and once for reads. (Of course, once can remove the fclose() done after write-block and fopen() done before read-block as well, by just getting the file-pointer to the beginning of file -- left as an exercise for you).

3. While testing, no one wants to enter so much data. So I've added a #define (and with a newer compiler you can replace it with a const definition as well), that defines a macro which holds the addressbook array size. For testing, I keep it at "2". For production you can just change that value to "1000" and it will still work. Again, this wasn't an error, just a better style, if you will.

4. Oh, and BTW, pls get your indentation right. Are you coming from Python world ? Or it could be an artifact of the indentation required by SO for posting code-blocks.

HTH

Answer 2

It seems to me that you are not including the required headers. For example, printf requires stdio.h, so at the beginning for your file, you will need

#include <stdio.h>

CPlusPlus.com provides pretty good and easy to search documentation for C, so if you wish to use a function, you can look it up and find out which header is required.