CODEWITHSUNDEEP
cdata-structures
Mezzoforte
Mezzoforte

Reputation: 69

Reading and writing structure to binary file in C, then print structure elements

I want to write structures into a Binary file in program A, and then read from it in program B, which prints the structures out. However, I am getting incorrect output.

I've tried fwrite in program A, and fread in program B, then subsequently printed the output using printf, only to get a string of nonsense numbers.

Program A:

  #include <stdio.h>
  #include <stdlib.h>

  struct record
  {
    int index;
    char name [100];
    int age;
  };

  int main ()
  {
     struct record *one=malloc(sizeof(struct record));
     FILE *in=fopen("records.bin", "ab+")l

     fgets(one->name, 100, stdin);
     one->index=100;
     one->age=50;

     fwrite(&one, sizeof(struct record), 1, in);
     fclose(in);
  }

Program B (all headers and struct the same, with the following code):

  int main ()
  {

     FILE *in=fopen("records.bin", "rb+");

     struct record array;

     fread (&array, sizeof(struct record), 1, in);
     printf ("%d\n", array.index);
     printf ("%s\n", array.name);
     printf ("%d\n", array.age);

     fclose(in);

  }

I expected the output to be

100
j
50

But the actual output is 172400800

Upvotes: 0

Views: 3847

Answers (1)

tadman
tadman

Reputation: 211740

You're writing trash to your file because you're using the pointer of a pointer, not the pointer.

So you can do a quick fix:

fwrite(one, sizeof(struct record), 1, in);

Or you can rewrite to avoid the allocation, something that's probably a good idea because it's completely unnecessary here:

#include <stdio.h>
#include <stdlib.h>

struct record
{
  int index;
  char name [100];
  int age;
};

int main ()
{
  struct record one;

  FILE *in = fopen("records.bin", "ab+");

  fgets(one.name, 100, stdin);
  one.index=100;
  one.age=50;

  fwrite(&one, sizeof(struct record), 1, in);
  fclose(in);
}

As an additional note, you really should be steering towards variable names that have intrinsic meaning. one is not meaningful. r is an improvement even if somewhat ambiguous, because at least "r" means something in relation to "record".

Upvotes: 3

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