Thread: Putting a set of structs in memory...how?

How can I malloc a set of memory and then put a set of structs into that memory?

Here is what I mean...let's say I want to put five integers into memory and then retrieve them (of course, I could use an array, linked list, etc.):

Code:

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

int main (void) {
  int * dataset;
  int i;

  dataset = malloc ( sizeof (int) * 5 );

  *(dataset)   = 9000;
  *(dataset+1) = 10000;
  *(dataset+2) = 11000;
  *(dataset+3) = 12000;
  *(dataset+4) = 13000;

  for (i=0; i<5;i++)
    printf ("position %i value %i\n",i,*(dataset+i));
}

That works fine. But now let's say instead of integers, I want to put structs in memory. And in fact I have several million of these structs, and I don't know the number ahead of time, so an array is not possible.

This won't work:

Code:

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

struct person_struct {
  char name[20];
  int age;
};

int main (void) {
  int * dataset;
  int i;
  struct person_struct this_person;

  dataset = malloc ( sizeof (struct person_struct) * 5 );

  strcpy(this_person.name,"Bob");
  this_person.age = 20;

  *(dataset) = this_person; // error: incompatible types in assignment
}

Of course, I could malloc each struct and put a pointer to it in dataset, but then I might as well use a linked list or other tree.

So how can I say "here is a struct, and here is a big pile of memory I've malloc'd. Store that struct at position 10. Now store the next one at position 11. Etc." Then once I have them, I can access them through pointer arithmetic and sort them.

Access it like an array (equivalent to pointer arithmetic, but clearer):

Code:

dataset = malloc(sizeof(*dataset) * 5); // this always makes sure you allocate the right amount of memory if dataset changes types
dataset[10] = this_person;

(forehead slap)

That was easy. Guess I need to take a break ;-)

Thanks all.