Thread: dynamic memory allocating error

Hi!
I'm trying to make an implementation of the datatype RATIONAL in ADT way. The probleme is when I try to allocate memory for a new struct that I create in main.c.
I send the adress of the struct to rational_create, where I try to allocate memory to it and after that set values to the integers in the struct, but the way I try to allocate doesn't work.
the problem is how to allocate the memory in rational_create(rational *r, int n, int t).

The code looks like this:

main.c
===========
int main(void)
{
Rational a;
rational_create(Rational a, 2, 2);

system("PAUSE");
return 0;
}

rational.c
===========
#include <stdio.h>
#include <stdlib.h>
#include "rational.h"

struct rat_
{
int a1, b1;
}

void rational_create(Rational *r, int n, int t)
{
*r = (Rational)malloc(sizeof(struct rat_));

r->a1=n;
r->b1=t;
}

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

struct rat_;

typedef struct rat_ Rational;

/*Allocate memory*/
void rational_create(Rational *r, int n, int t);

int gcd(int n, int t);
Rational add(Rational r);
Rational sub(Rational r);

and s on...

There are a few problems with the code... but to answer your main question: there is no need to allocate memory when you use the declaration

Code:

Rational a;

as it is done for you implicitly. You only really need to use malloc() / calloc() when you declare a pointer to some memory whose size will only be defined at run time.

It might also be a good idea to do the typedef like this:

Code:

typedef struct {
   int a1, b1;
} Rational;

otherwise you may have problems when you inlcude rational.h in main.c (which you will have to do I think). To prevent rational.h being compiled more than once, use:

Code:

#ifndef RATIONAL_H
#define RATIONAL_H
/*...... header declarations here ...... */
#endif /* RATIONAL_H */

There are still a few other things for you to find as well... !!!

DavT

the purpose is that someone else can't see the struct and funktions. He can only view the header file.
I can make changes in the functions without him knowing about it.
That's why the declarations of the struct has to be as it is.