dynamic memory allocation of an array of pointers to structures

[bimbam]

Hi,

I've been trying to allocate dynamicaly an array of pointers to structures. As it didn't work, I tried to reduce the problem as much as possible. This is what I have :

 
#include <stdio.h>
#include <stdlib.h>
 
typedef  struct
{
  int x;
  int y;
}struct_p;
 
 
int main()
{
  int i,n=2;
  struct_p *p;
  p=calloc(n,sizeof(struct_p*));
  for (i=0;i<n;i++)
    {
      p[i]->x=0;
      p[i]->y=1;
    }
 }
 
 

When compiling it with gcc, I get the errors:


new.c:18: error: invalid type argument of ‘->’
new.c:19: error: invalid type argument of ‘->’


I thought that by using calloc the way I did it, I had defined an array of pointers to structures. So the use of 'p[i]->x' for example, should give the content of the strucure field 'x' in the structure of adress 'p[i]'. Where am I wrong? What should I do?

Regards

[sanctus]

Hi,

I've been trying to allocate dynamicaly an array of pointers to structures. As it didn't work, I tried to reduce the problem as much as possible. This is what I have :

 
#include <stdio.h>
#include <stdlib.h>
 
typedef  struct
{
  int x;
  int y;
}struct_p;
 
 
int main()
{
  int i,n=2;
  struct_p *p;
  p=calloc(n,sizeof(struct_p*));
  for (i=0;i<n;i++)
    {
      p[i]->x=0;
      p[i]->y=1;
    }
 }
 
 

When compiling it with gcc, I get the errors:


new.c:18: error: invalid type argument of ‘->’
new.c:19: error: invalid type argument of ‘->’


I thought that by using calloc the way I did it, I had defined an array of pointers to structures. So the use of 'p[i]->x' for example, should give the content of the strucure field 'x' in the structure of adress 'p[i]'. Where am I wrong? What should I do?

Regards

p[i] is no more an adress...p[i] is an element of an array and not a pointer to an element so you can't use ->. How you use calloc you allocate memory for an array of 2 objects of size of struct_p, so that should be fine. I think it should work if you replace '->' by '.'
Regards

[bimbam]

p[i] is no more an adress...p[i] is an element of an array and not a pointer to an element so you can't use ->. How you use calloc you allocate memory for an array of 2 objects of size of struct_p, so that should be fine. I think it should work if you replace '->' by '.'
Regards

Thank you for your answer.

I used sizeof(struct_p *) and not sizeof(struct_p). Does'nt it make any difference? I thought that, this way, the elements of the array would be pointers.

Regards

[sanctus]

Thank you for your answer.

I used sizeof(struct_p *) and not sizeof(struct_p). Does'nt it make any difference? I thought that, this way, the elements of the array would be pointers.

Regards

Actually I don't know well the operator sizeof, but from what I understand sizeof doesn't define any type (i.e. pointer or variable) just giving the size in octets. Considering this you should use sizeof(struct_p), because you want allocate memory to p so that it can hold two objects of type struct_p.
Using sizeof(struct_p *) it means that p can hold 2 objects of size of a pointer to struct_p.
I'm not sure about this, but I think it is this way.

Did you still get errors using the '.' instead of the '->' ?

[bimbam]

Actually I don't know well the operator sizeof, but from what I understand sizeof doesn't define any type (i.e. pointer or variable) just giving the size in octets. Considering this you should use sizeof(struct_p), because you want allocate memory to p so that it can hold two objects of type struct_p.
Using sizeof(struct_p *) it means that p can hold 2 objects of size of a pointer to struct_p.
I'm not sure about this, but I think it is this way.

Did you still get errors using the '.' instead of the '->' ?

It works with 'sizeof(struct_p)' (and '.'). You were right. Thank you for your help.

Regards

[bimbam]

Actually I don't know well the operator sizeof, but from what I understand sizeof doesn't define any type (i.e. pointer or variable) just giving the size in octets. Considering this you should use sizeof(struct_p), because you want allocate memory to p so that it can hold two objects of type struct_p.
Using sizeof(struct_p *) it means that p can hold 2 objects of size of a pointer to struct_p.
I'm not sure about this, but I think it is this way.

Did you still get errors using the '.' instead of the '->' ?

But in that case, how would you allocate dynamically the memory of an array of pointers if not with 'p=calloc(n,sizeof(struct_p*))'?

Regards

[JosAH]

But in that case, how would you allocate dynamically the memory of an array of pointers if not with 'p=calloc(n,sizeof(struct_p*))'?

Regards

If you want to allocate n of those pointers, the way you described it is the way
to do it; but when you want to assign whatever calloc returned you need a
struct_p** (note the double star) because it points to zero or more of those
struct_p*s (not the single star).

If you do it like this you have an array of struct pointers but none of the pointers
point to such a struct yet, i.e. you have to initialize the array elements (the pointers)
if you want to dereference anything from them.

kind regards,

Jos

[bimbam]

If you want to allocate n of those pointers, the way you described it is the way
to do it; but when you want to assign whatever calloc returned you need a
struct_p** (note the double star) because it points to zero or more of those
struct_p*s (not the single star).

If you do it like this you have an array of struct pointers but none of the pointers
point to such a struct yet, i.e. you have to initialize the array elements (the pointers)
if you want to dereference anything from them.

kind regards,

Jos

Thanks a lot for your answer.

Everything seems to work. I just want to be sure that I did the memory allocation properly so that nothing will be overwriten later and no memory was wasted.

Could you quickly have a look at these few lines to see if everything is "perfect"? Do you have suggestions to improve it?

 
#include <stdio.h>
#include <stdlib.h>
 
typedef  struct
{
  int x;
  int *y;
}struct_p;
 
 
int main()
{
  int i,j,n=3;
  int x,y;
  struct_p **p;  
  p=malloc(n*sizeof(struct_p*));
 
  for (i=0;i<n;i++)
    {
      p[i]=malloc(sizeof(struct_p)); 
      p[i]->x=0;
      p[i]->y=malloc(n*sizeof(int));
    }
 
  for (i=0;i<n;i++)
    {
      printf("p[%d]->x=%d | ",i,p[i]->x);
      for (j=0;j<n;j++)
    printf("p[%d]->y[%d]=%d ",i,j,p[i]->y[j]);
      printf("\n");
    }
 }
 
 

The output is as expected:

p[0]->x=0 | p[0]->y[0]=0 p[0]->y[1]=0 p[0]->y[2]=0
p[1]->x=0 | p[1]->y[0]=0 p[1]->y[1]=0 p[1]->y[2]=0
p[2]->x=0 | p[2]->y[0]=0 p[2]->y[1]=0 p[2]->y[2]=0


Regards

[JosAH]

Yep, that's all correct as far as I can see now.

kind regards,

Jos

edit: you could've done 'calloc(n, sizeof(struct_p*))' instead of malloc(n*sizeof(struct_p*))'
but it doesn't matter.

[bimbam]

Yep, that's all correct as far as I can see now.

kind regards,

Jos

edit: you could've done 'calloc(n, sizeof(struct_p*))' instead of malloc(n*sizeof(struct_p*))'
but it doesn't matter.

Thanks.

To 'desallocate', is 'free(p)' enough? Or do I have to do everything in reverse to free each field of all the structures, free the structures abd then free the array?
Regards

[JosAH]

Thanks.

To 'desallocate', is 'free(p)' enough? Or do I have to do everything in reverse to free each field of all the structures, free the structures abd then free the array?
Regards

Yep, you have to do it all yourself. C's free() function is quite stupid: it frees
the memory directly pointed to by its parameter and that's about it. It doesn't
know that that memory contains pointers to other 'things' that should be free'd
as well. You need C++ if you want that type of functionality. Or you can build
a nice little function for exactly this purpose.

kind regards,

Jos