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,siz eof(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,siz eof(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