Double pointer in c, assignment warning for my code

Question

I don't know why I am getting warnings for the following code.

#include<stdio.h>
#include<malloc.h>
int main()
{
    int **p;
    int i,j;
    for(i=0;i<5;i++)
    {
        if(i==0)
            p=(int*)malloc(1*sizeof(int));
        else
            p=(int*)realloc(p,(i+1)*sizeof(int));
        p[i]=(int*)malloc(5*sizeof(int));

        for(j=0;j<5;j++)
        {
           p[i][j]=j;
        }
    }
    for(i=0;i<5;i++)
    {
        for(j=0;j<5;j++)
            printf("%5d",p[i][j]);
        printf("\n");
    }

    return 0;
}

Is there any other way to allocate memory dynamically for a double pointer.

why are you using dynamic allocation of `p` to begin with? Why not `int *p[5];`? — Ben Voigt, Aug 30 '13 at 03:46
@EricZ I am trying to implement a demo program by inserting values into 2d array which is created by allocating memory dynamically. — Sashank Alladi, Aug 30 '13 at 17:01
@BenVoigt, I am trying to use double pointer, why double pointer because the fragment in which you specified int *p[5] you have to specify a value i.e, the number of rows , what if you don't know how many rows you are going to create. — Sashank Alladi, Aug 30 '13 at 17:05
@SashankAlladi: Well, `5` was hard-coded in your program, so I thought you knew it in advance. If you don't know until after a few iterations, then I agree. — Ben Voigt, Aug 30 '13 at 18:08
@BenVoigt I got you, The program is just a demo, well its my fault I have to explicitly specify it. — Sashank Alladi, Aug 31 '13 at 12:34

score 3 · Answer 1 · answered Aug 30 '13 at 02:27

3

You get warnings because p is an int ** and you are allocating memory as if it was an int * in your if..else lines. Simply replace your lines as follows:

p=(int **)malloc(1*sizeof(int *));

p=(int **)realloc(p,(i+1)*sizeof(int *));

answered Aug 30 '13 at 02:27

trogdor

1,626
14
17

score 0 · Accepted Answer · answered Aug 30 '13 at 02:29

0

This compiles with no warning

  int **p;
  p = (int **) malloc(sizeof(void *));

answered Aug 30 '13 at 02:29

stark

12,615
3
33
50

What compiler will return a different size for a void* than an int**? – stark Aug 31 '13 at 02:25
Those examples are C++ – stark Aug 31 '13 at 03:03
So? C++ provides the same guarantees on primitive types as C. It's the platform, not the language, that determines pointer sizes. (You're welcome to pull a rule out of the C standard that says otherwise... but I don't think any exists) – Ben Voigt Aug 31 '13 at 03:21
Just checked the C standard. Section 6.2.5/26 is the relevant rule, and explicitly says that `void*` doesn't have to be the same size as any other pointer except `char*`. – Ben Voigt Aug 31 '13 at 03:25
malloc returns an untyped void * and you cast it to the pointer type that you want. How would that work with different pointer sizes? – stark Aug 31 '13 at 04:39
Try actually reading the explanations I linked to. Short version: `void*` needs to be able to contain any address, `int*` only needs to be able to contain aligned addresses, which means less information. – Ben Voigt Aug 31 '13 at 18:23
Since you've been given ample opportunity to correct your answer but persist in spreading wrong code, you've earned a -1 from me. – Ben Voigt Aug 31 '13 at 18:24

score 0 · Answer 3 · answered Aug 30 '13 at 03:08

You're getting warnings because you're (a) casting the return value of malloc() and realloc(), and (b) casting it to the wrong type. In C it's generally considered best not to cast the return value of malloc() and realloc() at all, because void* will automatically convert to any other pointer type, and putting in the unnecessary cast can lead to errors (such as yours, for example).

Double pointer in c, assignment warning for my code

3 Answers3