Thread: 2D array create using malloc: error - invalid type argument of unary *

Hi Members,

I'm new to 2D pointers, and C. I'm trying few basic things, of creating new pointer, printing, adding matrices.
Below is my code:

Code:

int main(){

  int (*add)[3]=malloc(3*3*sizeof(int));
  int i=0,j=0;
  int mat1[3][3]={{2,3,70},{20,-8,90},{40,-2,-1}};
  int mat2[3][3]={{8,17,-30},{80,12,-10},{-20,-8,100}};
  printf("Initialized 2 matrices\n");

  for(i=0;i<=2;i++){
    for(j=0;j<=2;j++){
      *(*(add+i)+j) = mat1[i][j]+mat2[i][j];
    }
  }

  for(i=0;i<=2;i++){

    for(j=0;j<=2;j++){

      printf("%d\t",*(*(add+i)+j));
    }
    printf("\n");
  }
  return 0;
}

I get the desired output from this, if I keep:
int (*add)[3]=malloc(3*3*sizeof(int));

If I use:

int *add=malloc(3*3*sizeof(int));
I get error as:

matrix_add.c:14: error: invalid type argument of ‘unary *’ (have ‘int’)

Why is so?
with *add=malloc(3*3*sizeof(int)); do I not get a pointer in contiguous memory?

(*add)[3]=malloc(3*3*sizeof(int)); this is same as creating a 2D pointer.

Please enlighten.

Originally Posted by laserlight

This is error prone:

Code:

int (*add)[3]=malloc(3*3*sizeof(int));

add is a pointer to an array of 3 ints. If you want to allocate space for 3 such arrays of 3 ints, then you should write:

Code:

int (*add)[3] = malloc(3 * sizeof(*add));

Since add is now a pointer to an int, *(*(add+i)+j) no longer makes sense. The compiler is basically complaining that you are trying to dereference an int.


You do get a pointer that points to the start of a block of contiguous memory (unless malloc returns a null pointer), but that is not the issue. The issue is that if add is a pointer to an int, *(*(add+i)+j) is invalid syntax.

By the way, there is no point writing *(*(add+i)+j). Just write: add[i][j].

Hi laserlight,

- Thanks for your attention to my post and correcting the 3*3 malloc part. I was oblivious of it.
- I could have used
add[i][j]

But I was trying to use pointer. Perhaps playing/messing around with it.
I'd need more understanding here:

*(*(add+i)+j)=mat1[i][j]+mat2[i][j]

Doesn't this mean that value at add[i][j] = sum of matrix1 and matrix2's respective row and column value?
I can print value as printf("%d",*(*(add+i)+j));

Sorry for my naive doubts.

Hi Laser light,
Thanks for your comments.

Originally Posted by laserlight

Oh, and remember to check the return value of malloc, and then to free what you malloc.

Code:

int (*add)[3]=malloc(3*sizeof(int));

  if(add==NULL){
    printf("Not succeeded\n");
    exit(-1);
  }
/*
*I used above code to check ..
*/
//But I'm unable to free the allocated memory..
free(add); //spits *** glibc detected ***

I'm having hard time to free this memory.

I used loop to free the memory too, but seems compiler is still complaining.

Originally Posted by laserlight

The problem lies with this loop:

Code:

  for(i=0;i<3;i++){
    free(*(add+i));
  }

Observe that you called malloc once. So, why do you call free in a loop?

Hi,
Thanks for pointing it.

If I don't use the loop, I get the same error, of libc.
How do I free 2D array's pointer?

Originally Posted by laserlight

The above is wrong. Basically, whenever you do sizeof(X), where X is a type, you run the risk of using the wrong type. This is exactly what happened here. Refer to my post #2 for a superior alternative.

Hi,
Ouch, that was too picky.

int (*add)[3]=malloc(3*sizeof(*add));

This worked. Thanks.

But I never read anything like this, before.
I mean, *add is being assigned memory of *add, weird, isn't it? *add is not created.
I'm lost.

Of course, I'm not trying to counter you, but trying to understand the subtleties of pointers.

Thanks much for your patience, and replies to answer my queries. Never seen someone with this depth of knowledge and concepts.
Looking at your answers, and building upon them, implies most of the code on 2D arrays, malloc; out there is incorrect.!