problem with pointers and bidimensional arrays

Hi all.

I have a problem with a pointer and a bidimensional array.
here's the code:

Code:

---


includes..

typedef char t_board[5][4];
int i,j;

t_board *copy (t_board b) {
t_board *ptr;

        ptr = (t_board *) malloc (sizeof(t_board));
        for (i=0; i<5; i++)
            for (j=0; j<4; j++)
                  *ptr[i][j] = b[i][j];

        return ptr;
}

int main () {
t_board some_board, *new_board;

      new_board = copy(some_board);
            .
            .
            code here
            .
            .
}

---

........

After this, I print new_board, and its ok; but the following code after
the call to copy, crashes. Don't know why.
if I remove the statment new_board = copy(some_board); , the code that follows runs ok.
Can someone tell me why?
Thanks;

> *ptr[i][j] = b[i][j];
You have to watch the precedence here.
You have a pointer to an array, not an array of pointers.

So you need
(*ptr)[i][j] = b[i][j];

Quote:

---

Originally Posted by Salem

> *ptr[i][j] = b[i][j];
You have to watch the precedence here.
You have a pointer to an array, not an array of pointers.

So you need
(*ptr)[i][j] = b[i][j];

---

the only difference between the code I posted, and the one i'm writting, is that some_board is now a parameter of a function.
I think it shouldn't be a mistake.

Code:

---

void func(t_board b) {
t_board *new board;

        new_board = copy(b);
}

---

Interesting.... do go on.

Code:

---

#include<stdio.h>
#include<stdlib.h>
typedef char t_board[5][4];
 
t_board *copy (t_board b) {
  int i, j;
  t_board *ptr;
 
        ptr = malloc (sizeof(*ptr));
        for (i=0; i<5; i++)
            for (j=0; j<4; j++)
                  (*ptr)[i][j] = b[i][j];
 
        return ptr;
}
 
int main () {
  int i,j;
  t_board some_board = {
    { 1, 2, 3, 4 },
    { 11, 12, 13, 14 },
    { 21, 22, 23, 24 },
    { 31, 32, 33, 34 },
    { 41, 42, 43, 44 },
  };
  t_board *new_board;
 
  new_board = copy(some_board);
  for ( i = 0 ; i < 5 ; i++ ) {
    for ( j = 0 ; j < 4 ; j++ ) {
      printf("%2d ", some_board[i][j] );
    }
    printf("\n");
  }
  for ( i = 0 ; i < 5 ; i++ ) {
    for ( j = 0 ; j < 4 ; j++ ) {
      printf("%2d ", (*new_board)[i][j] );
    }
    printf("\n");
  }
  free(new_board);
  return 0;
}

$ gcc -Wall bar.c
$ ./a.out
 1  2  3  4
11 12 13 14
21 22 23 24
31 32 33 34
41 42 43 44
 1  2  3  4
11 12 13 14
21 22 23 24
31 32 33 34
41 42 43 44

---

Quote:

---

Originally Posted by Gustaff

I think it shouldn't be a mistake.

---

Unfortunately, the C standard (by virtue of the precedence of operators) disagrees with you.

In a dispute between your expectations and behaviour of your compiler, the compiler almost always prevails. When the dispute is because your expectations differ from the standard, then there is almost 100% chance of your compiler prevailing.

Salem has pointed you in the right direction.