C : In function, malloc two dimensional array and duplicatating array of argument does not work

Question

#include <stdio.h>
#include <stdlib.h>

int** asd(int map[][7]);

int main(void)
{
    int **child_map = (int**)malloc(sizeof(int*) * 6);
    int i, k;
    for (i = 0; i < 6; i++)
        child_map[i] = (int*)malloc(sizeof(int) * 7);


    for (i = 0; i < 6; i++)
    {
        for (k = 0; k < 7; k++)
        {
            child_map[i][k] = 10;
        }
    }

    for (i = 0; i < 6; i++)
    {
        for (k = 0; k < 7; k++)
        {
            printf("%3d", child_map[i][k]);
        }
        printf("\n");
    }

    int **map = NULL;
    map = asd(child_map);

    for (i = 0; i < 6; i++)
    {
        for (k = 0; k < 7; k++)
        {
            printf("%3d", map[i][k]);
        }
        printf("\n");
    }

    return 0;
}

int** asd(int map[][7])
{
    int **a = (int**)malloc(sizeof(int*) * 6);
    int i, k;
    for (i = 0; i < 6; i++)
        a[i] = (int*)malloc(sizeof(int) * 7);


    for (i = 0; i < 6; i++)
    {
        for (k = 0; k < 7; k++)
        {
            a[i][k] = map[i][k];
        }
    }
    return a;
}

In function, a[i][k] and map[i][k] are not mapped. Value of array a is very strange like 99882978. Why doesn't it work?
I have been made tree of game like go and each tree node has copy of parent's map. So i made function malloc two dimesional array , copies map of parent and return pointer of copy map. But it do not work... I'm very stuck here...

[don't cast malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Barmar, May 03 '17 at 15:25
You should write `int **child_map = malloc(sizeof(int*) * 6);` without `(int **)` before `malloc`. Click on the link to learn why. — Barmar, May 03 '17 at 15:31
`int **` is no 2D array, nor can it point to one. If you need a 2D array, **use one**! — too honest for this site, May 03 '17 at 15:33
"Does not work" is not a **specific** problem statement as required by site-rules. Read [ask], provide a [mcve] and all required information. — too honest for this site, May 03 '17 at 15:47
And why don't you `malloc` a 2D array and just `memcpy` it? That`s just two function calls. Why simple if we can do it complicated? As a sidenote: don't use magic numbers! Pass teh lengths of each dimension as arguments or use constant-like macros. — too honest for this site, May 03 '17 at 16:16

score 2 · Answer 1 · answered May 03 '17 at 15:28

2

child_map is an array of pointers. The declaration int map[][7] is for a 2-dimensional array of int, not an array of pointers. You need to change asd to:

int **asd(int **map)

answered May 03 '17 at 15:28

Barmar

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This is surprisingly difficult to spot... – Eugene Sh. May 03 '17 at 15:31
OP wants a 2D array, so why not allocate and return one? – too honest for this site May 03 '17 at 15:34
@Olaf Dynamically allocating a 2D array is tricky. You can do it with `malloc(rows*cols*sizeof(int))`, but you can't access this with `array[i][j]`, you need to do the conversion from 2D to linear addressing yourself. – Barmar May 03 '17 at 15:37
@Barmar: Ehm, no! Use a pointer to a 1D VLA! `int (*int_alloc2D(size_t rows, size_t cols))[cols];`. Use normal index-operator. Note the index-operator anyway does not operate on arrays, but pointers **only**. – too honest for this site May 03 '17 at 15:41
If you have a better answer, post it. – Barmar May 03 '17 at 15:44
I wait until it is clear what OP actually wants/needs. The question is not clear if that is a debug request or a "how to do" question. For example: Since when has an array scalar "value"? – too honest for this site May 03 '17 at 15:46

C : In function, malloc two dimensional array and duplicatating array of argument does not work

1 Answers1