Two dimensional array malloc in C ---> error?

Question

I am trying to make two dimensional array function, but somehow it is not working. The code here:

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

int **multiTable (unsigned int xs, unsigned in ys)

{
unsigned int i, j;

int **table = int(**)malloc(xs * ys * sizeof(int*));
for(i = 0; i < ys; i++)
{
    for(j = 0; j < xs; j++)
    {
       table[i][j] = j * i;
    }


}
free(**table);
return table;


}

So first of all, should I also add inside the malloc the row (xs)? Or should it work, if I work only with the columns? --> like this:

int **table = int(**)malloc(ys * sizeof(int*));

you cannot do this in one shot. first allocate memory to table, then table[i](s). — Sourav Ghosh
– Sourav Ghosh, Commented Dec 2, 2014 at 12:31
what's the point of returning something that was just freed? — Karoly Horvath
– Karoly Horvath, Commented Dec 2, 2014 at 12:34

2501 · Accepted Answer · 2014-12-02 12:49:25Z

1

That is not going to work as an array of pointers int **table is not contiguous and it is not equivalent to a 2d array table[a][b].

You can however use a pointer to an array if you want to use a single malloc.

int (*table)[xs] = malloc( ys * sizeof(*table));
for( int i = 0; i < ys; i++)
{
    for( int j = 0; j < xs; j++)
    {
       table[i][j] = i;
    }
}
free( table ) ;

And do not return table after you free it as your return call does.

edited Dec 2, 2014 at 12:49

answered Dec 2, 2014 at 12:34

2501

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5 Comments

rcgldr Over a year ago

Note, on some compilers, xs would have to be a define or a constant.

2501 Over a year ago

@rcgldr Not true, compilers have nothing to do with that. The Standard does. C99 and onward has variable-length arrays( and pointers to them ).

rcgldr Over a year ago

I had the impression even with C99, only the first (or only) dimension of a multi (or single) dimensional array could be variable. The rest of the dimensions need to be fixed in size. For example twodarrray[var][const1] or threedarray[var][const1][const2] .

2501 Over a year ago

@rcgldr There is not such limit, you can try it for yourself.

rcgldr Over a year ago

Let us continue this discussion in chat.

Vlad from Moscow · Accepted Answer · 2014-12-02 16:42:01Z

1

It seems you mean the following

int ** multiTable( unsigned int xs, unsigned int ys )
{
    unsigned int i, j;

    int **table = malloc( ys * sizeof( int * ) );

    for ( i = 0; i < ys; i++ )
    {
        table[i] = malloc( xs * sizeof( int ) );
        for ( j = 0; j < xs; j++ )
        {
           table[i][j] = j * i;
        }
    }

    return table;
}

edited Dec 2, 2014 at 16:42

answered Dec 2, 2014 at 13:01

Vlad from Moscow

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Comments

Lavish Kothari · Accepted Answer · 2014-12-02 13:16:30Z

'table' is a 2-D pointer. First understand what is a 2D pointer. A pointer is a special type of variable that is used to store the address of another variable. So a 2D pointer is a special variable that is again used to store the address of a pointer variable.

Now imagine that you have a single array of pointers(collection of base address of different 1-D array), that will store the base address of many 1-D arrays.

To dynamically allocate memory to this array of pointers you need the statement

table=(int**)malloc(sizeof(int*)*xs);

Now you have an array with 'xs' number of elements, and you can access each element by table[0], table[1], table[2]..and so on..., but none of these arrays is allocated memory. So you need to allocate memory to each of the array individually using a loop like this:

for(i=0;i<xs;i++)
    table[i]=(int*)malloc(sizeof(int)*ys);

So your over-all program becomes:

int **table; // table is a 2D pointer

table=(int**)malloc(sizeof(int*)*xs);
for(i=0;i<xs;i++)
    table[i]=(int*)malloc(sizeof(int)*ys);

for(i = 0; i < ys; i++)
{
    for(j = 0; j < xs; j++)
    {
       table[i][j] = j * i;
    }
}
return table;

You don't need to free the array before returning it. Doing so will make your pointer 'table' a dangling pointer that is still referring to a memory that is no longer allocated, so just return 'table' without the statement:

free(table);

The above statement will lead to a dangling memory that has no access point, ans so is useless. This is a memory leakage problem that arises when memory gets accumulated, and this memory is no more accessible through your program and could not be relocated for other purpose, such a memory is called dangling memory.

Please do not cast the result of malloc. As of C89 it's no longer necessary, and under C89 it can suppress a useful diagnostic if you don't have a declaration for malloc in scope.

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