How can I malloc a struct array inside a function? Code works otherwise

Question

I'm trying to create a function that creates a variable sized 2D funct array. I'm using the following code, which seems to work just fine on its own:

typedef struct
{
    //Starter Properties
    int TypeB;
    int TypeF;
    int TypeW;
    //Randomized Properties
    int RandB;
    int RandF;
    int RandW;
    //Derived Properties
    int Speed;
} MapTileData;

MapTileData **Map;

int i, x=5, y=5;

//Allocate Initial Space
Map = (MapTileData**)calloc(x, sizeof(MapTileData));

for(i = 0; i < x; i++)
{
    Map[i] = (MapTileData*)calloc(y, sizeof(MapTileData));
}

So the above code creates a 2D struct array. My attempts to move the code to a function have been less successful, giving segmentation faults when trying to print the array:

void CreateMap(MapTileData **Map, int xSize, int ySize)
{
    //Variables
    int i;

    //Allocate Initial Space
    Map = (MapTileData**)calloc(xSize, sizeof(MapTileData));

    for(i = 0; i < xSize; i++)
        {
            Map[i] = (MapTileData*)calloc(ySize, sizeof(MapTileData));
        }
}

Used in the code:

MapTileData **MapTile;

CreateMap(MapTile,5,5);

Any and all help is greatly appreciated!

If you do `void f(int i) {i = /* something */;}`, why doesn't it change `i` outside the function? — user253751, Mar 18 '16 at 03:00
Unrelated, but `sizeof(MapTileData)` should be `sizeof(MapTileData*)` in the first `calloc` call. — user253751, Mar 18 '16 at 03:01
More or less a duplicate to allocating a "string"-array, (which in fact is as "2d-array" of `char`) inside a function: http://stackoverflow.com/q/25769443/694576 — alk, Mar 18 '16 at 08:02

MikeCAT · Accepted Answer · 2022-03-31T11:19:40.977

Function arguments are passed by value in C and modifying arguments in callee won't affect caller's local variables.

Use pointers to modify caller's local variables.

void CreateMap(MapTileData ***Map, int xSize, int ySize)
{
    //Variables
    int i;

    //Allocate Initial Space
    *Map = calloc(xSize, sizeof(MapTileData));

    for(i = 0; i < xSize; i++)
    {
        (*Map)[i] = calloc(ySize, sizeof(MapTileData));
    }
}

Usage in the code:

MapTileData **MapTile;

CreateMap(&MapTile,5,5);

Alternate way: Pass the allocated array via the return value.

MapTileData **CreateMap(int xSize, int ySize)
{
    //Variables
    MapTileData **Map;
    int i;

    //Allocate Initial Space
    Map = calloc(xSize, sizeof(MapTileData));

    for(i = 0; i < xSize; i++)
    {
        Map[i] = calloc(ySize, sizeof(MapTileData));
    }

    //Return the value
    return Map;
}

Usage in the code:

MapTileData **MapTile;

Maptile = CreateMap(5,5);

Also note that they say you shouldn't cast the result of malloc() and its family in C.

Up'ed, assuming you left out all error checking for the sake of readability ... ;-) — alk, Mar 18 '16 at 08:05
Thank you, this worked perfectly. I read up on using pointers in functions, I think I understand it a little better now. Cheers! — Danni111111, Mar 19 '16 at 02:48

How can I malloc a struct array inside a function? Code works otherwise

1 Answers1

Linked

Related