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? — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Mar 18, 2016 at 3:00
Unrelated, but sizeof(MapTileData) should be sizeof(MapTileData*) in the first calloc call. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Mar 18, 2016 at 3:01
More or less a duplicate to allocating a "string"-array, (which in fact is as "2d-array" of char) inside a function: stackoverflow.com/q/25769443/694576 — alk
– alk, Commented Mar 18, 2016 at 8:02

MikeCAT · Accepted Answer · 2022-03-31 11:19:40Z

1

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.

edited Mar 31, 2022 at 11:19

answered Mar 18, 2016 at 2:49

MikeCAT

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2 Comments

alk Over a year ago

Up'ed, assuming you left out all error checking for the sake of readability ... ;-)

Danni111111 Over a year ago

Thank you, this worked perfectly. I read up on using pointers in functions, I think I understand it a little better now. Cheers!

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