How can I malloc a struct array inside a function?

问题:

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!

回答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));
    }
}

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.