inicializing a structure with arrays of arrays [duplicate] - c

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!

Answer 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.