Assigning data to array using curly brackets

Question

In the C language, why does the following expression fail?

map = malloc(sizeof(Map) * tiles);
map = {
    0,2,0,0,0,0,0,0,2,0,
    0,1,0,0,0,0,0,0,1,0,
    0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,
    2,1,0,0,0,0,0,0,1,2,
    0,0,0,0,0,0,0,0,0,0
};

I just want to fill data in such a "row/column" format into a C array. However, the compiler fails with

error: expected expression before ‘{’ token

(in the map = { line). Filling the array in other ways works fine, and I am sure the brackets work for initializations...I can't do that after I have allocated memory?

EDIT: I solved it by making a temporary char array and then feeding the data to the malloced map in a for loop. Still. I'd like to know why the above code would leak memory as pointed below. And would my fix (parsing the temp array and setting the map data with it) leak memory as well?

Solution

Your Map * map; is a pointer, not an array. Curly braces are for aggregate initialization:

int x[3] = { 1, 2, 3 };

Pointers are not arrays, and you cannot fill memory with the aggregate initialization syntax.

Here is the closest construct that would work:

typedef struct Map_ { int a; int b; } Map; // some struct

Map m[] = { {1,2}, {3,4}, {5,6} };  /* we initialized "Map m[3]",
                                       it has automatic storage! */

Note that each element of the brace-list must itself initialize the base type of the aggregate.

Comments

[Marlon]

Not only is it wrong, you are going to cause a memory leak.

[roger_rales]

Can you elaborate on that, Marlon? Knowing it causes one won't help unless I know why...

[Marlon]

Even though your code doesn't compile, it demonstrates how a real memory leak is caused. The address returned by malloc must be freed. Since you are are reassigning a different address to map, you lose the address returned by malloc and thus you can no longer free it.

[roger_rales]

Ah I see. Makes sense actually. I have seen a few pieces of code assigning map/pixel data in a similar way, so I thought it was possible, although after double-checking, I realized it's as you say.

[trusktr]

@Kerrek, Can you do something like int x[] = {1,2,3,4,5,6} to make the array an arbitrary size when you assign the values (omitting the size between the square brackets)?

[trusktr]

@KerrekSB oh, sweet. If you do int x[]; would x just be an int pointer not pointing to anything?