C, memory leak? Can not free some memory. Beginner

Question

Hello

I can't truly understand why the free process returns in an error. I got this code in C:

int LuffarschackStart(void)
{
/* to avoid the program from closing */
char readEnd;
int i = 0;    

board_type *board = malloc(sizeof(square_type));
if (board == NULL)
{
    printf("Could not allocate the memory needed...");
    scanf("%c", &readEnd);         
    return 0;
}

for(i = 0; i < 9; i = i + 1)
    board->square[i].piece_type = NO_PIECE;

board_play_game(board);    

free(board);
printf("Press any key and enter to quit the program...");
scanf("%c", &readEnd);         
return 0;
}

The board struct that i am allocating looks like this:

typedef struct
{
    /* flag to indicate if a square is free or not */  
    int free;
    /* the type of piece stored on the square if the 
       square is not free, in this case the admissible 
       values are CROSS_PIECE and CIRCLE_PIECE, 
       otherwise the value NO_PIECE is used */ 
    int piece_type; 
} square_type; 

typedef struct
{
    square_type square[N_SQUARES]; 
    int computer_type;
    int player_type;
} board_type;

Could the problem be that i need to free the square_type inside the board first? If thats the case, how do i free that?

Answer 1

I think your malloc is wrong. It should be

board_type *board = malloc(sizeof(board_type)); /* instead of sizeof(square_type) ...*/

Besides this I think your code is correct ...

Answer 2

First, you are allocating the wrong size here:

board_type *board = malloc(sizeof(square_type));

It needs to be

board_type *board = malloc(sizeof(board_type));

You probably didn't saw this problem, but I suspect you are writing to unallocated memory. (Potential memory exception).

You don't need to free the inner array, because it is a fixed size array, and when you allocate a board_type, it will be ready with the entire array.

Fix the malloc, it will solve the free.

Answer 3

Others have already pointed out the error, but here's a macro that will help catch those errors:

#define NEW(type)   (type *)malloc(sizeof(type))

You would then use it like this:

// Correct usage
board_type *board = NEW(board_type);

What nice about this is that if you make a mistake like you did, you should get a compiler warning about mismatched pointers due to the cast inside the macro:

// Incorrect usage, a decent compiler will issue a warning
board_type *board = NEW(square_type);

Answer 4

Another nitpick, nothing to do with your memory problem: you probably don't need an extra flag to mark free squares if you already distinguish between the three possible pieces CROSS / CIRCLE / NONE...