Program shows memory leak, even after using free()

Question

I am new to malloc() and been reading my books and some tutorials about it. Now that I have written my assignment, it's required that you do not have any memory leaks.

This is a piece of my code how I have written it in main:

#define _CRTDBG_MAP_ALLOC
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <crtdbg.h>
int main()
{   
    _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
    srand((unsigned)time(NULL)); //Kallas för randomisera ett nummer tidsmässigt
    struct CardDeck * deck; //Dynamiskt allokerad array
    int index;
    int suit_index;
    /*int i;  kan användas om man vill loopa kortleken*/
    deck = (struct CardDeck *)malloc(sizeof(struct CardDeck)); //Bör bara användas en gång med tanke på att vi bara har en pekare under vår struct i cardeck
    deck->last_Index = 0; //Pekar på senaste indexen för att se så att kortleken är full. 
    for (suit_index = 0; suit_index < NR_OF_SUITS; suit_index++)    //Loopar 4 (avser färgerna)
        for (index = 0; index < COLOR_SIZE; index++) //Loopar 13 gånger för varje värde i kortleken
        {
            deck->cards[suit_index*COLOR_SIZE + index].suit = suit_index;
            deck->cards[suit_index*COLOR_SIZE + index].value = index;
        }
    /*for (i = 0; i < 52; i++) {
    printCards(deck);
    }  Om man vill loopa igenom hela kortleken (sorterad) så finns den möjlig*/

    shuffleCards(deck->cards); 
    play(deck);
    system("pause");
    return 0;
    free(deck);
}

As you see, I used free(deck) at the bottom, is it something that I am missing or what am I doing wrong?

Don't cast the return value of malloc - http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — Idos, Jan 03 '16 at 21:27
Why does `int main() {printf("AAA\n"); return 0; printf("BBB\n");}` print AAA but not BBB? — user253751, Jan 04 '16 at 00:00

score 6 · Answer 1 · answered Jan 03 '16 at 21:26

6

Your free() statement is after return, do you think it executes? You can try formatting your code carefully and then this kind of mistake might be avoided.

answered Jan 03 '16 at 21:26

Iharob Al Asimi

52,653
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Oh thank you! Because this code comes along when i put it after the return "Detected memory leaks! Dumping objects ->" However when i put it before the return, it doesnt say anything? – William hart Jan 03 '16 at 21:39
But this is what happens when i use it before the return: The thread 0x420c has exited with code -1073741510 (0xc000013a). – William hart Jan 03 '16 at 21:41
Is there a chance you free the pointer in `play()`? – Iharob Al Asimi Jan 03 '16 at 21:42
Yes i do free the pointer in play, so how do i make it work without doing that? – William hart Jan 03 '16 at 21:54
You can't free it twice. The memory leak must be originated somwhere else or by another reason. – Iharob Al Asimi Jan 03 '16 at 21:55

score 1 · Answer 2 · answered Jan 03 '16 at 21:26

The problem is, you attempt to call free() after the return statement.

Once the execution reach the unconditional return statement, the control will return to the caller and any statement placed after the return statement will be unreachable code. So, essentially, your call to free() does not get a chance to execute, at all.

To quote the standard, chapter §6.8.6.4

A return statement terminates execution of the current function and returns control to its caller. [...]

So basically, your free() does not get a chance o free-up the dynamically allocated memory, resulting in a memory leak.

You need to call free() before the return statement, like

. . . . 
free(deck);
return 0;

Program shows memory leak, even after using free()

2 Answers2