C: dynamically sized array cannot be accessed in another function

Question

I am working on a connect-four game simulator in C.

https://en.wikipedia.org/wiki/Connect_Four

The first step is to create a board environment for the game. I went ahead and made a data type board_t which is a struct that includes a dynamically sized array that will save moves played in a single dimension array. Board_t also includes height and width information of the board, so things can be retrieved in a correct manner.

I initialize this board in board_create() function, and use this initialized board_t variable in a board_can_play() function to check whether any play is possible in a given play. Here is the code.

#include <stdlib.h>
#include <assert.h>
#define PLAYER_BLUE    2
#define PLAYER_YELLOW  1
#define PLAYER_EMPTY   0

typedef unsigned char player_t;

typedef struct board_t
{
    unsigned int width;
    unsigned int height;
    unsigned int run;
    player_t * moves;
} board_t;

bool board_create (board_t ** b, unsigned int height, unsigned int width, unsigned int run, const player_t * i)
{
    //Declare a board_t variable temp_b where parameters will be saved.
    board_t temp_b;
    //Create a pointer and malloc a memory location based on width and height.
    temp_b.moves = malloc(sizeof(unsigned char)*(height*width));
    //Itereate through the moves and initialize with the given player_t
    int j;
    for (j = 0; j < width*height; j++)
    {
        temp_b.moves[j] = PLAYER_EMPTY;
    }
    //Input all the values to temp_b
    temp_b.height = height;
    temp_b.width = width;
    temp_b.run = run;
    //Make a temporary pointer and assign that pointer to *b.
    board_t * temp_b_ptr = malloc(sizeof(board_t));
    temp_b_ptr = &temp_b;
    *b = temp_b_ptr;
    return true;
};

/// Return true if the specified player can make a move on the
/// board
bool board_can_play (const board_t * b, player_t p)
{
    unsigned int i;
    unsigned int height = board_get_height(b);
    unsigned int width = board_get_width(b);
    for(i = (height-1)*width; i < height*width; i++)
    {
        if (b->moves[i] == PLAYER_EMPTY)
        {
            return true;
        }
    }
    return false;
}

However, whenever I call the board_t *b from board_can_play(), the program gives segmentation fault. More specifically,

        if (b->moves[i] == PLAYER_EMPTY)

This line is giving me a segmentation fault. Also, functions that worked well in main(), is not working here in board_can_play(). For instance,

    unsigned int height = board_get_height(b);
    unsigned int width = board_get_width(b);

Are supposed to get 3 and 3, but getting 2 and 419678? I spent about 7 hours now figuring out, but cannot figure out what is going on.

P.P · Accepted Answer · 2015-11-14 21:09:24Z

2

In the if statement that gives you segfault,

    if (b->moves[i] == PLAYER_EMPTY)

The problem is not how moves was allocated, but how b itself was allocated. In board_create(), you are returning a temporary object in here:

    board_t * temp_b_ptr = malloc(sizeof(board_t));
    temp_b_ptr = &temp_b;
    *b = temp_b_ptr;

The malloc'ed pointer is lost (you are overwriting it) and simply returning (through *b) a pointer to a local variable.

So the move the allocation to the top and use temp_b_ptr instead of temp_b:

    board_t *temp_b_ptr = malloc(sizeof(board_t));
    if( !temp_b_ptr ) {
       /* error handling */
       }
    ....
    ....
    *b = temp_b_ptr;

edited Nov 14, 2015 at 21:09

answered Nov 14, 2015 at 20:57

P.P

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3 Comments

Yong Jun Kim Over a year ago

How do I allocate memory for board_t,then? Should I just malloc on *b?

P.P Over a year ago

Exactly how you are doing already. Except move the temp_b_ptr allocation to the top and use it in place of temp_b. I have updated with a little snippet, which hopefully helps understand what you need to change.

Yong Jun Kim Over a year ago

Wow it works! Thank you! I still have a long way to go in understanding pointers and mallocs.

thurizas · Accepted Answer · 2015-11-14 22:29:38Z

I would approach you problem in the following way. Not that I have stubbed-in some error handling, as well as adding a method to destroy the board when done.

The following code compiles without warning in Ubuntu 14.01 LTS, using gcc-4.8.2. I compile the code with the following command line:

     gcc -g -std=c99 -pedantic -Wall connect4.c -o connect4

Now, on to the code. You didn't provide a main, so I created a quick stub main:

    #include <stdlib.h>
    #include <stdbool.h>
    #include <stdio.h>
    #include <assert.h>
    #define PLAYER_BLUE    2
    #define PLAYER_YELLOW  1
    #define PLAYER_EMPTY   0

    typedef unsigned char player_t;

    typedef struct board_t
    {
        unsigned int width;
        unsigned int height;
        unsigned int run;
        player_t * moves;
    } board_t;


    bool board_create(board_t** b, unsigned int height, unsigned int width);
    void board_destroy(board_t** b);

    int board_get_height(const board_t* b);
    int board_get_width(const board_t* b);

    int main(int argc, char** argv)
    {
         board_t*   pBoard = NULL;
         if(board_create(&pBoard, 4, 4))
         {
             printf("board dimensions: %d by %d\n", board_get_height(pBoard), board_get_width(pBoard));
             // TODO : put game logic here...

            board_destroy(&pBoard);
         }
         else
         {
             fprintf(stderr, "failed to initialize the board structure\n");
         }

         return 0;
   }

Not a lot to see in main, much like you would expect. Next is the board_create function. Note that I deleted the run and the player_t parameters because i didn't see you use them in your code.

    bool board_create(board_t** b, unsigned int height, unsigned int width)
    {
        bool   bRet = false;

        if(*b != NULL)    // we already have a board struct laying about
        {
            board_destroy(b);
        }

        if(NULL != (*b = malloc(sizeof(board_t))))
        {
            (*b)->width = width;
            (*b)->height = height;

            if(NULL != ((*b)->moves = malloc(sizeof(unsigned char*)*(height * width))))
            {
                for(int j = 0; j < height * width; j++)
                    (*b)->moves[j] = PLAYER_EMPTY;

                bRet = true;
            }
            else
            {
                /* TODO : handle allocation error of moves array */
            }
        }
        else
        {
            /* TODO : handle allocation error of board struct */
        }

        return bRet;
    }

Couple of comments on this function;

First a bit of defensive programming, I check to see that the board structure has not be previous allocated. If it was I proceed to destroy the previous board prior to creating a new one. This prevent us leaking memory in that is there was a board allocated and then we recalled this function we would over write the pointer to the original board, and this would mean that we would lose our 'handle' to the first board.
Notice that every call to malloc is check to make sure that we actually got the memory that we wanted. I tend to place the check in the same statement as the malloc, but that is personal preference.
I now actually have a significant return value. In you original code, you would just return true regardless if all the allocations succeeded or not. Notice, that I only return true after both allocations are performed, and they succeeded.

Ok, on the the new function I added, board_destroy:

    void board_destroy(board_t** b)
    {
        if(*b != NULL)   // no board struct, nothing to do..
        {
            if((*b)->moves != NULL)
            {
                free((*b)->moves);
            }
            free(*b);
           *b = NULL;
        }
    }

Some comments on this function;

a bit more defensive programming, I check to make sure we actually have a board structure to get rid of prior to doing any work.
remember that in your board structure, you have a dynamic array, so you need to free that array first. (free-ing the board structure first would mean that you lost your only reference to the moves array, and you would be leaking memory then).
Prior to free-ing the moves array, I again check to see that it exists.
Once the moves array is destroyed, I proceed to destroy the board structure, and set the pointer back to NULL (in case we want to reuse the board pointer in main).

You didn't provide implementation details of board_get_* functions, but from their usage, I suspect that you have them implemented as:

int board_get_height(const board_t* b)
{
    return (b->height);
}

int board_get_width(const board_t* b)
{
    return (b->width);
}

I didn't do anything with your board_can_more function due to not being sure how you intend to use it.

A quick run of the above code:

******@ubuntu:~/junk$ ./connect4
board dimensions: 4 by 4
******@ubuntu:~/junk$

My personal opinion is that when doing lots of memory allocations, frees in C or C++ you should run your program under valgrind periodically to make sure you are not leaking memory or have other memory related errors. Below is a sample of running this code under valgrind:

    *****@ubuntu:~/junk$ valgrind --tool=memcheck --leak-check=full ./connect4
    ==4265== Memcheck, a memory error detector
    ==4265== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
    ==4265== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
    ==4265== Command: ./connect4
    ==4265== 
    board dimensions: 4 by 4
    ==4265== 
    ==4265== HEAP SUMMARY:
    ==4265==     in use at exit: 0 bytes in 0 blocks
    ==4265==   total heap usage: 2 allocs, 2 frees, 152 bytes allocated
    ==4265== 
    ==4265== All heap blocks were freed -- no leaks are possible
    ==4265== 
    ==4265== For counts of detected and suppressed errors, rerun with: -v
    ==4265== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Hope this helps, T.

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