question about my sendings - dynamic allocation

Question

I'm working on my student project about dynamic allocation and I think I have a problem with my sending, someone can help?

void buildBoard(int*** mat, int size);
void initMat(int*** mat, int size);

int main()
{
    int size;
    int** mat;
    printf("Please enter a size of the matrix:\n");
    scanf("%d", &size);
    buildBoard(&mat, size);
    initMat(&mat, size);
    for (int i = 0; i < size; ++i)
    {
        for (int j = 0; j < size; ++j)
        {
            printf("%d ", mat[i][j]);
        }
        printf("\n");
    }
    return 0;
}

void buildBoard(int*** mat, int size)
{
    *mat = (int**)malloc(size * sizeof(int*));
    if (*mat == NULL)
    {
        printf("Bye\n");
        exit(1);
    }
    for (int i = 0; i < size; ++i)
    {
        mat[i] = (int*)malloc(size * sizeof(int));
        if (mat[i] == NULL)
        {
            printf("Bye\n");
            free(mat[i]);
        }
    }
}

void initMat(int*** mat, int size)
{
    int num;
    printf("Please enter a numbers:\n");
    for (int i = 0; i < size; ++i)
    {
        for (int j = 0; j < size; ++j)
        {
            scanf("%d", &mat[i][j]);
        }
    }
}

In the main function I want to check my allocation, and it crashes all the time on when printing.

Please read this: [Do I cast the result of malloc?](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — P.W, Feb 05 '19 at 10:49
you missed to dereference _mat_ in some places, see my answer — bruno, Feb 05 '19 at 11:05

bruno · Accepted Answer · 2019-02-05T11:04:24.163

You missed to dereference mat several times, I mean some mat must be (*mat) :

void buildBoard(int*** mat, int size)
{
    *mat = (int**)malloc(size * sizeof(int*));
    if (*mat == NULL)
    {
        printf("Bye\n");
        exit(1);
    }
    for (int i = 0; i < size; ++i)
    {
        (*mat)[i] = (int*)malloc(size * sizeof(int));
        if ((*mat)[i] == NULL)
        {
            printf("Bye\n");
            exit(1);
        }
    }
}

void initMat(int*** mat, int size)
{
    printf("Please enter a numbers:\n");
    for (int i = 0; i < size; ++i)
    {
        for (int j = 0; j < size; ++j)
        {
            scanf("%d", &(*mat)[i][j]);
        }
    }
}

I also removed the useless variable num

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -g -pedantic -Wall c.c
pi@raspberrypi:/tmp $ ./a.out
Please enter a size of the matrix:
2
Please enter a numbers:
1
2
3
4
1 2 
3 4

Under valgrind :

pi@raspberrypi:/tmp $ valgrind ./a.out
==4232== Memcheck, a memory error detector
==4232== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==4232== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==4232== Command: ./a.out
==4232== 
Please enter a size of the matrix:
2
Please enter a numbers:
1
2
3
4
1 2 
3 4 
==4232== 
==4232== HEAP SUMMARY:
==4232==     in use at exit: 24 bytes in 3 blocks
==4232==   total heap usage: 5 allocs, 2 frees, 2,072 bytes allocated
==4232== 
==4232== LEAK SUMMARY:
==4232==    definitely lost: 8 bytes in 1 blocks
==4232==    indirectly lost: 16 bytes in 2 blocks
==4232==      possibly lost: 0 bytes in 0 blocks
==4232==    still reachable: 0 bytes in 0 blocks
==4232==         suppressed: 0 bytes in 0 blocks
==4232== Rerun with --leak-check=full to see details of leaked memory
==4232== 
==4232== For counts of detected and suppressed errors, rerun with: -v
==4232== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)
pi@

Of course valgrind indicates memory leaks because you do not free the allocated memory

Note that it is useless to give the address of the variable into initMat, can be :

void initMat(int** mat, int size)
{
    printf("Please enter a numbers:\n");
    for (int i = 0; i < size; ++i)
    {
        for (int j = 0; j < size; ++j)
        {
            scanf("%d", &mat[i][j]);
        }
    }
}

of course also changing its declaration and the call in main

Previously I also removed the free you had in :

    if (mat[i] == NULL)
    {
        printf("Bye\n");
        free(mat[i]);
    }

because in fact you free NULL

it's help me so much, theank you! but can you explain me what did you mean in (*mat)[i]? i need to understand thet for sure! — Avihay Maman, Feb 05 '19 at 11:11

question about my sendings - dynamic allocation

1 Answers1