CODEWITHSUNDEEP
cgraphdynamic-memory-allocation
Boukharist
Boukharist

Reputation: 1229

How to allocate memory to struct in C

Hello I have this struct below :

typedef struct GRAPHE{
        int n;    /* Nombre de sommets */
        int **M;  /* Matrice d'Adjacence */
    } GRAPHE;

I need to allocate memory ,I tried this function but it doesn't seem to work

void reservation_en_memoire(int n, GRAPHE *g) {
    int i, j;
    g = (GRAPHE *) malloc(n * sizeof (struct GRAPHE));
    g->M = (int **) malloc(n * sizeof (int*));
    for (i = 0; i < n; i++)
        g->M[i][j] = (int) malloc(n * sizeof (int));

    g->n = n;

}

And in the main I'm just doing like this :

int main(int argc, char** argv) {

    GRAPHE *g;
    reservation_en_memoire(3, g);
    printf("%d", g->n);
    //  printf("%d", g->M[0][0]);

    return (EXIT_SUCCESS);
}

Can you please give me a little help here please !!

Upvotes: 1

Views: 489

Answers (2)

JimR
JimR

Reputation: 16153

There are several problems I see:

  • j is uninitialized
  • If M is supposed to be per-instance of GRAPHE, your code will not work
  • g = ... malloc( n * ... ) is allocating n copies of GRAPHE, which you then access as g->(struct_member). g points to an array after a successful call to malloc. You should be accessing it as g[index] if an array was truly your intent.
  • q is not saved after the code in reservation_en_memoire completes so you'll never see the result of reservation_en_memoire reflected in main
  • If you truly intend to allocate an array of GRAPHE, you need to loop through and initialize each member of the array.

That should be enough to get you on the right track...

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409374

You have a beginners mistake here, in that you pass the pointer g from main by-value. That means that in the reservation_en_memoire function you are only allocating memory to a copy of the pointer, a copy which disappears when the function returns (but the memory the pointer points to will not be unallocated, so you have a memory leak). When you then access g in the main function, it's still the uninitialized pointer you declared there, and you have undefined behavior.

To make it work you have to pass the pointer by reference, i.e. as a pointer to the pointer.

void reservation_en_memoire(int n, GRAPHE **g) {
    *g = malloc(sizeof (GRAPHE));

    (*g)->M = malloc(n * sizeof (int*));
    for (int i = 0; i < n; i++)
        (*g)->M[i] = malloc(n * sizeof (int));

    (*g)->n = n;
}

And call it using the address-of operator &:

GRAPHE *g;
reservation_en_memoire(3, &g);

As you can see I have made some substantial changes to your reservation_en_memoire function, except the ones needed by passing the pointer by reference. The first is that I only allocate one GRAPHE structure instead of n structures. The second is that in the loop I no longer index the second dimension of the matrix, as it's what's being allocated in the loop.

Upvotes: 5

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