CODEWITHSUNDEEP
cstructmalloc
Vinicius Garcia
Vinicius Garcia

Reputation: 1748

C: error using struct with variable arrays

I have a struct like this:

typedef struct tgPoligono {
    int numero_de_lados;
    CvPoint** cantos;
    float* lados;
    double* angulos;
} *tgPoligono;

In a function, I'm using it like this:

tgPoligono criar_tgPoligono(CvSeq* poligono){
    int i;
    tgPoligono tg_poligono = (tgPoligono) malloc(sizeof(tgPoligono));

    tg_poligono->numero_de_lados = poligono->total;

    tg_poligono->cantos = (CvPoint**) malloc(tg_poligono->numero_de_lados * sizeof(CvPoint*));
    for(i= 0; i < tg_poligono->numero_de_lados; i++){
        tg_poligono->cantos[i] = (CvPoint*)cvGetSeqElem(poligono, i);
    }

    tg_poligono->lados = (float*) malloc(tg_poligono->numero_de_lados * sizeof(float));
    tg_poligono->angulos = (double*) malloc(tg_poligono->numero_de_lados * sizeof(double));
    double* angulos = (double*) malloc(tg_poligono->numero_de_lados * sizeof(double));
    for(i = 0; i < tg_poligono->numero_de_lados; i++){
        CvPoint* pt_0 = tg_poligono->cantos[((i == 0)?(tg_poligono->numero_de_lados - 1):(i - 1))];     //Canto anterior
        CvPoint* pt_1 = tg_poligono->cantos[i];                                                         //Canto atual
        CvPoint* pt_2 = tg_poligono->cantos[(i + 1) % tg_poligono->numero_de_lados];                    //Canto seguinte

        //Calculo a distancia entre dois cantos, o que retorna o comprimento do lado do poligono
        tg_poligono->lados[i] = sqrt(((pt_1->x - pt_2->x)*(pt_1->x - pt_2->x)) + ((pt_1->y - pt_2->y)*(pt_1->y - pt_2->y)));

        //Calculo o cosseno do angulo correspondente ao ponto atualmente avaliado
        tg_poligono->angulos[i] = cosseno(pt_0, pt_2, pt_1);
        angulos[i] = cosseno(pt_0, pt_2, pt_1);
    }

    return tg_poligono;
}

Ok, my problem is with the tg_poligono->angulos tg_poligono->lados works fine but tg_poligono->angulos fails with apparently no reason!

when I use de function in my program (inside a for statment), the code above works for a while and then fails with no message.

If I comment the following lines, the program work fine, with no errors:

tg_poligono->angulos = (double*) malloc(tg_poligono->numero_de_lados * sizeof(double));
...
tg_poligono->angulos[i] = cosseno(pt_0, pt_2, pt_1);

If you look at my code, you'll see a test that i do:

double* angulos = (double*) malloc(tg_poligono->numero_de_lados * sizeof(double));
...
angulos[i] = cosseno(pt_0, pt_2, pt_1);

Its the same thing but not using the struct and it works fine.

ps.: I'm using opencv functions on this code, but it's unrelated with the problem. the function cosseno(pt_0, pt_2, pt_1) works fine, I sure of that. It's a C code, I can't use C++.

Upvotes: 0

Views: 143

Answers (2)

noelicus
noelicus

Reputation: 15055

This is how you want to define your structure, I think:

typedef struct _tgPoligono {
    int numero_de_lados;
    CvPoint** cantos;
    float* lados;
    double* angulos;
} tgPoligono;

tgPoligono criar_tgPoligono(CvSeq* poligono){
    int i;
    tgPoligono *tg_poligono = (tgPoligono*) malloc(sizeof(tgPoligono));

etc...

Note the removal of the star from the end of your structure definition and typedef. That's what's confusing you I think.

Upvotes: 1

fizzer
fizzer

Reputation: 13796

tgPoligono tg_poligono = (tgPoligono) malloc(sizeof(tgPoligono));

only allocates space for the pointer. Use sizeof(*tgPoligono).

Upvotes: 1

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