free() in char dynamic stack does not work - C

Question

Hello,

I’m having trouble trying to release dynamically allocated memory. For small string there is time that works, but for large string the program tops. There’s a time when a 3-character or 4-character string works, there’s a time when.

int main()
{
    TPilha* p;
    char str[10];

    scanf("%s",&str);

    p = cria_pilha(10);

    empilha(p,str);
    printf("\n\n");
    desempilha(p);

    libera_pilha(p,0);
    libera_pilha(p,1);
    libera_pilha(p,2);
    libera_pilha(p,3);
    libera_pilha(p,4);
    libera_pilha(p,5);
    libera_pilha(p,6);
    libera_pilha(p,7);
    libera_pilha(p,8);
    libera_pilha(p,9);
    libera_pilha(p,10);

    return 0;
}

At the time of releasing the memory I went position by vector position and was giving free(). I did it to try to understand the mistake.

The problem starts when I try to type something like filipi, and sometimes even something with three characters, like fhg, but strangely it works with fil or fili.

My structure is this:

typedef struct pilha TPilha;
struct pilha{
    int topo;
    int prox;
    int tamanho;
    char** PILHA;
};

I initialize her like this:

TPilha*cria_pilha(int tam){

    TPilha* aux = (TPilha*)malloc(sizeof(TPilha));
    aux->PILHA = (char**)malloc(sizeof(char*) * tam);
    aux->topo = NULL;
    aux->prox = 0;
    aux->tamanho = tam;

    return aux;
}

To stack I do so:

void empilha(TPilha* aux,char* num){

    if(pilha_vazia(aux)!=2){// Só posso empilhar se não for 2, 2 = pilha cheia

        for (int i=0; i<strlen(num); i++){
            aux->PILHA[aux->prox] = num[i];
            aux->topo = aux->prox;
            aux->prox++;
            printf("PILHA[%d] = %c EMPILHADO\n",aux->prox-1,aux->PILHA[aux->topo]);
        }
    }
}

To unstack I do:

int desempilha(TPilha* aux){
    for (int i=aux->prox; i!=0; i++){
        if(pilha_vazia(aux)==1){
            printf("Pilha vazia\n");
            return 0;
        }
        printf("PILHA[%d] = %c REMOVIDO\n",aux->topo,aux->PILHA[aux->topo]);
        aux->prox = aux->topo;
        aux->topo--;
    }
}

To check if the stack is empty:

int pilha_vazia(TPilha* aux){
    if(aux->prox == 0){
        return 1;
    }
    else if(aux->prox == aux->tamanho){
        printf("Pilha cheia\n");
        return 2; 
    }
    return 0;
}

And to release I do:

void libera_pilha(TPilha* aux,int i){
   //for (int i=0; i<aux->tamanho; i++){
        //free(aux->PILHA[i]);
        printf("liberado - %d\n",aux->PILHA[i]);
        free(aux->PILHA[i]);
    //}
    //free(aux->PILHA);
    //free(aux);
}

The idea is to do the free() with the loop, but I removed it to do one by one and understand where the error is happening, but there is a pattern in the error, there is time that works and there is time that does not. What’s going on?

Who cares I left the code in a . rar here.

Answer 1

From the information that is in the question it is possible to see that there is (at least) one thing wrong: in the creation of the pointer vector it is necessary to initialize the pointers, which is not being done:

TPilha* cria_pilha(int tam) {
    TPilha* aux = (TPilha*)malloc(sizeof(TPilha));
    aux->PILHA = (char**)malloc(sizeof(char*) * tam);
    memset(aux->PILHA, 0, sizeof(char*) * tam);        // faltando
    aux->topo = NULL;
    aux->prox = 0;
    aux->tamanho = tam;
    return aux;
}

Note: It is not customary to use uppercase names for variable or type names, only for names defined by #defines.

UPDATE

In the stack function there is also an error

void empilha(TPilha* aux, char* num ) {
  if (pilha_vazia(aux) !=2 ) { // Só posso empilhar se não for 2, 2 = pilha cheia
    for (int i = 0; i < strlen(num); i++) {
      aux->PILHA[aux->prox] = num[i]; // <------- ERRO
      aux->topo = aux->prox;
      aux->prox++;
      printf("PILHA[%d] = %c EMPILHADO\n",aux->prox-1,aux->PILHA[aux->topo]);
    }
  }
}

On the highlighted line there is an assignment of a character to a pointer:

aux->PILHA[aux->prox] = num[i]; // <------- ERRO  

I haven’t analyzed the logic of this command, but most likely there’s something wrong there.