Pointers and data type hidden in C

Question

EDIT: I’ll leave the codes right below in text same.

Good evening. Please see attached image.

It is the implementation of a function whose prototype is defined in another file named "stack. h". As it stands, it works perfectly.

However, if I delete line 11 and modify line 5 to typedef struct { .... } stack; i have a compilation error:

pile. c: In Function 'breed:

stack. c:15:6: error: Dereferencing Pointer to incomplete type ːstruct pile'

 p->v = malloc(tamanho * sizeof(char));
  ^~

Could someone help me understand this mistake?

From now on, thank you!

--------------- pile. h -----------------

typedef struct pilha *Pilha;

Pilha criaPilha(int tamanho);

int pilhaCheia(Pilha p);

int pilhaVazia(Pilha p);

void empilha(Pilha p, char elemento);

char desempilha(Pilha p);

int topo(Pilha p);

char eleTopo(Pilha p);

---

----------- pile. c -----------

#include <stdio.h>
#include <stdlib.h>
#include "pilha.h"

struct pilha {
    char *v;
    int topo;
    int tamanho;
};

typedef struct pilha pilha;

Pilha criaPilha(int tamanho) {
    Pilha p = malloc(sizeof(pilha));
    p->v = malloc(tamanho * sizeof(char));
    p->topo = 0;
    p->tamanho = tamanho;

    return p;
}

int pilhaCheia(Pilha p) {
    if (p->topo > p->tamanho)
        return 1;
    return 0;
}

int pilhaVazia(Pilha p) {
    if (p->topo == 0)
        return 1;
    return 0;
}

void empilha(Pilha p, char elemento) {
    if (!pilhaCheia(p)) {
        p->topo++;
        p->v[p->topo] = elemento;
    }
    else
        printf("Pilha cheia!\n");
}

char desempilha(Pilha p) {
    if (!pilhaVazia(p)) {
        p->topo--;
        return p->v[p->topo + 1];
    }
    else
        return '\0';
}

int topo(Pilha p) {
    return p->topo;
}

char eleTopo(Pilha p) {
    return p->v[p->topo];
}

Answer 1

typedef struct Pilha {
    char *v;
    int topo;
    int tamanho;
} Pilha;

Pilha *criaPilha(int tamanho) {
    Pilha *p = (Pilha *) malloc(sizeof(Pilha));
    p->v = (char *) malloc(tamanho);
    p->topo = 0;
    p->tamanho = tamanho;
    return p;
}

There are some things here to be observed:

1. You mustn’t confuse pilha with Pilha. Remember that C is case sensitive. Have Pilha and pilha how different things it is to ask to do lambança.

2. The function malloc always returns a pointer. So don’t forget the * on the pointer.

3. Note the strange syntax of typedef. She says the nickname given to struct Pilha {...} is Pilha.

4. Place Casts in the malloc is a good idea. The reason for this is that there are some interoperability problems between C and C++ if you don’t do this.

5. The standard C language says that sizeof(char) always is 1, regardless of the platform it is. Therefore, you do not need to multiply by sizeof(char) within the malloc.

6. Hide that a data type is a pointer by means of a typedef is another way to ask to make lambança.

7. In the header file pilha.h, you would only use typedef struct Pilha Pilha;, while putting the struct complete in pilha.c.