Difficulty debugging my linked-list in my C program

Question

I have this structure containing links to the same structure type (peca). The problem is that when I try and use peca->penext in a different function, the debugger shows that the structure peca is not linked to the next or previous. What am I doing wrong?

typedef struct peca{
    int index;
    int esquerda;
    int direita;
    int disponibilidade;
    struct peca *penext;
    struct peca *peant;
}PECA;

typedef struct mao{
    int index;
    int tamanho;
    PECA *ppeca;
    struct mao *pnext;
}MAO;

typedef struct jogada{
    int index;
    PECA *ppeca;
    struct jogada *pnext;
}JOGADA;

typedef struct jogo{
    int nr_maos;
    MAO *pmao;
    JOGADA *pjog;
}JOGO;

void init_jogada(JOGADA *pj) {
pj->ppeca = (PECA *) malloc(sizeof(PECA) * 28);
PECA *paux = pj->ppeca;
  for (int i = 0; i < 28; ++i) {
    paux->index = i;
    paux->disponibilidade = 1;
    if (i == 0)
    {
        paux->peant = NULL;
        paux->penext = paux++;
    }
    else if (i == 27)
    {
        paux->peant = paux--;
        paux->penext = NULL;
    }
    else
    {
        paux->penext = paux++;
        paux->peant = paux--;
    }
    paux++;
  }


}

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, Jan 01 '17 at 12:25
why not? this way when the peca is the next one, paux-- will point to the previous. — João Rodrigo Castro, Jan 01 '17 at 12:30
`paux->penext = paux++;` is equivalent to `paux->penext = paux; paux += 1; ` probably not what you want? — Unimportant, Jan 01 '17 at 12:32
so I should do first paux++; and then paux->penext = paux right? — João Rodrigo Castro, Jan 01 '17 at 12:34
@João Rodrigo Castro : or `paux->penext = ++paux; `. Same problem with the `paux--`. — Unimportant, Jan 01 '17 at 12:35
I know. when the for loop ends I iterate the peca struct, so that when I use peant in the next iteration of the loop the struct will be the next one and I need it to be the previous again. I know it sounds confusing but, when the for loop iterates, I need to go get the previous iteration for the peant. — João Rodrigo Castro, Jan 01 '17 at 12:45
@JoãoRodrigoCastro The title of your question is wrong, can you change it to better describe what you are really asking? It is important for the next ppl that end up here by searching. — Felipe Lavratti, Jan 01 '17 at 13:25

score 3 · Answer 1 · answered Jan 01 '17 at 13:23

The way you increment pointers to PECA is just cunfusing, that's probably what is breaking your code. Since I speak Portuguese, I can tell peant means pelast, so I can infer it is a double linked list.

Your double Linked List is absolutely redundant since your pieces (PECA is piece in english) are in a array. Linked lists are used to iterate over a sequence where items are stored sparse on memory, arrays put items next to each other, that's why you can dereference items with a index.

This is what I think you are doing worng:

    paux->penext = paux++; 
    /* Here you set the next, why increment the pointer while assigning?
       paux++ changes the value of paux. You might not want that here */

Maybe you are not aware that paux++ or ++paux or paux-- or --paux does change where paux point to.

This is your code corrected:

  for (int i = 0; i < 28; ++i) {
    paux->index = i;
    paux->disponibilidade = 1;
    if (i == 0)
    {
        paux->peant = NULL;
        paux->penext = paux +1;
    }
    else if (i == 27)
    {
        paux->peant = paux - 1;
        paux->penext = NULL;
    }
    else
    {
        paux->penext = paux + 1;
        paux->peant = paux - 1;
    }
    /* Ok! Now we really want to increment paux, to use in the next iteration */
    paux++;
  }

score 1 · Answer 2 · answered Jan 01 '17 at 13:22

As a general rule, Ddn't make special cases in the for loop. Take the code inside if (i==0) and put it before the loop, and the code inside if (i==27) outisde the loop. It makes the loop itself more simple to the reader and avoid problems in case you change the number of iterations and forget to change the conditions.

Your code will look much simpler if you don't modify paux inside the loop. Use the iterator i to access the objects. If you really want to use paux to simplify your syntax, either make it the iterator or assign it at the beginning of the loop:

paux = pj->ppeca + i;

Then you can find the next and the previous with paux + 1 and paux - 1.

But do you really need next/previous pointers? Without seeing the rest of the code, this may sound a silly question to you, but why do you have a structure that resembles a doubly-linked list when your elements are already allocated into an array?

Difficulty debugging my linked-list in my C program

2 Answers2