This code gives me undefined behaviour on Windows, but works fine on Linux

Question

I'm trying to write a small doubly linked lists program in C, but for some reason it gives me undefined behavior for the first element. I want it to have an empty cell at the beginning which links the first and last elements. So it's like this: ... <-> Second Last <-> Last <-> Empty Cell <-> First <-> Second <->...

The first element is a random value but the next ones work. For example, if my input file is 1 2 3 4 5, the output will be <undefined> 2 3 4 5, where undefined can be any number C wishes to give me.

The weird part is that it works flawlessly in debug mode too (using MinGW Developer Studio as I got used to it from school). It also works good under Linux (using gcc for compilation).

This is the code:

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

typedef struct Nod {
struct Nod *next, *ant;
int x;
} Nod_t, *List_t, **AList_t;

void PrintList (List_t sant){
List_t lista = sant->next;
while(lista != sant){
    printf("%i ", lista->x);
    lista = lista->next;
}
}

List_t PopulateList(char* fis){

List_t lista, sant;
int nr;
FILE *f = fopen(fis, "rt");

sant = (List_t)malloc(sizeof(List_t));
sant->next = sant->ant = NULL;
lista = sant;
//First node

while(!feof(f)){
    fscanf(f, "%i", &nr);
    lista->next = (List_t)malloc(sizeof(List_t));
    lista->next->x = nr;
    lista->next->ant = lista;
    lista = lista->next;
}

sant->ant = lista;
lista->next = sant;

return sant;

}

int main (){

List_t lista1;
lista1 = PopulateList("1.txt");
PrintList(lista1);
return 0;
}

Way better indentation here: http://pastebin.com/NVQqaYHK

Answer 1

Try changing your mallocs from

sant = (List_t)malloc(sizeof(List_t));

to

sant = malloc(sizeof(Nod_t));

sant = malloc(sizeof(List_t)); returns a pointer to area the size of a pointer.

sant = malloc(sizeof(Nod_t)); returns a pointer to area the size of Nod_t.

By the way, it's a matter of style, but your type aliases are confusing and probably led to your typo in the mallocs. I'd suggest just using the Nod_t type and get rid of List_t and AList_t. Consider these declarations:

List_t foo; // is this a pointer or a struct? Can't tell from the decl, need to know the typedef.
Nod_t *foo; // obviously a pointer

See also Zack's suggestions, especially concerning the feof() issues.

Answer 2

The bug is here:

sant = (List_t)malloc(sizeof(List_t));
sant->next = sant->ant = NULL;
lista = sant;

You do not initialize sant->x, so you have an entry in the list (before all the entries containing real data from the file) with an uninitialized x.

When you later print out x for all the entries in the list, that triggers undefined behavior (on both Linux and Windows, to be clear; undefined behavior includes the possibility that the value printed will always be zero).

You should restructure the loop immediately after the code shown, so that you don't allocate the first entry in the list until after you've read the first line in the file, and you don't have this initial list entry with an uninitialized value.

You also need the bugfix mentioned by Charlie Burns.