pointer dereferencing outside of function call doesnt compile, but inside does

Question

I have come across this error i can not explain. Was working on a liked list:

main.c

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

int main(int argc, char *argv[]) {
    
    Container* c = initList((void*)0x1);

    void* value1 = list_get_content(c,0);  //1.
    void* value2 = list_get(c,0)->content; //2.

    printf("%p",value1);
    printf("%p",value2);
    
    return 0;
}

1. Works fine, but 2. doesn't compile, eventhough they should do the same thing.
Why does the function call make a diffrence here?

list_get_content is just a wrapper for list_get()->content

Compiled using MinGW GCC 4.7.2 in Dev-C++.
No parameters or settings

list.h:

struct _Node;
typedef struct _Node Node;
 
struct _Container;
typedef struct _Container Container;
 
Node* createNode(Node* last, Node* next, void* content);
Container* initList(void* firstValue);
 
void list_add(Container* list,void* e);
void* list_get_content(Container* list,int index);
Node* list_get(Container* list,int index);
void list_remove(Container* list,int index);

list.c:

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

struct _Node{
    void* content;
    struct Node* last;
    struct Node* next;
};

struct _Container{
    Node* start;
    Node* end;
};

Node* createNode(Node* last, Node* next, void* content){
    Node *n = (Node *) malloc(sizeof(Node));
    n->content = content;
    n->last = last;
    n->next = next;
    return n;
}

Container* initList(void* firstValue){
    Container *c = (Container *) malloc(sizeof(Container));
    Node* n = createNode(NULL,NULL,firstValue);
    c->start = n;
    c->end = n;
    return c;
}

Node* list_get(Container* list,int index){
    Node* n = list->start;
    for(int i=0;i<index;i++){
        n = n->next;
    }   
    return n;
}

void* list_get_content(Container* list,int index){
    return list_get(list,index)->content;
}

Answer 1

Your header file only contains declarations for struct _Node and struct _Container, while the definitions reside in list.c. So for main.c those types are incomplete.

This means that in main.c you can use pointers to those types but cannot dereference them because it doesn't have the definition of those structures.

If you want the definitions of struct _Node and struct _Container to be viewable outside of list.c then you need to move them into list.h. Otherwise you need to rely on functions in list.c to return any data inside of them.

You also have a problem here:

struct _Node{
    void* content;
    struct Node* last;
    struct Node* next;
};

The next and last fields are pointing to a different struct type which has not been defined. This results in a type mismatch when you later attempt to assign values to those pointers. You want:

struct _Node{
    void* content;
    struct _Node* last;
    struct _Node* next;
};

Answer 2

I know it doesn't provide a solution for your problem, but please please please stop using Dev-C++. It might still receive updates (from whom I don't know, since it's said to be unmaintained), but the compiler version it ships with is completely obsolete (even for toy projects or school assignments) and last time I've heard about it, even the IDE experience was very lacking.

If you're on Windows you can install a more recent version of gcc with MinGW and run it manually while programming on something like VSCode; it's not very straightforward to configure, but it's probably the best "enhanced code editor" around. If you want a full fledged IDE there's Visual Studio Community shipped with (but not limited to) the MSVC++ compiler, which might not be 100% compatible with the more recent C standards but at least it doesn't require a convoluted setup.

On Mac there's LLVM's clang compiler preinstalled and you could use XCode, while in Linux the preinstalled one is gcc and it seems that KDevelop is a very good IDE. Of course you can always use VSCode as it's multiplatform.