Incompatible pointer types - linked list struct inside another struct

Dev Shed Forums Sponsor:

October 20th, 2012, 11:42 PM

MuchJokes

Registered User

Join Date: Oct 2012

Posts: 3

Time spent in forums: 1 h 18 m 50 sec
Reputation Power: 0

Incompatible pointer types - linked list struct inside another struct

First, my linked list:

Code:

struct sr_tt { 
    struct sr_tt * next; 
    ... 
};

Now, due to reasons I wont disclose I've been shoving what I assume is an instance of the above in another struct:

Code:

struct sr_inst { 
    struct sr_tt * my_tt; 
    ... 
};

What happens next is in my code I have this permanent instance of sr_inst (let's call it 's') being passed around everywhere, so I would do the following to get at my_tt:

Code:

struct sr_tt * curr = s->my_tt;
...
curr = curr->next
...

Now there were no complaints from the compiler about the first line. Declaration went along just fine, apparently. But I keep getting the following warning:

"warning: assignment from incompatible pointer type" on the line that should, by all means, just let me iterate through my linked list.

Why is it doing this and how do I make it stop that.

October 21st, 2012, 12:08 AM

salem

Contributed User

Join Date: Jun 2005

Posts: 3,840

Folding Points: 153 Folding Title: Novice Folder

Time spent in forums: 2 Months 3 Weeks 2 Days 19 h 39 m 7 sec
Reputation Power: 1774

Is struct sr_tt an in-scope declaration when it gets to struct sr_inst ?
That is, in the same source file, or the same header file.

It it isn't, then the struct sr_tt in sr_inst will be distinct from your actual struct st_tt.

Can you produce a short example which fails to compile?
Because I can't see how it fails at the moment.

Code:

$ cat bar.c
#include <stdio.h>
#include <stdlib.h>

struct sr_tt { 
    struct sr_tt * next; 
};

struct sr_inst { 
    struct sr_tt * my_tt; 
};

struct sr_inst *s;

int main()
{
    struct sr_tt * curr = s->my_tt;
    curr = curr->next;
    return 0;
}
$ gcc -Wall bar.c

__________________
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper

October 21st, 2012, 12:17 AM

MuchJokes

Registered User

Join Date: Oct 2012

Posts: 3

Time spent in forums: 1 h 18 m 50 sec
Reputation Power: 0

Quote:

Originally Posted by salem

Code:

$ cat bar.c
#include <stdio.h>
#include <stdlib.h>

struct sr_tt { 
    struct sr_tt * next; 
};

struct sr_inst { 
    struct sr_tt * my_tt; 
};

struct sr_inst *s;

int main()
{
    struct sr_tt * curr = s->my_tt;
    curr = curr->next;
    return 0;
}
$ gcc -Wall bar.c

They are indeed in the same header file, sr_tt is defined right above sr_inst.

I don't know if I can produce an example per-say, because this is basically an enormous pile of code and I'm not sure I'm allowed to post any of it...

in more detail: I'm passing struct sr_inst* s into a function and then doing exactly this:

Code:

struct sr_tt * curr = s->my_tt;
    curr = curr->next;

and getting that warning.

If you can't make head or tails of what's going on, that's ok and thank you very much for trying. I'm sorry I can't provide any more information.

I should note it compiles just fine. I just keep getting that warning.

Last edited by MuchJokes : October 21st, 2012 at 12:22 AM. Reason: clicked reply early

October 21st, 2012, 12:43 AM

salem

Contributed User

Join Date: Jun 2005

Posts: 3,840

Time spent in forums: 2 Months 3 Weeks 2 Days 19 h 39 m 7 sec
Reputation Power: 1774

> I don't know if I can produce an example per-say, because this is basically an enormous pile of code and I'm not sure I'm allowed to post any of it...
Create a copy of the project and start hacking away at it to reduce it's size.

An empty main(), and delete every other function except the one in question would be a good start.

Another thing to try is just examine what the code looks like after it's been through the C pre-processor. This checks to see if anyone has been doing stupid things with #defines.

Code:

$ cat bar.c
#include <stdio.h>
#include <stdlib.h>

struct sr_tt { 
    struct sr_tt * next; 
};

//!! WTF is this?
#define sr_tt   foo

struct sr_inst { 
    struct sr_tt * my_tt; 
};

struct sr_inst *s;

int main()
{
    struct sr_tt * curr = s->my_tt;
    curr = curr->next;
    return 0;
}
$ gcc -Wall bar.c
bar.c: In function ‘main’:
bar.c:20:16: error: dereferencing pointer to incomplete type
$ gcc -Wall -E -o bar.i bar.c
$ tail -20 bar.i

struct sr_tt {
    struct sr_tt * next;
};




struct sr_inst {
    struct foo * my_tt;
};

struct sr_inst *s;

int main()
{
    struct foo * curr = s->my_tt;
    curr = curr->next;
    return 0;
}

As you can see here, what you thought was struct sr_tt has "not so mysteriously" changed into struct foo.

October 21st, 2012, 12:47 AM

MuchJokes

Registered User

Join Date: Oct 2012

Posts: 3

Time spent in forums: 1 h 18 m 50 sec
Reputation Power: 0

UPDATE:

I am the worlds biggest idiot.

I accidentally wrote

struct st_tt * next;

and even after staring at it for bloody hours I only now noticed that.

Thank you so much for the help, though! Sorry for wasting you time like that D:

Incompatible pointer types - linked list struct inside another struct

Developer Shed Advertisers and Affiliates