"Error: unknown type name" - Using a struct alias within function declaration

Question

In the file: "priority_queue.h", I have defined the struct:

#include "astar.h"
typedef struct p_q{
struct astar_node* first;
struct astar_node* last;
int size;
 }P_Q;

The file "astar.h" looks like this:

#include "priority_queue.h"

typedef struct astar_node{
struct astar_node* next;
struct astar_node* prev;
struct astar_node* PQ_next;
struct astar_node* PQ_prev;
int x;
int y;
int c;
int h;
int tot_cost;
}Astar_node;

int func(P_Q* P);

Why do I get the error: "Unknown typename 'P_Q' ?

If i redefine "funk()" to :

 int func(struct p_q* P);

the error disappears but i get the warning:"Description Resource Path Location Type 'struct p_q' declared inside parameter list will not be visible outside of this definition or declaration"

Anyone knows why?

in the header "astar.h" - This header is also included in "priority_queue.h" the function: int func(P_Q* P) is implemented in "priority_queue.c" - There are no errors connected to the type "P_Q" in the implementation — Erlend
– Erlend, Commented May 26, 2018 at 5:28
So priority_queue.h includes astar.h and astar.h includes priority_queue.h, right? Sounds problematic. — 4386427
– 4386427, Commented May 26, 2018 at 5:30
That is correct. My intuition is that since I need "P_Q" in astar.h, I must include priority_queue.h in astar.h , and since I need "struct astar_node" in priority_queue.h I must include astar.h — Erlend
– Erlend, Commented May 26, 2018 at 5:33
Circular includes shall be avoided. Instead you can use a forward declaration in priority_queue.h, i.e. instead of including astart.h, try struct astar_node; before defining P_Q — 4386427
– 4386427, Commented May 26, 2018 at 5:36

4386427 · Accepted Answer · 2018-05-26 08:08:24Z

1

Your includes are circular, i.e priority_queue.h includes astar.h which includes priority_queue.h. That is something you should avoid.

Try changing:

#include "astar.h"
typedef struct p_q{
    struct astar_node* first;
    struct astar_node* last;
    int size;
} P_Q;

into

// Forward declaration
struct astar_node;

typedef struct p_q{
    struct astar_node* first;
    struct astar_node* last;
    int size;
} P_Q;

You can do this because you only use pointers to struct astar_node inside priority_queue.h. So the compiler doesn't need to know what is inside struct astar_node. All it needs to know is that there is a struct named astar_node somewhere in your project.

answered May 26, 2018 at 8:08

4386427

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