Programming Forums > Application Development > C

mevuorin · Jul 6th, 2005, 9:22 AM

Hi

I'm having problems compiling structs. I tried to find a solution from my K&R C book and the web, but couldn't.

Basically I'm trying to have two structs that both contain member whose type is the other struct. I've tried a number of ways to get it to work, but have had no luck.

Here's my latest attempt ( simplified from my code) :

a.h:

(Toggle Plain Text)

#ifndef _A_H_ 
#define _A_H_ 
#include"b.h"

typedef struct{
    b_struct bs; 
} a_struct;
#endif

b.h:

(Toggle Plain Text)

#ifndef _B_H_
#define _B_H_ 
#include"a.h"

typedef struct{
    a_struct as;
} b_struct;
#endif

c.c:

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#include"a.h"

When I try to compile, I get:

$ gcc c.c
In file included from a.h:4,
from c.c:1:
b.h:7: error: parse error before "a_struct"
b.h:7: warning: no semicolon at end of struct or union
b.h:8: warning: data definition has no type or storage class
In file included from c.c:1:
a.h:7: error: parse error before "b_struct"
a.h:7: warning: no semicolon at end of struct or union
a.h:8: warning: data definition has no type or storage class

If someone could tell me how to modify my code so that it would compile, or propose a different way of creating two structs that both have a member whose type is the other, it would be greatly appreciated. It would be even better if someone could explain to me, why my code doesn't compile but the correct code having those two structs does.

Thanks for reading and for any advice/pointers to advice in advance.

L7Sqr · Jul 6th, 2005, 1:01 PM

(Toggle Plain Text)

struct first;

struct second {
   int x;
   struct first * f;
};

struct first {
   int y;
   struct second * g;
};

Works just fine for me.....

jim mcnamara · Jul 6th, 2005, 5:55 PM

L7sqr - the reason it works is because you created a pointer to a struct, pointers have size which is always known by the compiler. Drop the * and see if you can make it work with a forward reference.

uman · Jul 6th, 2005, 6:12 PM

The problem is because the include files can only be included once. That's your syntax errors. The other problem is the fact that you can't do that in C. A b_struct contains an a_struct which contains a b_struct which contains an a_struct, forever.

Ooble · Jul 6th, 2005, 6:51 PM

Yes you can. L7Sqr demonstrated it quite well. If you want to apply it to mevuorin's stuff, here's how it works:

(Toggle Plain Text)

/* globals.h */
struct _a_struct;
struct _b_struct;

/* a.h */
#ifndef _A_H_ 
#define _A_H_ 

#include "b.h"

typedef struct _a_struct {
    b_struct bs; 
} a_struct;

#endif

/* b.h */
#ifndef _B_H_
#define _B_H_ 

#include "a.h"

typedef struct _b_struct {
    a_struct as;
} b_struct;

#endif

uman · Jul 6th, 2005, 8:22 PM

He did it with pointers you commie bastard!

Animatronic · Jul 6th, 2005, 9:18 PM

ooble your code will not work, forward defines can only be used to declare references (c++) or pointers. In your code the compiler work know the size of the forward declared structures. This problem can only be solved by using pointers in one, or both, of the structues (as shown in L7Sqr's code).

uman · Jul 6th, 2005, 9:53 PM

nyah, nyah.
Oobs was wrong!
(note to self: increase quality of future posts.)

L7Sqr · Jul 6th, 2005, 10:16 PM

Quote:

Originally Posted by jim mcnamara

L7sqr - the reason it works is because you created a pointer to a struct, pointers have size which is always known by the compiler. Drop the * and see if you can make it work with a forward reference.

c'mon, don't ruin ALL the fun

I was merely showing how the problem could be solved - no constraint was applied

Although, doing 'exactly' what was asked is not possible - as is well pointed out by previous posts.

mevuorin · Jul 7th, 2005, 3:58 AM

Thanks to everyone for help. Now I understand what the problem was and managed to make my structs work.

Here's the solution I ended up with:

(Toggle Plain Text)

//A.h
#ifndef _A_H_
#define _A_H_

#include"b.h"

typedef struct _a_struct{
	struct _b_struct *bs;
} a_struct;

#endif /*_A_H_*/

//B.h
#ifndef _B_H_
#define _B_H_

#include"a.h"

typedef struct _b_struct{
	struct _a_struct *as;
} b_struct;

#endif /*_B_H_*/

//C.c
#include"a.h"

Compiles and works. Finally.

Big thanks to everyone again. That struct problem had made my head hurt for few days. You cured it. I think I'll post here more.

Jul 6th, 2005, 9:22 AM	#1
mevuorin Newbie Join Date: Jul 2005 Posts: 10 Rep Power: 0	c struct : problems whit structs containing each other Hi I'm having problems compiling structs. I tried to find a solution from my K&R C book and the web, but couldn't. Basically I'm trying to have two structs that both contain member whose type is the other struct. I've tried a number of ways to get it to work, but have had no luck. Here's my latest attempt ( simplified from my code) : a.h: (Toggle Plain Text) #ifndef _A_H_ #define _A_H_ #include"b.h" typedef struct{ b_struct bs; } a_struct; #endif #ifndef _A_H_ #define _A_H_ #include"b.h" typedef struct{ b_struct bs; } a_struct; #endif b.h: (Toggle Plain Text) #ifndef _B_H_ #define _B_H_ #include"a.h" typedef struct{ a_struct as; } b_struct; #endif #ifndef _B_H_ #define _B_H_ #include"a.h" typedef struct{ a_struct as; } b_struct; #endif c.c: (Toggle Plain Text) #include"a.h" #include"a.h" When I try to compile, I get: $ gcc c.c In file included from a.h:4, from c.c:1: b.h:7: error: parse error before "a_struct" b.h:7: warning: no semicolon at end of struct or union b.h:8: warning: data definition has no type or storage class In file included from c.c:1: a.h:7: error: parse error before "b_struct" a.h:7: warning: no semicolon at end of struct or union a.h:8: warning: data definition has no type or storage class If someone could tell me how to modify my code so that it would compile, or propose a different way of creating two structs that both have a member whose type is the other, it would be greatly appreciated. It would be even better if someone could explain to me, why my code doesn't compile but the correct code having those two structs does. Thanks for reading and for any advice/pointers to advice in advance.

Jul 6th, 2005, 1:01 PM	#2
L7Sqr Hobbyist Programmer Join Date: Jun 2005 Location: here Posts: 146 Rep Power: 4	(Toggle Plain Text) struct first; struct second { int x; struct first * f; }; struct first { int y; struct second * g; }; struct first; struct second { int x; struct first * f; }; struct first { int y; struct second * g; }; Works just fine for me..... __________________ "...and though our kids are blessed their parents let them shoulder all the blame." - The Quiet Things That No One Ever Knows [BrandNew]

Jul 6th, 2005, 6:12 PM	#4
uman Expert Programmer Join Date: Dec 2004 Posts: 794 Rep Power: 4	The problem is because the include files can only be included once. That's your syntax errors. The other problem is the fact that you can't do that in C. A b_struct contains an a_struct which contains a b_struct which contains an a_struct, forever. __________________ Few people deserve to be compared to (Rush) Limbaugh, most of them were convicted at the Nuremburg trials. --WilliamSChips on Slashdot

Jul 6th, 2005, 6:51 PM	#5
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	Yes you can. L7Sqr demonstrated it quite well. If you want to apply it to mevuorin's stuff, here's how it works: (Toggle Plain Text) /* globals.h / struct _a_struct; struct _b_struct; / a.h / #ifndef _A_H_ #define _A_H_ #include "b.h" typedef struct _a_struct { b_struct bs; } a_struct; #endif / b.h / #ifndef _B_H_ #define _B_H_ #include "a.h" typedef struct _b_struct { a_struct as; } b_struct; #endif / globals.h / struct _a_struct; struct _b_struct; / a.h / #ifndef _A_H_ #define _A_H_ #include "b.h" typedef struct _a_struct { b_struct bs; } a_struct; #endif / b.h */ #ifndef _B_H_ #define _B_H_ #include "a.h" typedef struct _b_struct { a_struct as; } b_struct; #endif __________________ Me :: You :: Them

Jul 6th, 2005, 8:22 PM	#6
uman Expert Programmer Join Date: Dec 2004 Posts: 794 Rep Power: 4	He did it with pointers you commie bastard! __________________ Few people deserve to be compared to (Rush) Limbaugh, most of them were convicted at the Nuremburg trials. --WilliamSChips on Slashdot

Jul 6th, 2005, 5:55 PM	#3
jim mcnamara Hobbyist Programmer Join Date: Jun 2005 Location: New Mexico Posts: 228 Rep Power: 4	L7sqr - the reason it works is because you created a pointer to a struct, pointers have size which is always known by the compiler. Drop the * and see if you can make it work with a forward reference.

Jul 6th, 2005, 9:18 PM	#7
Animatronic Programmer Join Date: Jun 2005 Posts: 99 Rep Power: 4	ooble your code will not work, forward defines can only be used to declare references (c++) or pointers. In your code the compiler work know the size of the forward declared structures. This problem can only be solved by using pointers in one, or both, of the structues (as shown in L7Sqr's code).

Jul 6th, 2005, 9:53 PM	#8
uman Expert Programmer Join Date: Dec 2004 Posts: 794 Rep Power: 4	nyah, nyah. Oobs was wrong! (note to self: increase quality of future posts.) __________________ Few people deserve to be compared to (Rush) Limbaugh, most of them were convicted at the Nuremburg trials. --WilliamSChips on Slashdot

Jul 7th, 2005, 3:58 AM	#10
mevuorin Newbie Join Date: Jul 2005 Posts: 10 Rep Power: 0	Thanks to everyone for help. Now I understand what the problem was and managed to make my structs work. Here's the solution I ended up with: (Toggle Plain Text) //A.h #ifndef _A_H_ #define _A_H_ #include"b.h" typedef struct _a_struct{ struct _b_struct bs; } a_struct; #endif /_A_H_/ //B.h #ifndef _B_H_ #define _B_H_ #include"a.h" typedef struct _b_struct{ struct _a_struct as; } b_struct; #endif /_B_H_/ //C.c #include"a.h" //A.h #ifndef _A_H_ #define _A_H_ #include"b.h" typedef struct _a_struct{ struct _b_struct bs; } a_struct; #endif /_A_H_/ //B.h #ifndef _B_H_ #define _B_H_ #include"a.h" typedef struct _b_struct{ struct _a_struct as; } b_struct; #endif /_B_H_/ //C.c #include"a.h" Compiles and works. Finally. Big thanks to everyone again. That struct problem had made my head hurt for few days. You cured it. I think I'll post here more.

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Programming Forums > Application Development > C

c struct : problems whit structs containing each other