How do I reference a datatype that hasn't yet been declared?

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If I have the following C code:

C++

struct A{struct B* element;}
struct B{struct A* element;}
main(){}

How do I compile it if when I declare the variable element of type "struct B*" this type wasn't defined yet?

And how would I make the makefile and the header files if I had those 2 structs defined in diferent files:

fileA.h:

C++

#ifndef _FILEA_H_
#define _FILEA_H_
#include "fileB.h"
struct A{struct B* element;}
#endif

fileB.h:

C++

#ifndef _FILEB_H_
#define _FILEB_H_
#include "fileA.h"
struct B{struct A* element;}
#endif

MAKEFILE:

CC = gcc
CFLAGS = -Wall -g
LDFLAGS = -lm

all: example

program.o: program.c

fileA.o: fileA.c fileA.h

fileB.o: fileB.c fileB.h

example: principal.o fileA.o fileB.o
	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)

Can you guys help me? In both cases the compiler doesn't recognize the type inside the struct because it is only defined later.

Posted 19-Jun-15 19:59pm

Member 11779222

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Sergey Alexandrovich Kryukov · Accepted Answer · 2015-06-19T20:23:00

Solution 1

First, write forward declaration of one of the struct, then fully define the other struct below, and, finally, fully define the first structure represented by the forward declaration above. The C syntax is pretty awkward; please see:
https://en.wikipedia.org/wiki/Struct_%28C_programming_language%29#Struct_initialization[^].

—SA

Posted 19-Jun-15 20:23pm

Sergey Alexandrovich Kryukov

Updated 19-Jun-15 20:24pm

v2

Comments

CPallini 20-Jun-15 4:45am

5.

Sergey Alexandrovich Kryukov 20-Jun-15 8:36am

Thank you, Carlo.
—SA

CPallini · Accepted Answer · 2015-06-19T22:51:00

Sergey already gave you the correct answer, I just add an example:

File a.h

C

#ifndef __A_H__
#define __A_H__
struct A; // forward declaration of A
struct B { struct A * element; };
#endif // __A_H__

File b.h

C

#ifndef __B_H__
#define __B_H__
struct B; // forward declaration of B
struct A { struct B * element; };
#endif // __B_H__

File m.c

C

#include "a.h"
#include "b.h"
int main()
{
  struct A a;
  struct B b;
  a.element = &b;
  b.element = &a;
  return 0;
}

An alternative would be putting forward declarations in the C source file, before actually including the headers:

C

#ifndef __A_H__
#define __A_H__
// requires the forward declaration of A
struct B { struct A * element; };
#endif // __A_H__

C

#ifndef __B_H__
#define __B_H__
// requires the forward declaration of B
struct A { struct B * element; };
#endif // __B_H__

C

// forward declarations
struct A;
struct B;
// headers
#include "a.h"
#include "b.h"

int main()
{
  struct A a;
  struct B b;
  a.element = &b;
  b.element = &a;
  return 0;
}