> -Original Message-
> From: kernelnewbies-boun...@kernelnewbies.org [mailto:kernelnewbies-
> boun...@kernelnewbies.org] On Behalf Of valdis.kletni...@vt.edu
> Sent: Sunday, April 26, 2015 7:23 PM
> To: harshkdev
> Cc: kernelnewbies@kernelnewbies.org
> Subject: Re: Structure declaration without its members variables
>
> On Mon, 27 Apr 2015 00:16:59 +0530, harshkdev said:
>
> > struct kvm;
> > struct kvm_vcpu;
> >
> > Generally we declare structure and its member at same place.
>
> It's a forward declaration.
>
> Consider two structures that have pointers to each other:
>
> struct a {
> int b, c, d;
> struct *b b_ptr;
> }
>
> struct b {
> int foo, bar, baz;
> struct *a a_ptr;
> }
>
> Now, the struct *b in the first structure won't compile because it hasn't seen
> b yet. So we stick a 'struct b;' in front both of them so struct a can
> compile
> successfully.
This does not seem to be true, though I had to fiddle with the definitions of
structs a and b to get gcc to not generate syntax errors:
[jharan@js1]~/dev/c_fun/3$ cat main.c
#include
struct a {
int b, c, d;
struct b *b_ptr;
};
struct b {
int foo, bar, baz;
struct a *a_ptr;
};
int main(int argc, char *argv[])
{
struct a a1;
struct b b1;
printf("address of a1 %p, address of b1 %p\n", &a1, &b1);
return 0;
}
[jharan@js1]~/dev/c_fun/3$ gcc main.c
[jharan@js1]~/dev/c_fun/3$ ./a.out
address of a1 8047c90, address of b1 8047c80
[jharan@js1]~/dev/c_fun/3$ type gcc
gcc is hashed (/usr/local/bin/gcc)
[jharan@js1]~/dev/c_fun/3$ gcc --version
gcc (GCC) 3.4.6
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
[jharan@js1]~/dev/c_fun/3$
I've seen forward declarations used when the definition of struct a is in one
.h file and the definition of struct b is in another and you have source code
that wants to declare an instance of struct a or b without needing to include
both include files.
Jeff Haran
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