-----Original Message----- From: kernelnewbies-bounces@kernelnewbies.org [mailto:kernelnewbies- bounces@kernelnewbies.org] On Behalf Of Valdis.Kletnieks@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 <stdio.h> 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