NULL is defined within the kernel code in linux\stdfed.h, so the result should be platform independent. Thanks & regards, Suvidh Mathur -----Original Message----- From: Anupam Kapoor [mailto:anupam.kapoor@gmail.com] Sent: Wednesday, June 30, 2004 2:22 PM To: Nir Tzachar Cc: Borislav Petkov; kernelnewbies@nl.linux.org Subject: Re: inline asm question(s) On Wed, 30 Jun 2004 11:38:47 +0300 (IDT), Nir Tzachar <tzachar@cs.bgu.ac.il> wrote: > hello there ;) > > > 211 #define container_of(ptr, type, member) ({ \ > > 212 const typeof( ((type *)0)->member ) *__mptr = (ptr); \ > > 213 (type *)( (char *)__mptr - offsetof(type,member) );}) > > > > I can't understand the ((type *)0) part - type is passed as an argument and it > > is some struct pointer but the trailing 0 ... what does it actually do? > > you need the type of the member, so you can have a proper pointer to it. > you could achieve this by supplying member_type directly, but you dont > need to. by using typeof ((type *)0)->member we get the type of the member > ('type' is the type of the container). > > > > offsetof is similar: > > > > <from include/linux/stddef.h> > > 12 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) > > well, lets say you have a structure defined like this: > struct foo { > int a; > int b; > int c; > char d; > }; > > to get the offset of member c in this struct, we need to size of all > members which come b4 c: offset_of_c = sizeof(a)+sizeof(b) . no, this is not correct. the compilers are free to add padding to meet alignment requirements of a given platform. > however, a more generic and _much_ better way: > lets say you had a pointer to a struct foo (foo_ptr), so you can get the > offset like this: > > offset_of_c = &foo_ptr->c - foo_ptr > > the macro actually saves the subtraction, by letting the compiler think > that foo_ptr is 0 ((TYPE *)0) (located at start of memory). the assumption here is that null is repersented as a bunch of zeros on a platform. if that is not the case, you will get strange results. kind regards anupam > > -- > ======================================================================== > nir. -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
