a literal '0' in pointer context always means a null pointer. what happens if for example on some platform, a null pointer is defined as 0xdeadbeef ? kind regards anupam On Wed, 30 Jun 2004 14:29:33 +0530, Suvidh Mathur <suvidhm@centillium.com> wrote: > 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/
