hi > > 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. you are correct, maybe i oversimplified a bit.... > > 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. i dont see your point. this has nothing to do with the NULL pointer, but with a pointer whose base address is 0, which saves the subtraction... as far as this macro concerns, NULL may be defined like this: #define NULL ((void *)400) comments?? -- ======================================================================== nir. -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
