Messages in this thread | | | Date | Sun, 1 Feb 2009 11:59:04 +0100 (CET) | From | Geert Uytterhoeven <> | Subject | Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls |
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On Sun, 1 Feb 2009, Boaz Harrosh wrote: > Geert Uytterhoeven wrote: > > On Sun, 1 Feb 2009, Boaz Harrosh wrote: > >> Arnd Bergmann wrote: > >>> +struct space_resv { > >>> + __s16 l_type; > >>> + __s16 l_whence; > >>> + __s64 l_start; > >>> + __s64 l_len; /* len == 0 means until end of file */ > >>> + __s32 l_sysid; > >>> + __u32 l_pid; > >>> + __s32 l_pad[4]; /* reserve area */ > >>> +}; > >> What about telling the compiler exactly what you said above, just > >> to be sure we all mean the same thing. (And as documentation for new > >> comers): > >> > >> +struct space_resv_64 { > >> + __s16 l_type; > >> + __s16 l_whence; > >> + __u32 reserved; > >> + __s64 l_start; > >> + __s64 l_len; /* len == 0 means until end of file */ > >> + __s32 l_sysid; > >> + __u32 l_pid; > >> + __s32 l_pad[4]; /* reserve area */ > >> +} __packed; > > > > Because the compiler will assume all fields are always unaligned and will use very > > suboptimal code to access them? > > This discussion comes up every once in a while. I'm using an old FC7 compiler > (gcc (GCC) 4.1.2 20070925 (Red Hat 4.1.2-27)) And tests show that when the layout > of a structure is exactly the same the "__packed" on structure declarations does > nothing. It only starts to affect when there are real differences in alignment. > Also tests with gcc 3.4.x showed the same effect. > > On previous discussions no one could come forward and say what compiler version > breaks when __packed is applied on structure definition. I'm afraid your statement > above is a myth.
FC7, targeting ia32? Sure, ia32 has no alignment restrictions. Try e.g. MIPS.
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
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