Messages in this thread | | | Date | Sun, 01 Feb 2009 14:32:45 +0200 | From | Boaz Harrosh <> | Subject | Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls |
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Geert Uytterhoeven wrote: > 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 >
I don't understand
if you have a structure like struct foo { u32 one; u32 two; }; vs struct foo_packed { u32 one; u32 two; } __packed;
Just adding an __attribute__((packed)) to it clearly does not change the layout of the structure. Are you saying the __attribute__((packed)) is an hint to the compiler that foo_packed might be used unaligned. This is just brain-dead, because I can use an unaligned pointer to foo just as I can to foo_packed. Otherwise there is no difference what-so-ever between the two. I have to see it to believe. It is totally the wrong hint in the wrong place taking away valuable meaning of saying "please don't use padding holes in this structure"
Sorry for been so slow, I just don't get it. Boaz
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