>>>>> "Olaf" == Olaf Petzold <[EMAIL PROTECTED]> writes:
>> : So the best way is the way how did you suggested. Anyway is
>> there a performance : factor to use unsigned char versus unsigned
>> int ? I believe to remember that : one should use basic types (int
>> and double) on performance programs ? Any : experience? I compiled
>> the kernel program (written in C) with -align-double etc.
>>
>> The perfomance difference not so significant, rewriting critical
>> paths in asm is more effective usually. It should be noted that
>> using -malign-double option can lead to different struct fields
>> aligning, even in the same app.
Olaf> Oh dear... Can I avoid this by using
Olaf> typedef struct { flag_t inuse; flag_t valid; double um[65536] }
Olaf> __attribute__ ((aligned)) shm_slut_t;
Olaf> ??? Is it guarantee that all structures (inside shm, inside
Olaf> local static copy, inside kernel space code produced by Gnu-C
Olaf> and user space program compiled by Gnu-C++) have the same
Olaf> aligment - even if compiled with the Option -align-double?
Same alignment as what?
If you compile the same structure declaration n times with the same
compiler alignment controls, for the same platform, it should produce
the same alignment. That seems rather obvious; the compiler is broken
if that fails.
If you change the structure, the offsets generally change; the changes
should be predictable but they may not always be what you would
expect, because the rules aren't necessarily what you might think they
are or should be. The C standard does constrain the rules but doesn't
nail them down fully. In particular, compiler options may affect the
outcome, as may moving to another platform.
As far as I know (I'm not a language lawyer) these are the rules, in
the absence of "packed":
1. Fields must be in the declared order.
2. Each field is aligned on a multiple of its alignment. Note that it
does NOT have to be the smallest possible multiple of its alignment.
2a. The alignment of a primitive datatype is its size
2b. The alignment of a struct or union is the largest of the
alignments of its members (recursively)
3. The size of a struct or union is a multiple of the largest of the
alignments of its members.
So, if you have a struct that contains a char, a uint32, and a struct
of two uint16s, in that order. If the minimum alignments are used,
the offsets will be 0, 4, and 8, with the inner structure having a
total length of 4 and its components are at offsets 8 and 10 from the
start of the outer struct. The struct size is 12, which is a multiple
of 4.
It would also be legal for everything to be rounded up to a multiple
of 8, making the inner struct size 16 (with alignment 8) and the outer
struct having its fields at 0, 8, and 16, for a total size of 32.
paul
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