Memory Alignment question.

Thu Jul 22 13:20:00 GMT 2004

Greetings everyone,

I am using gcc 3.3.4 on a pentium M and noticed a
behaviour in memory allocation that confused me. So
far, the only thing I knew of memory
allocation/alignment was this: memory areas can only
be addressed by a multiple of the word size (in my
case a word is 4 bytes).

I did some testing to verify what I knew was correct
and was proved totally wrong. Here are two basic
examples

void function() {
char buffer1;
char buffer2;
}

Only 4 bytes are allocated for buffer1 & buffer2 (sub
$0x4 %esp) while I would have expected 8 bytes.
Someone told me it was for memory optimization which
seems rational but then... I guess one of the buffer
cannot be addressed by a multiple of 4 bytes, right?
Is not it a problem?

There is something even more confusing. I have been
chewing on it for a few days and came up with no
possible explanation. Take this:

void function() {
char buffer1[5];
char buffer2[6];
}

28 bytes are allocated for both buffers! I would have
expected 16 at most (8 for each). I did other tests
and came up with as surprising results.

Could anyone explain me how this works please and/or
point me to some documentation concerning this?

Thanks in advance,

          Barlad.

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