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From: awdorrin <awdorrin@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Interfacing between Ada and C: records and structs
Date: Tue, 31 Jul 2012 12:08:10 -0700 (PDT)
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Date: 2012-07-31T12:08:10-07:00
List-Id: <comp.lang.ada>

I am wondering if the Dummy element in the Mutex record is not being treated as a null, but as a null pointer, adding 8 bytes to the record size.

I tried forcing the sizes to '192' in Ada by adding the rep spec and got the following error messages:

size for "LOCK_SHM_1" too small, minimum allowed is 224
position of aliased field "LOCK_SHM_1" must be multiple of 64 bits

So I'm guessing 192 (pthread_mutex_t) + 32 ( tagged null record) = 224 bits, rounded to 256 due to the address position requirement of an aliased field. (Still not sure I understand what a tagged or aliased field is, will have to read up on it.)

So, I guess this turns out to be more of a C question - on if I can pad/align the structs via compiler flags, or if I am better off sticking with manually adding the padding.

Not sure if there is a standard method used 'in modern times' to do this, other than defining a record in Ada and a corresponding struct in C.