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From: awdorrin <awdorrin@gmail.com>
Newsgroups: comp.lang.ada
Subject: Interfacing between Ada and C: records and structs
Date: Tue, 31 Jul 2012 09:38:51 -0700 (PDT)
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Date: 2012-07-31T09:38:51-07:00
List-Id: <comp.lang.ada>

I have some legacy code that I am working with, they share common data structures.

Simplifying my code, In Ada, I have a record:

type FP_SHARED_BUF_TYPE is
record
  LOCK_SHM_1 : aliased Mutex;
  LOCK_SHM_2 : aliased Mutex;
  LOCK_SHM_3 : aliased Mutex;
end record;
pragma Convention(C,FP_SHARED_BUF_TYPE);

In C:

struct FP_SHARED_TYPE {
  pthread_mutex_t LOCK_SHM_1;
  pthread_mutex_t LOCK_SHM_2;
  pthread_mutex_t LOCK_SHM_3;
};

Now, the Mutex/pthread_mutex_t is 192bits, not 255bits, but when I compile the program.

The Ada code defines the record as:

for FP_SHARED_BUF_TYPE'Alignment use 9;
for FP_SHARED_BUF_TYPE use record
  LOCK_SHM_1 at - range  0 .. 255;
  LOCK_SHM_1 at - range 32 .. 255;
  LOCK_SHM_1 at - range 64 .. 255;
end record;

While the C code defines the structure elements as 192bits, and packs them right next to each other

Addresses of:
LOCK_SHM_1	0xF5650008
LOCK_SHM_2	0xF5650020
LOCK_SHM_3	0xF5650038

The only way I can get this to work, is to add padding to my C struct definition:

struct FP_SHARED_TYPE {
  pthread_mutex_t LOCK_SHM_1;
  int pad_for_m1[2];
  pthread_mutex_t LOCK_SHM_2;
  int pad_for_m2[2];
  pthread_mutex_t LOCK_SHM_3;
  int pad_for_m3[2];
};

Which, makes the C code really ugly.

Is there a way to make the Ada code pack the record?

I have been looking through the docs and haven't seen it yet.