misaligned actual cannot be passed by reference

misaligned actual cannot be passed by reference

[awdorrin]

I have the following record definition that is used as a shared data
area:

  type FP_SHARED_BUF_TYPE is
    record
      -- POSIX.Mutexes
      LOCK_SHM_ACT_NRT      : Mutex;
      LOCK_SHM_ACT_RT       : Mutex;
      LOCK_SHM_EXIT_NRT     : Mutex
      LOCK_SHM_EXIT_RT      : Mutex;
      LOCK_SHM_RT_STATUS    : Mutex;
      LOCK_SHM_NRT_STATUS   : Mutex;
      LOCK_SHM_NRT_FILENAME : Mutex;
      LOCK_SHM_REINIT_FLAG  : Mutex;
      LOCK_ERROR            : Mutex;
      FP_PIDS               : FP_PID_ARRAY_TYPE := (others => 0);
      CYCLE_CTR             : INT32 := 0;
      CMN_SHM_READY         : INT8;
      MSCS_PER_SEC          : INT8;
      CURRENT_AC_TYPE       : GLOBAL_TYPES.AIRCRAFT_TYPE :=
GLOBAL_TYPES.NONE;
      SIM_STATUS            : INT32 := 0;
      CHAN_RATES            : M1553_RATE_TYPE;
      DCFSHM                : DCFSHM_TYPE;
      ERRORS_SHM            : ERRORS_SHM_TYPE;
      PERFORMANCE           : PERFORM_ARRAY_TYPE;
      LOCK_MISSION_VARS   : Mutex;
      MISSION_START_TIME  : INT32;
      MISSION_TIME        : INT32;
      MISSION_FRAME_COUNT : INT32;
      LOCK_BUS_DATA      : Mutex;
      LOCK_CHNG_SCREEN   : Mutex;
      LOCK_STACK_DATA    : Mutex;
      LOCK_START_TIME    : Mutex;
      LOCK_SHM_TC_UPDATE : Mutex;
      LOCK_UPDATE_RATE   : Mutex;
      OISHM            : OISHM_TYPE;
      LOCK_NAV         : Mutex;
      LOCK_PB          : Mutex;
      NAV              : NAV_SHM_TYPE;
      NAV_SERV         : NAV_SERV_REQ_TYPE;
      PB               : PB_SHM_TYPE;
      MISSION_DESC     : MISSION_DESC_TYPE;
      COMPANION_ID     : INT32;
      CAPTURE_TIME     : INT32;
    end record;

Compiling with the -gnatR3 option I get the following rep spec:

for FP_SHARED_BUF_TYPE'Size use 7693248;
for FP_SHARED_BUF_TYPE'Alignment use 8;
for FP_SHARED_BUF_TYPE use record
   LOCK_SHM_ACT_NRT      at      0 range  0 .. 255;
   LOCK_SHM_ACT_RT       at     32 range  0 .. 255;
   LOCK_SHM_EXIT_NRT     at     64 range  0 .. 255;
   LOCK_SHM_EXIT_RT      at     96 range  0 .. 255;
   LOCK_SHM_RT_STATUS    at    128 range  0 .. 255;
   LOCK_SHM_NRT_STATUS   at    160 range  0 .. 255;
   LOCK_SHM_NRT_FILENAME at    192 range  0 .. 255;
   LOCK_SHM_REINIT_FLAG  at    224 range  0 .. 255;
   LOCK_ERROR            at    256 range  0 .. 255;
   FP_PIDS               at    288 range  0 .. 511;
   CYCLE_CTR             at    352 range  0 .. 31;
   CMN_SHM_READY         at    356 range  0 ..  7;
   MSCS_PER_SEC          at    357 range  0 ..  7;
   CURRENT_AC_TYPE       at    358 range  0 ..  7;
   SIM_STATUS            at    360 range  0 .. 31;
   CHAN_RATES            at    364 range  0 .. 127;
   DCFSHM                at    380 range  0 .. 196255;
   ERRORS_SHM            at  24912 range  0 .. 219903;
   PERFORMANCE           at  52400 range  0 .. 20479;
   LOCK_MISSION_VARS     at  54960 range  0 .. 255;
   MISSION_START_TIME    at  54992 range  0 .. 31;
   MISSION_TIME          at  54996 range  0 .. 31;
   MISSION_FRAME_COUNT   at  55000 range  0 .. 31;
   LOCK_BUS_DATA         at  55008 range  0 .. 255;
   LOCK_CHNG_SCREEN      at  55040 range  0 .. 255;
   LOCK_STACK_DATA       at  55072 range  0 .. 255;
   LOCK_START_TIME       at  55104 range  0 .. 255;
   LOCK_SHM_TC_UPDATE    at  55136 range  0 .. 255;
   LOCK_UPDATE_RATE      at  55168 range  0 .. 255;
   OISHM                 at  55200 range  0 .. 703;
   LOCK_NAV              at  55288 range  0 .. 255;
   LOCK_PB               at  55320 range  0 .. 255;
   NAV                   at  55352 range  0 .. 799999;
   NAV_SERV              at 155352 range  0 .. 48063;
   PB                    at 161360 range  0 .. 6401407;
   MISSION_DESC          at 961536 range  0 .. 895;
   COMPANION_ID          at 961648 range  0 .. 31;
   CAPTURE_TIME          at 961652 range  0 .. 31;
end record;

Since I have a struct defined in C code which must map this, I don't
want the GNAT compiler moving things around in future builds, so I cut
and pasted this rep spec into the Ada code.

The next time I compile, I get the error message mentioned in the
subject:
"misaligned actual cannot be passed by reference"

There are several locations in the code being flagged but an example
is:

POSIX.Mutexes.Lock( POSIX.Mutexes.Descriptor_Of( CMN_SHM.LOCK_MISSION_VARS ) );

Looking at the LOCK_MISSION_VARS in the rep spec, it is defined as:
LOCK_MISSION_VARS at 54960 range 0 .. 255;

54960 is an 8 byte boundary, so I am not sure why GNAT thinks this is
misaligned, especially since this error is not flagged if I do not
include the rep spec in the code.

Re: misaligned actual cannot be passed by reference

[awdorrin]

I spent a long time today stepping through the debugger and looking at
the address locations of the record elements in both Ada and the
corresponding C Structure.

It looks like the rep spec that GNAT generated was not aligning some
of the values the way it wanted to. I was able to manually adjust the
rep spec (and the C struct) - and now GNAT is happy with the results.

Not sure why GNAT didn't align the items properly to begin with...

Re: misaligned actual cannot be passed by reference

[Robert Matthews]

To share with your C code, this is normally sufficient:

pragma Convention(C, FP_SHARED_BUF_TYPE);

with no need for the representation clause.
GNAT should then give a compatable layout.

In addition, I presume: "POSIX.Mutexes.Descriptor_Of (CMN_SHM.LOCK_MISSION_VARS)"
is needing the address of the parameter - so shouldn't that component of the
record be declared as aliased?


Robert