Re: Q: How to do memory access with gnat

[stt@houdini.camb.inmet.com (Tucker Taft)]

Michael Erdmann (boavista@berlin.snafu.de) wrote:

: i like to do something like this in Ada95  (excuse the C-example):

: 	typedef struct  {  ..,other_data;   char data[1];  }  Msg_data; 
: 	Msg_Data	*shared;

: 	// get some memory from the system
: 	DosGetSharedMemory( ..., &shared... )

: 	// copy data into shared memory 
: 	for( i ,,,, )
: 	   shared->data[i] := some_data[i];

: In  Ada95  i am not able to copy data from an Ada object into a some external
: memory.
: Any idea ?

You can use an address clause to associate an Ada name with a given
address.  E.g.:
    Shared : Msg_Data;
    for Shared'Address use <arbitrary computation>;
  begin
    Shared.data(I) := ...

Alternatively, the most direct analogy to the C approach is to declare
an access type, and initialize a variable of the type using 
unchecked_conversion or the Address_To_Access_Conversion generic, or by
passing its 'Address to some external routine.

    type Msg_Data_Ptr is access all Msg_Data;
    Shared : Msg_Data_Ptr;

  begin
    DosGetSharedMemory(..., Shared'Address, ...);
    Shared.data(I) := ...

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA