From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,b93f800319d6e6e7 X-Google-Attributes: gid103376,public From: stt@houdini.camb.inmet.com (Tucker Taft) Subject: Re: Aligning record components Date: 1997/07/08 Message-ID: #1/1 X-Deja-AN: 255471812 Sender: news@inmet.camb.inmet.com (USENET news) X-Nntp-Posting-Host: houdini.camb.inmet.com References: <01bc8b3a$d9161b80$7175b89e@Ken'slaptop.ssd.loral.com> Organization: Intermetrics, Inc. Newsgroups: comp.lang.ada Date: 1997-07-08T00:00:00+00:00 List-Id: kgamble (kgamble@ricochet.net) wrote: : I am porting Ada code from a 16-bit machine to a PC. The original code had : record structures that had components aligned on 16-bit boundaries. The : components were 16 and 32 bit integers and 32 bit floats. I need to : maintain the exact alignments on the new PC target. There does not appear : to be a way to do this using the GNAT compiler without resorting to record : representation clauses. I understand the benefits of using such clauses, : but the scope of the work to create thousands of these component clauses is : significant, so I am looking for a work around. : If I could force 2-byte alignments, then my problem would be solved. : However, GNAT only allows 4-byte alignments. In the following data : structure I would need to see components at positions at (0,2,6,10,14). : What GNAT gives is (0,2,8,12,16), or by using pragma pack (0,2,6,12,16). It : is interesting that the pack version does align 32-bit integers correctly, : but has no affect on 32-bit floats. Have you tried specifying the alignment of your scalar types, such as via: for Integer_16'Alignment use 2; This might cause GNAT to lay out the records as you wanted. : type rec_type is : record : I16: Integer_16; : I32: Integer_32; : J32: Integer_32; : f: float_32 ; : H: float_32; : end record; : I understand that relying on implicit characteristics of a compiler is not : good for portability. But a required goal in the port is to make minimal : changes to the original source code when doing the port. Fixing the : original source code to use representation clauses is not possible at this : time. : I do not know why GNAT does not allow 2-byte alignments. There are still : plenty of 16-bit target machines that Ada supports. GNAT probably uses 4-byte alignment by default since it is more efficient on modern x86 chips. However, the alignment clause (exemplified above) is one way to override this choice, at least for you own scalar types. -- -Tucker Taft stt@inmet.com http://www.inmet.com/~stt/ Intermetrics, Inc. Burlington, MA USA