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From: ijah400%ivax.DECnet@GOLD.BACS.INDIANA.EDU ("IVAX::IJAH400")
Newsgroups: comp.os.vms
Subject: Re: LIB$GETJPI
Message-ID: <8807181443.AA06135@ucbvax.Berkeley.EDU>
Date: 13 Jul 88 18:28:00 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 44

Christopher Chiesa <bsu-cs!cfchiesa@IUVAX.CS.INDIANA.EDU> writes (about
passing arguments by reference to library routines):
< In article <8807071431.AA22249@ucbvax.Berkeley.EDU>, sysmgr@JALCF.WPAFB.AF.MIL
< ("SYSTEM") writes:
< ...
<        [whatever you have to do to put a value in PID, and
<         invoke LIB$GETJPI...]
<
<          STATUSCODE = LIB$GETJPI( item_code, %LOC PID, .... )
<
< The %LOC modifier forces the compiler to code for passing of PID "by
< reference," rather than "by value" which is apparently the default.
< Like I said, it's been a while; if any more up-to-date FORTRANers
< would care to correct me, that's fine!

OK, here goes:  %LOC is an built-in function that gives the address of its
argument as the result.  But
         STATUSCODE = LIB$GETJPI( item_code, %LOC PID, .... )
gives a syntax error, because %LOC(PID) is legal but %LOC PID is not.
Furthermore, that isn't what the guy wants, because %LOC(PID) ends up passing
the ADDRESS of PID by reference (i.e., the address of a longword containing
the address of PID).  What he wants is:
         STATUSCODE = LIB$GETJPI( item_code, %REF(PID), .... )
or       STATUSCODE = LIB$GETJPI( item_code, PID, .... )
which generate the same code, as the default argument passing mechanism in
VAX FORTRAN is by REFERENCE.  The built-in function %REF may be used in
actual argument lists to be picky about the fact that you want something
passed by reference.  VAX FORTRAN also has an built-in function %VAL to
specify that an argument be passed by value, %DESCR to pass the address of
a descriptor (the default for CHARACTER data, etc., RTFM).

Note that the FORTRAN-77 standard does not REQUIRE that arguments be passed
by reference; the value-result (copy-in, copy-out) mechanism is OK too.
For example, the PDP-10 (TOPS-10 and TOPS-20) FORTRAN compilers use the
value-result mechanism for scalar data items.  It really only makes a
difference if your programmers are doing things prohibited by the FORTRAN
standard (like passing a COMMON variable to a routine that refers to it both
by the COMMON variable name and the formal argument name).  All the other
FORTRAN compilers I've ever encountered, including VAX FORTRAN, use the
reference mechanism by default.

Hope this helps....

        James Harvey
        ijah400@indyvax (bitnet) or ijah400%ivax.decnet@gold.bacs.indiana.edu