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From: HELLER%cs.umass.edu@RELAY.CS.NET (Stride 440 User)
Newsgroups: mod.computers.vax
Subject: RE: VMS, FORTRAN & CTRL-c AST
Message-ID: <8611151312.AA28958@ucbvax.Berkeley.EDU>
Date: Fri, 14-Nov-86 08:05:00 EST
Article-I.D.: ucbvax.8611151312.AA28958
Posted: Fri Nov 14 08:05:00 1986
Date-Received: Sun, 16-Nov-86 01:33:23 EST
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The problem is FORTRAN I/O is non-"atomic", that is a FORTRAN statement
like:

	accept 10,x,y,x

does not generate a single call to some FOR$blah routine, but in fact
something like 5 calls to FOR$blah routines.  This sequence is really a
"critical section" of sorts.  At least for you purposes it is.  If your ^C
handler bobs along in this critical section, you will end up with a pending
I/O operation on the unit number (-3 in the case of accept) associatiated
with the interupted I/O statement.  The next I/O request on that unit number
can't be processed, and fails with a recursive I/O operation.  That unit
number is now dead.  The only thing you can do now is exit back to DCL and
start over.  So much for FORTRAN I/O.

What you have to do is poll for ^C. Your ^C handler simply sets a logical
variable in a common block. This variable is tested at "reasonable" points
in the code, and a non-local exit is done at these points. (Probably by
doing a lib$signal with some made up signal which is caught by your
condition handler, which does a sys$unwind.

There is a routine called FOR$IO_END (something like that), but you can only
call this if there is an I/O operation in progress AND if you know the unit
# of the pending I/O. There is no universal "clean up any I/O, if any" type
function. Once a unit has been bashed with a "Recursive I/O operation", it
is completely dead, with no recovery posible. The only safe thing you can do
is the ^C polling method. Also, if you setup an out-of-band AST (like DCL's
^T) to report status, DON'T do FORTRAN I/O in it, use SYS$QIOW only, using
SYS$FAO(L) or C's sprintf, not FORTRAN's ENCODE to format the message
buffer(s).

		Robert Heller
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