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This appendix contains two programs (Example C-1 in COBOL,
and Example C-2 in FORTRAN) which illustrate the use of NOWAIT
I/O in Interprocess Communication. Software interrupts are not
used in these examples; see Appendix D for sample programs
illustrating their use. Example C-1.
001000*
001100* NOWAITRD
001200*
001300* Compiled with COBOLII.
001400*
001500* This program has a background task that it does in a loop. After
001600* each pass through the loop it checks a message file to see if a
001700* special request has been made. The check for the special request
001800* is made only at the completion of a pass through the loop, for two
001900* reasons. First, the time it takes to make a pass through the loop
002000* is not too long for the special request to wait to be read. Second,
002100* the special request may require the use of some of data structures
002200* used by the background task, and those data structures may be in an
002300* inconsistent state part way into a pass through the loop. The
002400* message file is checked for records containing the special requests
002500* using NOWAIT FREADs. Standard (wait) FREADs were not used because
002600* they would have caused this program to wait if the message file did
002700* not contain any records (and there was another program with this
002800* file open for write access), when what we want is to continue doing
002900* the background task in the loop. Software interrupts were not used
003000* because they would try to receive a special request anywhere in the
003100* loop, and would have added complexity to provide features that this
003200* program does not need.
003300*
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003400 IDENTIFICATION DIVISION.
003500 PROGRAM-ID. NOWAITRD.
003600*
003700 ENVIRONMENT DIVISION.
003800 CONFIGURATION SECTION.
003900 SOURCE-COMPUTER. HP-SYSTEM.
004000 OBJECT-COMPUTER. HP-SYSTEM.
004100 SPECIAL-NAMES.
004200 CONDITION-CODE IS CC.
004300*
004400 DATA DIVISION.
004500 WORKING-STORAGE SECTION.
004600 01 IN-REC PIC X(80).
004700 01 TERM-REC PIC X(80).
004800 01 BACK-GROUND-MSG PIC X(17) VALUE "BACKGROUND WORK ".
004900 01 ERROR-MSG PIC X(17) VALUE "UNEXPECTED ERROR ".
005000 01 IN-FILE-NAME PIC X(9) VALUE "MSGFILE1 ".
005100 01 DONE PIC X.
005200 88 FINISHED VALUE IS "T".
005300 88 NOTFINISHED VALUE IS "F".
005400 01 PARM PIC S9(4) COMP.
005500 01 LNGTH PIC S9(4) COMP.
005600 01 IN-FILE PIC S9(4) COMP.
005700 01 LOOP-COUNTER PIC S9(4) COMP.
005800 01 MSG-FLAG PIC S9(4) COMP.
005900*
006000 PROCEDURE DIVISION.
006100*
006200 100-START-OF-PROGRAM.
006300 MOVE 0 TO LOOP-COUNTER.
006400 CALL INTRINSIC "FOPEN" USING IN-FILE-NAME %5 %4000
006500 GIVING IN-FILE.
006600 IF CC NOT = 0 THEN
006700 PERFORM 900-ERROR-CONDITION.
006800*
006900* Set up extended waits on read-file.
007000*
007100* This will cause IODONTWAIT to indicate that the record is still
007200* unavailable rather than returning an End-Of-File error if the
007300* writer program terminates.
007400*
007500 MOVE 1 TO PARM.
007600 CALL INTRINSIC "FCONTROL" USING IN-FILE 45 PARM.
007700 IF CC NOT = 0 THEN
007800 PERFORM 900-ERROR-CONDITION.
007900*
008000* Start the first read on the message file.
008100*
008200 PERFORM 600-START-NEXT-READ.
008300*
008400 MOVE "F" TO DONE.
008500 PERFORM 200-PROCESSING-LOOP UNTIL FINISHED.
008600*
008700* Abort the outstanding read (PARM is ignored) and close the msg file.
008800*
008900 CALL INTRINSIC "FCONTROL" USING IN-FILE 43 PARM.
009000 IF CC NOT = 0 THEN
009100 PERFORM 900-ERROR-CONDITION.
009200 CALL INTRINSIC "FCLOSE" USING IN-FILE 0 0.
009300 IF CC NOT = 0 THEN
009400 PERFORM 900-ERROR-CONDITION.
009500 STOP RUN.
009600*
009700 200-PROCESSING-LOOP.
009800*
009900* Each pass through this loop we do one iteration of the "background
010000* task" and then we test to see if a message has come in.
010100*
010200 PERFORM 300-BACKGROUND-TASK.
010300 PERFORM 400-CHECK-FOR-MSG.
010400*
010500 300-BACKGROUND-TASK.
010600*
010700* This could be any background processing, but in our case it is
010800* just a display.
010900*
011000 PERFORM 700-CPU-WASTER 10000 TIMES.
011100 ADD 1 TO LOOP-COUNTER.
011200 IF LOOP-COUNTER = 100 THEN
011300 MOVE "T" TO DONE.
011400 DISPLAY BACK-GROUND-MSG.
011500*
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011600 400-CHECK-FOR-MSG.
011700*
011800* Call IODONTWAIT to see if a record has been written to the
011900* message file. Whether or not a message is there, IODONTWAIT
012000* will always return immediately.
012100*
012200 CALL INTRINSIC "IODONTWAIT" USING IN-FILE IN-REC LNGTH
012300 GIVING MSG-FLAG.
012400 IF CC NOT = 0 THEN
012500 PERFORM 900-ERROR-CONDITION.
012600*
012700* MSG-FLAG will be non-zero (= file number) if a message was received.
012800* If a message was received, handle it and re-start the next read on
012900* the message file.
013000*
013100 IF MSG-FLAG NOT = 0 THEN
013200 PERFORM 500-HANDLE-MSG
013300 PERFORM 600-START-NEXT-READ.
013400*
013500 500-HANDLE-MSG.
013600*
013700* Do any processing that is required to handle the incoming message.
013800*
013900 MOVE IN-REC TO TERM-REC.
014000 DISPLAY TERM-REC.
014100*
014200 600-START-NEXT-READ.
014300*
014400* Start the NOWAIT FREAD. It will be completed by IODONTWAIT. Note
014500* that NOWAIT FREADs on message files do not require Priv Mode.
014600*
014700 MOVE -80 TO LNGTH.
014800 CALL INTRINSIC "FREAD" USING IN-FILE IN-REC LNGTH.
014900 IF CC NOT = 0 THEN
015000 PERFORM 900-ERROR-CONDITION.
015100*
015200 700-CPU-WASTER.
015300*
015400* This is here just to burn up time. Real work should be done here.
015500* DO NOT put a "CPU waster" like this in a real program.
015600*
015700 MOVE SPACES TO TERM-REC.
015800*
015900 900-ERROR-CONDITION.
016000 DISPLAY ERROR-MSG.
016100 CALL INTRINSIC "PRINTFILEINFO" USING IN-FILE.
016200 STOP RUN.
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Example C-2.
$CONTROL USLINIT
$STANDARD_LEVEL SYSTEM
C
PROGRAM NOWAITREAD
C
C Compiled with FORTRAN 77.
C
C This program reads messages from both a terminal and a message
C file, and processes them. When not processing a message, the
C program just waits for the next message. This program uses
C NOWAIT I/O because it allows it to start FREADs on both the
C terminal and the message file, and then wait in a single
C "IOWAIT(0,..." statement for whichever FREAD is finished first.
C
INTEGER*2 fnum,fnumterm,fnuminfile,fnumoutfile
INTEGER*2 tcount,length,condcode
LOGICAL buf(40)
LOGICAL eof
SYSTEM INTRINSIC GETPRIVMODE,GETUSERMODE,FOPEN,FREAD,IOWAIT
SYSTEM INTRINSIC FWRITE,FCLOSE,PRINTFILEINFO
C
C
C Priv Mode needed to open the terminal for NOWAIT I/O. Must
C also PREP with PM.
C foption = $STDIN, ascii, old. aoption = no wait I/O, read access.
C
CALL GETPRIVMODE
fnumterm = FOPEN( , 45B, 4000B)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnumterm )
STOP ' Error occured during terminal FOPEN '
END IF
CALL GETUSERMODE
C
C Open the input message file.
C foption = ascii, old. aoption = no wait I/O, read access.
C
fnuminfile = FOPEN( "MSGFILE1", 5B, 4000B)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnuminfile )
STOP ' Error occured during input message file FOPEN '
END IF
C
C Open the output message file.
C foption = ascii, old. aoption = write access.
C
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fnumoutfile = FOPEN( "MSGFILE2", 5B, 1B)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnumoutfile )
STOP ' Error occured during output message file FOPEN '
END IF
C
C Start the read on the terminal. No Wait FREADs always return
C a length of 0. The real data length is returned by IOWAIT.
C
tcount = -80
length = FREAD( fnumterm, ibuf, tcount)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnumterm )
STOP ' Error occured during the terminal FREAD '
END IF
C
C Start the read on the message file. No Wait FREADs always return
C a length of 0. The real data length is returned by IOWAIT.
C
tcount = -80
length = FREAD( fnuminfile, buf, tcount)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnuminfile )
STOP ' Error occured during the message file FREAD '
END IF
C
eof = .FALSE.
DO WHILE (.NOT. eof)
C
C An IOWAIT with file-number = 0 will complete whichever
C FREAD is ready to be finished first.
C
fnum = IOWAIT( 0, buf, tcount)
condcode = CCODE()
IF (condcode .EQ. -1) THEN ! Error
CALL PRINTFILEINFO( fnum )
STOP ' Error occured during IOWAIT '
END IF
IF (condcode .EQ. 1) THEN ! EOF
eof = .TRUE.
END IF
IF (condcode .EQ. 0) THEN
C
C Process the message that came in; in this case write it to
C "message file 2". FREAD and FWRITE want byte lengths to be
C negative. IOWAIT returns a positive byte length. We have
C to change the sign.
C
tcount = - tcount
CALL FWRITE( fnumoutfile, buf, tcount, 0)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnumoutfile )
STOP ' Error occured during FWRITE '
END IF
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C
C Re-start the FREAD that the IOWAIT just completed.
C
tcount = -80
length = FREAD( fnum, buf, tcount)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnum )
STOP ' Error occured during FREAD '
END IF
END IF
ENDDO
C
C Time to shut down.
C Call FCONTROL-43 to abort the no wait read that is pending
C against the terminal and the message file. If CCG is returned,
C then the abort could not complete, and an IOWAIT must be called
C to clear the I/O. CCE means aborted OK. CCG means nothing to
C abort, which is OK here because the read is not restarted if
C there was an error.
C
CALL FCONTROL( fnumterm, 43, 0)
IF (CCODE() .EQ. 1) THEN
fnum = IOWAIT( fnumterm, buf, tcount)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnumterm )
STOP ' Error occured during terminal FCONTROL/IOWAIT '
END IF
END IF
CALL FCONTROL( fnuminfile, 43, 0)
IF (CCODE() .EQ. 1) THEN
fnum = IOWAIT( fnuminfile, buf, tcount)
IF (CCODE() .NE. 0) THEN
CALL PRINTFILEINFO( fnuminfile )
STOP ' Error occured during message file FCONTROL/IOWAIT '
END IF
END IF
C
CALL FCLOSE( fnumterm, 0, 0)
CALL FCLOSE( fnuminfile, 0, 0)
CALL FCLOSE( fnumoutfile, 0, 0)
C
STOP 'Successful'
END
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