Exchange/SNA Manual

CANCEL Procedure
Exchange/SNA System Procedure Calls
5–6 104700 Tandem Computers Incorporated
CANCEL Procedure The CANCEL procedure cancels the oldest outstanding I/O operation against a file
opened for nowait I/O.
CALL CANCEL ( filenum );
filenum
INT:value
is the file number of a previously opened subdevice supported by the
Exchange/SNA line server. The number you specify here is the number returned
by the call to OPEN that opened the subdevice.
Considerations The CANCEL procedure cancels the oldest outstanding I/O operation on the specified
file. In the case of Exchange/SNA subdevices, there can be only one outstanding I/O
operation at a time. The CANCEL procedure always cancels the most recently
requested I/O operation against the specified subdevice.
If the operation has already taken place (the record has been sent, for example), the
CANCEL procedure has no effect on the physical I/O operation. If the operation has
not yet occurred (the host has not yet sent anything during an initial receive operation,
for example), the CANCEL procedure does cancel the physical I/O operation. There is
no way to guarantee that the CANCEL procedure can stop the physical I/O operation.
However, the CANCEL procedure always eliminates the need to call AWAITIO to
complete the canceled I/O operation.
A wait I/O operation cannot be canceled; it will already have completed before control
is returned to your program.