Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

70 Chapter3

Common Device Control Functions

Reading From Asynchronous Devices

Setting a Read Time Limit

FCONTROL can also be used to set a time limit on a succeeding read,

by specifying 4 as the value of controlcode and specifying a time limit,

in seconds, through param (up to 65535 seconds). If the read

immediately following the call takes more than the time speciﬁed, the

read will terminate in error and any data already received will be

ﬂushed. The read limit remains in effect only for the next read. You

must issue a separate FCONTROL(4) call prior to every read on which

you want to impose a time limit.

There are many circumstances in which it is advisable to set a timeout

value. Timeouts should always be used with devices that operate

without a dedicated attendant, to prevent hangs that could occur if the

device were to fail in some way.

Timeouts are effective in all data editing modes. This makes it possible

to terminate even a binary mode read should it exceed the speciﬁed

time limit. During block mode reads, however, the timer is halted upon

receipt of the DC2 character and the block mode read timer takes over.

When a read terminates as the result of exceeding the timeout set by

FCONTROL(4), the read fails with a CCL condition and all data is lost.

A call to FCHECK returns an FSERR 22, Software Timeout.

The code segment shown in Figure 3-4 sets a 30-second time limit for

the next read posted against

fileid_in

. The sample includes an error

checking routine which veriﬁes whether or not the read terminated

normally. If not, the program calls FCHECK. If FCHECK returns an

FSERR 22, the program will inform the user that a read timeout

occurred.