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Next: How do I decode encrypted password files?
This depends on which operating system your program is running on. In the
case of Unix, the serial ports will be accessible through files in /dev; on
other systems, the devices names will doubtless differ. Several problem
areas common to all device interaction are the following
Your system may use lockfiles to control multiple access. Make sure you
follow the correct protocol. Unpredictable behaviour can result from
multiple processes reading from one device.
- open mode
If you expect to use both read and write operations on the device, you'll
have to open it for update (see open for details). You may wish to open it without running the risk of blocking
O_RDWR|O_NDELAY|O_NOCTTY from the Fcntl module (part of the standard perl distribution). See
sysopen for more on this approach.
- end of line
Some devices will be expecting a ``\r'' at the end of each line rather than a ``\n''. In some ports of perl, ``\r'' and ``\n'' are different from their usual (Unix)
ASCII values of ``\012'' and ``\015''. You may have to give the numeric values you want directly, using octal (``\015''), hex (``0x0D''), or as a control-character specification (``\cM'').
print DEV "atv1\012"; # wrong, for some devices
print DEV "atv1\015"; # right, for some devices
Even though with normal text files, a ``\n'' will do the trick, there is
still no unified scheme for terminating a line that is portable between
Unix, DOS/Win, and Macintosh, except to terminate ALL line ends with ``\015\012'', and strip what you don't need from the output. This applies especially to socket
I/O and autoflushing, discussed next.
- flushing output
If you expect characters to get to your device when you
print() them, you'll want to autoflush that filehandle, as in
and the newer
You can use
select() and the
$| variable to control autoflushing (see $| and select):
$oldh = select(DEV);
$| = 1;
You'll also see code that does this without a temporary variable, as in
select((select(DEV), $| = 1));
As mentioned in the previous item, this still doesn't work when using socket
I/O between Unix and Macintosh. You'll need to hardcode your line terminators, in that case.
- non-blocking input
If you are doing a blocking
you'll have to arrange for an alarm handler to provide a timeout (see
alarm). If you have a non-blocking open, you'll likely have a non-blocking read, which means you may have to use a 4-arg
select() to determine whether
I/O is ready on that device (see
Source: Perl FAQ: System Interaction
Copyright: Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
Previous: How do I ask the user for a password?
(Corrections, notes, and links courtesy of RocketAware.com)
Up to: Serial I/O
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