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How do I read and write the serial port?

by faq_monk (Initiate)
on Oct 08, 1999 at 00:29 UTC ( #715=perlfaq nodetype: print w/replies, xml ) Need Help??

Current Perl documentation can be found at

Here is our local, out-dated (pre-5.6) version:

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 by using sysopen() and 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. You can use select() and the $| variable to control autoflushing (see perlvar/$ and select):

    $oldh = select(DEV);
    $| = 1;

You'll also see code that does this without a temporary variable, as in

    select((select(DEV), $| = 1)[0]);

Or if you don't mind pulling in a few thousand lines of code just because you're afraid of a little $| variable:

    use IO::Handle;

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 read() or sysread(), 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 select.

While trying to read from his caller-id box, the notorious Jamie Zawinski <>, after much gnashing of teeth and fighting with sysread, sysopen, POSIX's tcgetattr business, and various other functions that go bump in the night, finally came up with this:

    sub open_modem {
        use IPC::Open2;
        my $stty = `/bin/stty -g`;
        open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
        # starting cu hoses /dev/tty's stty settings, even when it has
        # been opened on a pipe...
        system("/bin/stty $stty");
        $_ = <MODEM_IN>;
        if ( !m/^Connected/ ) {
            print STDERR "$0: cu printed `$_' instead of `Connected'\n";

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