FAQ 8.10 How do I read and write the serial port?

Discussion in 'Perl Misc' started by PerlFAQ Server, Jan 15, 2011.

  1. This is an excerpt from the latest version perlfaq8.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .


    8.10: How do I read and write the serial port?

    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, device 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 behavior 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" in perlfunc 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" in perlfunc 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

    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 $| variable to control autoflushing (see "$|" in
    perlvar and "select" in perlfunc, or perlfaq5, "How do I
    flush/unbuffer an output filehandle? Why must I do this?"):

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

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

    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 hard
    code 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" in
    perlfunc). 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" in perlfunc.

    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";


    The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
    are not necessarily experts in every domain where Perl might show up,
    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
    operating system or platform, so please include relevant details for
    corrections to examples that do not work on particular platforms.
    Working code is greatly appreciated.

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