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


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PerlFAQ Server

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 .

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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:

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

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

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;
DEV->autoflush(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 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 "<[email protected]>", 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>;
chomp;
if ( !m/^Connected/ ) {
print STDERR "$0: cu printed `$_' instead of `Connected'\n";
}
}



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