Hello monks,

That is my first question and I hope to do everything properly.

I want to write an NMEA modul that reads the heading strings from a serial device. It is working properly but I'm using a tied filehandle for the strings and can't figure out how to autoflush the buffer. I have to destroy the object each time before I read. When I'm getting a value with readline I don't get the most recent value that I can see with minicom at the serial device.

On top I don't know if I created a proper filehandle in the module with gensym() and also don't know how to bless a global filehandle properly. I can't use the star notation (*)

Here's the module. I cut the part where I do checksum things etc. with the line. I'm also using the log4perl module but I replaced it with warn/die.

#!/usr/bin/perl

#
### this module
package GPS::NMEAserial;

use strict;
use warnings;

# serial port communication
use Device::SerialPort qw( :PARAM :STAT 0.07 );

# flush output of filehandle
use IO::Handle;

# create filehandles
use Symbol qw( gensym );

sub new_nmea_port {
    my $class = shift;

    # open the serial port attached to control unit
    # $quiet parameter just for compatability
    my ( $port, $config_file, $quiet ) = @_;

    # this tests for an existing config file,
    # and creates one if necessary...
    if ( !-e $config_file ) {
        my $PortObj = new Device::SerialPort( $port, $quiet )
          or die("Can't open $port: $!");
        $PortObj->baudrate(4800);
        $PortObj->parity('none');
        $PortObj->databits(8);
        $PortObj->handshake('none');

        # when nmea data is available every second
        $PortObj->read_const_time(1100);
        $PortObj->read_char_time(700);
        $PortObj->stopbits(1);
        $PortObj->write_settings or undef $PortObj;
        $PortObj->save($config_file)
          or die("Can't save $config_file: $!");

        # close serial port
        $PortObj->close
          or warn("Close $PortObj at $port failed: $!");

        # destroy serial object
        # frees memory back to perl
        undef $PortObj;
    } else {
        print("$config_file already exists!");
    }

    # tie filehandle to read/write from serial port (nmea device)
    my $fh = gensym();
    tie( *$fh, 'Device::SerialPort', $config_file )
      or die("Can't tie $port: $!");
    my $self = bless {
                       PortName              => $port,
                       FileHandle            => $fh,
                       ConfigurationFileName => $config_file
    }, $class;
    $self->{'FileHandle'}->autoflush(1);
    return $self;
}

# this shouldn't be necessary
sub destroy_create_fh {
    my $self    = shift;
    my $fh_self = $self->{'FileHandle'};
    tie( *$fh_self, 'Device::SerialPort', 
$self->{'ConfigurationFileName'} )
      or die("Can't tie $self->{'PortName'}: $!");
    $self->{'FileHandle'} = $fh_self;
}


sub get_heading {
    my $self = shift;
    if ( !-e $self->{'PortName'} ) {
        warn("Device $self->{'PortName'} doesn't exist");
    }
    $self->destroy_create_fh();
    while ( my $line = readline( $self->{'FileHandle'} ) ) {
        print "Do something with $line";
        return $line;
    }
}

1;

[download]

Here's the script that calls the module. The device /dev/nmea is a symlink to /dev/ttyUSBX and created with an udev rule. The strings that appear look like: http://www.hemispheregps.com/gpsreference/GPHDT.htm

use strict;
use warnings;

use lib '/usr/local/lib';
use GPS::NMEAserial;

my $nmea_unit =
  GPS::NMEAserial->new_nmea_port( '/dev/nmea', 
                   "/var/run/.nmea_unit.conf" );

my $heading_string = $nmea_unit->get_heading();
print "$heading_string\n";
sleep(10);
my $heading_string2 = $nmea_unit->get_heading();
print "$heading_string2 is not most recent value\n";

[download]

I tried to leave it as small as possible. Mainly following part is bugging me.

    # tie filehandle to read/write from serial port (nmea device)
    my $fh = gensym();
    tie( *$fh, 'Device::SerialPort', $config_file )
      or die("Can't tie $port: $!");
    my $self = bless {
                       PortName              => $port,
                       FileHandle            => $fh,
                       ConfigurationFileName => $config_file
    }, $class;
    $self->{'FileHandle'}->autoflush(1);
    return $self;

[download]

How can I create a filehandle for this module that it will always return me the last line from the serial port output? Or should I use a different approach for serial ports when using it in a module?

In reply to serial port tied filehandle autoflush by kkaiser

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