in reply to Re^4: Measuring the sound level (dB(A)) with PERL
in thread Measuring the sound level (dB(A)) with PERL

Can you show me the output of the following code? It's a slightly modified version of your original code. According to you the else-branch should trigger if there's nothing to be read from /dev/dsp1.

#!/usr/bin/perl

use strict;
use warnings;

# input device
my $input = "/dev/dsp1";

# open filehandler
open(my $fh, '<', $input) or die("ERROR open $input ($!)\n");
binmode($fh);

while(1) {
    my $buffer;

    # read one byte
    read($fh, $buffer, 1);

    if(defined($buffer)) {
        # ignore
    }
    else {
        print "nothing to collect\n";
    }
}

close($fh);

[download]

If the else-branch does indeed trigger then I wonder what's going on. From my understanding of Perl the if-condition is always true, even if there's nothing to be read.

#!/usr/bin/perl

use strict;
use warnings;

open my $fh, '<', '/dev/null';
binmode($fh);

my $buffer;

# read one byte
read($fh, $buffer, 1);

if(defined $buffer) {
    print "read something\n";
}
else {
    print "nothing to collect\n";
}

[download]

returns

read something

Which is a lie. According to read you're supposed to check the return value, modifiying the code accordingly

#!/usr/bin/perl

use strict;
use warnings;

open my $fh, '<', '/dev/null';
binmode($fh);

my $buffer;

# read one byte
my $r = read($fh, $buffer, 1);

if($r > 0) {
    print "read something\n";
}
else {
    print "nothing to collect\n";
}

close($fh);

[download]

yields:

nothing to collect

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Re^6: Measuring the sound level (dB(A)) with PERL
by John-Robie (Novice) on Nov 10, 2016 at 13:19 UTC

    your code included:

    
[...]
    # read one byte
    read($fh, $buffer, 1);

    if(defined($buffer)) {
        # get an unsigned char
        my $v = unpack("C", $buffer);

        # sum the squared distance from the theoretical mean
        $sse += ($v-127.5)**2;
        $cnt++;
    } else {
        print "No byte here\n";
    }
[...]

    [download]

    The ELSE condition does not show up a single time:

    OUTPUT:

root@node:/srv/perl# ./dba2.pl
dbFSrms: -20.9475641203622, dba: 89.0524358796378
dbFSrms: -21.662467195052, dba: 88.337532804948
dbFSrms: -21.5176778028847, dba: 88.4823221971153
dbFSrms: -24.4135001203418, dba: 85.5864998796582
dbFSrms: -23.0140885363105, dba: 86.9859114636895
dbFSrms: -18.5448040737999, dba: 91.4551959262001
dbFSrms: -21.636493545482, dba: 88.363506454518
dbFSrms: -20.3636900956917, dba: 89.6363099043083
dbFSrms: -21.3307953726968, dba: 88.6692046273032
dbFSrms: -22.1928440805489, dba: 87.8071559194511
 [...]

    [download]

    If there is nothing to read, the read command seems to for something to read.... this seems to be a special behavior of the read command on devices (at least on /dev/dsp1). If there is something to read it will do the calculation as 8000 values were reached (which is 8000 values per second == 8KHz...

[reply]
[d/l]
[select]

      Ah. So the read() simply blocks until there actually is something to read and then continues.

      Which indicates you can either remove the if-condition completely (since it does nothing) -or- replace it with something that acts on the return value (in case there is an actual i/o error).

[reply]
        That's correct!
[reply]

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