in reply to Read from a Linux pipe hangs.

Hello.
First of all using the diamond operator (<>) is *not* non-blocking.
Also, using print means that you use buffers; the problem is not in the consumer, but in the producer.
See the following working examples:
#!/usr/bin/perl -w
# producer.pl

use strict;

while(1){
        syswrite(STDOUT,"Hello\n",length("Hello\n"));
        sleep(2);
}

[download]

#!/usr/bin/perl -w
# consumer.pl

use strict;

my $line;

while ($line = <STDIN>){
    print "$line";
}
=pod
my $line;
my $buf;

while (sysread(STDIN,$buf,1)){
    if ($buf =~ /\n|\r|\r\n/o){
        print "$line\n";
        $line = "";
    }else{
        $line.=$buf;
    }
}
=cut

[download]
Hope this helps ;-)
Dodge This!

Replies are listed 'Best First'.
Re^2: Read from a Linux pipe hangs.
by dmor4477 (Initiate) on Feb 23, 2005 at 20:18 UTC 
    
Yes they work very well. Now there's the second part of the problem:

I need the read non-blocking of check if there's any
data ready before doing the 'sysread()'.

The complete consumer is a program that reads account
packets from a radius server (this problem has been solved)
and stores the Radius attributes (MAC,IP,User_name) in a 
List of Hashes. This list of hashes has to be processed 
every 60 seconds.

The consumer only reads from stdin in order to add new
packets to the listofHashes.


Here there's the consumer that works with:
#cat accounts| ./consumer.pl
where accounts has:
MAC1;IP1;USer1
MAC2;IP2;USer2


#consumer.pl
#!/usr/bin/perl -w
                                                                                                                            
my (%account,@accountList,@accountListTemp);
                                                                                                                            
while(1){
    my (@lines,$line,@values);
    sleep(60);
    @lines = <STDIN>;
    foreach $line (@lines){
        #MAC;IP;User_Name\n
        chomp($line);
        @values = split ';',$line;
        $account{"MAC"} = $values[[0]];
        $account{"IP"} = $values[1];
        $account{"Name"} = $values[2];
        push @accountList, {%account};
    }
                                                                                                                            
    my $i;
    for $i (0 .. $#accountList){
            #Process @accountList <-- IMPORTANT !!                                                                                                                            
    }
}
[reply]
      You can read non blocking using select, and looping trough select until there is some data to read.
      Reading using the diamond operator or sysread is blocking. The difference between the two is that using sysread you can control how much data to read at once.
      Also, i see you are using cat some_file|consumer.pl, so i don't understand why you are looping and sleeping in consumer, as cat happens to give you data only once ;-) (if you "cat" a closed file)
      I also think that you are using the wrong approach here. If you want the consumer to read data at certain amounts of time, a better approach IMHO is to do blocking reads in a while(1) without sleeping, and feeding the consumer at the needed intervals. I.E: 
      shell> for ((;;)); do cat /usr/share/dict/words;sleep 10 ;done |./cons
+umer.pl

      [download]
      Note that by not running cat just once, you will not receive end-of-file, and as such you need to read using sysread from the consumer.
      Dodge This!
[reply]
[d/l]
[select]
        Hi,
Using Select and syswrite(), the system works better but there's a buffer problem:

If the producer prints the first line, the consumer only reads the first character. When the producer prints the
second line, then the consumer reads the rest of the fist 
line characters but can't read the second line so the 
consumer never reads the last line.


Here's the code and the result that illustrates the problem:

#producer.pl                                                                                                                            
my $i;
for $i (0 .. 3){
        syswrite(STDOUT,"Hello_$i\n",length("Hello_$i\n"));
        $i=$i+1;
        sleep(5);
}
while(1){} #The real producer never ends.

#Consumer.pl
$select = IO::Select->new();                                                                                                                            
$select->add(\*STDIN);                                                                                                                           
while (1){
    if(@ready=$select->can_read(0)){
       if(read($ready[0], $tmp, 1))
       {
                print "Data read $tmp\n";
                if ($tmp =~ /\n|\r|\r\n/o){
                         print "$line\n";
                         $line = "";
                }else{
                        $line.=$tmp;
                }
       }
                                                                                                                                   if(eof($ready[0])){
          last;
       }
   }else{
       print "Process.\n";
       sleep(5);
   }
}

#Result
./producer.pl | ./consumer.pl
Data read H
Process.
Data read e
Data read l
Data read l
Data read o
Data read _
Data read 0
Data read
 
Hello_0
Process.
Data read H
Data read e
Data read l
Data read l
Data read o
Data read _
Data read 1
Data read
 
Hello_1
Process.
Data read H
Data read e
Data read l
Data read l
Data read o
Data read _
Data read 2
Data read
 
Hello_2
Process.
Process.

If the producer dies, the consumer can read the last line,
but it never happens.

Thanks,
[reply]
        Hi,
Using Select and syswrite(), the system works better but there's a buffer problem:

If the producer prints the first line, the consumer only reads the first character. When the producer prints the
second line, then the consumer reads the rest of the fist 
line characters but can't read the second line so the 
consumer never reads the last line.


Here's the code and the result that illustrates the problem:

#producer.pl                                                                                                                            
my $i;
for $i (0 .. 3){
        syswrite(STDOUT,"Hello_$i\n",length("Hello_$i\n"));
        $i=$i+1;
        sleep(5);
}
while(1){} #The real producer never ends.

#Consumer.pl
$select = IO::Select->new();                                                                                                                            
$select->add(\*STDIN);                                                                                                                           
while (1){
    if(@ready=$select->can_read(0)){
       if(read($ready0, $tmp, 1))
       {
                print "Data read $tmp\n";
                if ($tmp =~ /\n|\r|\r\n/o){
                         print "$line\n";
                         $line = "";
                }else{
                        $line.=$tmp;
                }
       }
                                                                                                                                   if(eof($ready0)){
          last;
       }
   }else{
       print "Process.\n";
       sleep(5);
   }
}

#Result
./producer.pl | ./consumer.pl
Data read H
Process.
Data read e
Data read l
Data read l
Data read o
Data read _
Data read 0
Data read
 
Hello_0
Process.
Data read H
Data read e
Data read l
Data read l
Data read o
Data read _
Data read 1
Data read
 
Hello_1
Process.
Data read H
Data read e
Data read l
Data read l
Data read o
Data read _
Data read 2
Data read
 
Hello_2
Process.
Process.

If the producer dies, the consumer can read the last line,
but it never happens.

Thanks,
[reply]

In Section Seekers of Perl Wisdom