http://qs321.pair.com?node_id=11123410


in reply to eof() blocking

From perldoc -f eof

Note that this function actually reads a character and then "ungetc"s it, so isn't useful in an interactive context.
What I would do instead: 

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11123397
use warnings;
use Socket;

my $child;      #filehandle to child process
my $parent;     #filehandle to parent process
my $pid;        #Process ID of child process

socketpair($child, $parent, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
                                                or die "socketpair: $!
+";
$child->autoflush(1);
$parent->autoflush(1);

if ($pid = fork())
  {
  close $parent  or die "close: $!\n";
  sleep 1;
  print STDOUT time%1000, ": polling child\n";
  my $buf = '';
  while( sysread $child, $buf, 4096, length $buf )
    {
    while( $buf =~ s/(.*)\n// )
      {
      print STDOUT time%1000, ": received <$1>\n";
      }
    }
  print STDOUT time%1000, ": end of input from child\n";
  wait;
  }
elsif( defined $pid ) 
  {
  close $child or die "close: $!\n";
  print STDOUT time%1000, ": child started\n";
  sleep 5;
  print $parent time%1000, ": child printed\n";
  close $parent  or die "close: $!\n";
  exit;
  }
else
  {
  die "cannot fork: $!";
  }

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Replies are listed 'Best First'.
Re^2: eof() blocking
by azadian (Sexton) on Nov 05, 2020 at 10:07 UTC
    Thanks for your input. I have a hard time regarding this as an "interactive" context, but I guess the moral of the story is that eof() only works if it's an honest-to-God file. The fact that it blocks seems to me completely contrary to the whole point of the function. The system IO is what I was hoping to avoid, but it looks like I can't.
[reply]

      While I'm not completely sure of what you want, it appears to me you are looking for something like a "datawaiting()" call instead of an end-of-file.
      Here's a (somewhat) minimal change to your program with the end-of-file replaced :) 

      #!/usr/bin/perl

use diagnostics;
use strict 'subs';
use strict 'refs';
use Socket;
use IO::Handle;

my $child;      #filehandle to child process
my $parent;     #filehandle to parent process
my $pid;        #Process ID of child process

socketpair($child, $parent, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
                                                or die "socketpair: $!
+";
$child->autoflush(1);
$parent->autoflush(1);
if ($pid = fork()) {                    #parent
    close $parent  or die "close: $!\n";
    sleep 1;
    print STDOUT time%1000, ": polling child\n";
    if (not datawaiting($child)) {                        ##### CHANGE
+D
        print STDOUT time%1000, ": no input from child yet\n";
    } else {
        my $line = <$child>;
        chomp $line;
        print STDOUT time%1000, ": received <$line>\n";
    }
} else {                                #child
    die "cannot fork: $!"  unless defined $pid;
    close $child or die "close: $!\n";
    print STDOUT time%1000, ": child started\n";
    sleep 5;
    print {$parent} time%1000, ": child printed\n";
    close $parent  or die "close: $!\n";
    exit;
}

sub datawaiting  ####### CHANGED see perldoc -f select
  {
  my $rin = my $win = my $ein = '';
  vec($rin, fileno(shift), 1) = 1;
  return scalar select my $rout = $rin, my $wout = $win, my $eout = $e
+in, 0;
  }

      [download]

      Outputs: 

      277: child started
278: polling child
278: no input from child yet

      [download]

      Is that what you were looking for ?

[reply]
[d/l]
[select]

      With IO::Select, this isn't too much to avoid, is it? 

      #!/usr/bin/perl

use diagnostics;
use strict 'subs';
use strict 'refs';
use Socket;
use IO::Handle;
use IO::Select; #### ADDED

my $child;      #filehandle to child process
my $parent;     #filehandle to parent process
my $pid;        #Process ID of child process

socketpair($child, $parent, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
                                                or die "socketpair: $!
+";
$child->autoflush(1);
$parent->autoflush(1);
if ($pid = fork()) {                    #parent
    close $parent  or die "close: $!\n";
    sleep 1;
    print STDOUT time%1000, ": polling child\n";
    if ( IO::Select->new($child)->can_read(0) ) {  ############### CHA
+NGE
        my $line = <$child>;
        chomp $line;
        print STDOUT time%1000, ": received <$line>\n";
    } else {
        print STDOUT time%1000, ": no input from child yet\n";
    }
} else {                                #child
    die "cannot fork: $!"  unless defined $pid;
    close $child or die "close: $!\n";
    print STDOUT time%1000, ": child started\n";
    sleep 5;
    print {$parent} time%1000, ": child printed\n";
    close $parent  or die "close: $!\n";
    exit;
}

      [download]
[reply]
[d/l]

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