how to use O_ASYNC and fcntl in perl?

Question

i want to use O_ASYNC option and when the pipe can read , the SIGIO's handler will run .

but the following code are not work . any one can help me ?

#!/bin/env perl
use Fcntl;
 $SIG{IO}= sub {
   print "catch SIGIO!\n";
};

my $flags=0;

open(FH,"-|","sleep 4 ;echo aaa") or die "$!";

fcntl(FH,F_GETFL,$flags) or die "$!";
fcntl(FH,F_SETFL,$flags | O_NONBLOCK | O_ASYNC) or die "$!";

sleep(5);

print  "complete\n";

my perl version is 5.16.1 , operation system is Redhat 5u4 ,kernel 2.6.18, x86_64

Answer 1

Under Linux, you must both request asynchronous notification (O_ASYNC) and specify a recipient (F_SETOWN). So, you need only add one line to your example to make it work:

#!/bin/env perl

use Fcntl;

$SIG{IO}= sub {
   print "catch SIGIO!\n";
};

my $flags=0;

open(FH,"-|","sleep 4 ;echo aaa") or die "$!";

fcntl(FH,F_GETFL,$flags) or die "$!";
fcntl(FH,F_SETFL,$flags | O_NONBLOCK | O_ASYNC) or die "$!";
fcntl(FH,F_SETOWN,0 + $$) or die "$!";  # <-- Note that we force $$ to be numeric

sleep(5);

print  "complete\n";

Running the above:

$ perl so-12640993.pl
catch SIGIO!
complete

Answer 2

SIGIO based asynchronous IO is edge-triggered, not level triggered.

Before any filehandle will send you a SIGIO you must first "arm" it. To do that, you need to perform whatever is the operation - sysread() in this case - until you get an undef/EAGAIN. At that point the filehandle will now be armed for SIGIO and will send a signal when it is next read-ready. You can then read it until it yields EAGAIN, which will arm it once more.