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

I write a C version, but It was also not work: can you give me a C example ? — Chinaxing, Sep 29 '12 at 12:13

pilcrow · Accepted Answer · 2012-09-30T18:48:14.237

4

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

edited Sep 30 '12 at 18:48

answered Sep 29 '12 at 17:13

pilcrow

56,591
13
94
135

Why use the fancy `"-|"` argument instead of a normal pipe open? Good catch on nummifying `$$`, BTW. – tchrist Sep 30 '12 at 19:20
@tchrist, re: `"-|"`, just copying the OP's fanciness. Re: `0+$$`, thanks. – pilcrow Sep 30 '12 at 19:32
the default owner of a file is who ? is not the process who open it ? – Chinaxing Oct 03 '12 at 14:23

score 1 · Answer 2 · answered Sep 30 '12 at 17:10

1

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.

answered Sep 30 '12 at 17:10

LeoNerd

8,344
1
29
36

This analysis is mistaken. If I `open(F, '-|')` and have the child `for ('foo', 'bar') { say; sleep 2; } exit`, and the parent will get three SIGIOs without doing anything to `F` other than O_ASYNC/F_SETOWN. [Example here](http://pastebin.com/CyjGJtEA), also under kernel 2.6.18. – pilcrow Sep 30 '12 at 19:11

how to use O_ASYNC and fcntl in perl?

2 Answers2

Linked