morgon has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I would like to grab a Linux input-device via the EVIOCGRAB-ioctl call.

This is what's in my /usr/include/linux/input.h: 

#define EVIOCGRAB               _IOW('E', 0x90, int)                  
+  /* Grab/Release device */

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So I tried the naive approach first: 
open my $dev, "<", "/dev/input/event14" or die $!;
my $r = ioctl $dev, 0x90, 1;
print "$!\n" unless defined $r;

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But that prints "Invalid argument".

So then I generated the ph-files (they were not generated when I installed perl via perlbrew) with h2ph and ran it like this: 

require "linux/input.ph";

open my $dev, "<", "/dev/input/event14" or die $!;
my $r = ioctl $dev, EVIOCGRAB(), 1;
print "$!\n" unless defined $r;

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But this again results in "Invalid argument".

So how do I do this?

I am running 5.18.1 on Debian testing.

Replies are listed 'Best First'.
Re: How to do ioctl?
by zentara (Cardinal) on May 15, 2014 at 20:04 UTC
[reply]
      I know that posting - how does it help me?

      All that "documents" is that you can (in C) use a 1 (rather than a pointer to 1) as argument.

      If I am missing what you try to say please post some working example in Perl.

[reply]
Re: How to do ioctl?
by morgon (Priest) on May 16, 2014 at 00:49 UTC
    I have now tried to following which does what I want: 
    use Inline "C";

open my $dev, "<", "/dev/input/event14" or die $!;
grab($dev->fileno, 1);

__END__
__C__

#include <linux/input.h>

void grab(int fd, int act) {
   int r =  ioctl(fd, EVIOCGRAB, act);
   printf("grab: %d\n", r);
}

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    But it seems silly to me to resort to C just to issue an ioctl-call.

    Does anybody know how to do this in pure Perl?

[reply]
[d/l]
      What number is EVIOCGRAB? What does Inline think it is, and what does input.ph think it is?
[reply]
        Good point.

        I've tried the following: 

        printf("ph: %u\n", EVIOCGRAB());

open my $dev, "<", "/dev/input/event7" or die $!;
grab($dev->fileno, 1);

__END__
__C__

#include <linux/input.h>

void grab(int fd, int act) {
   printf("c: %u\n", EVIOCGRAB);
   int r =  ioctl(fd, EVIOCGRAB, act);
   printf("grab: %d\n", r);
}

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        And sure enough it prints:

        ph: 1073759632
        c: 1074021776

        So for whatever reason there is a mismatch.

        If I don't use the .ph-files but simply do ioctl $dev, 1074021776, 1;it works.

        So it seems the problem lies with h2ph....

        The way I generated the phs was I went to /usr/include and from there I issued "h2ph -a -d <some output-dir> linux/input.h"

        Was that the wrong way to do it? And is it normal that I have to do that on my own (as I said I installed via perlbrew and there were no .phs generated then?

        But at least I know how to do it now...

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[d/l]
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