Linux device filehandle roadblock

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

Hi,

I'm trying to capture data from a linux device file; in the form:

#!/../../perl -w

open(FF, "< /dev/mouse") || die "cannot open handle \n";
while (<FF>)  {
    $vr = $_;
    print $vr;

 }
close (FF);
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I keep getting the error "readline() on closed filehandle FF on line 3"

I've tried this with /dev/ttyS1 , /dev/usb/USB/scanner0 and /dev/fd0

This works fine if I use a test file, like :

   open(FF, "< /path/to/file.txt" );
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Also, I tried this in the form: sysopen(FF, "/dev/fd0", O_RDONLY); but I always get the error that says "item O_RDONLY isn't numeric at line 3"

Why doen't the sysopen work ? why does the device file always seem closed or non-existent ?

I am using Linux 9 and Perl 5.8 . I would hope not to need any Perl modules for something this basic... Help Help..... Happi..

Edited by Chady -- code tags and minor formatting.

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Re: Linux device filehandle roadblock by amw1 (Friar) on Jun 11, 2004 at 18:42 UTC
this appears to work: or at least stuff is output. I've never used the POSIX::read/open stuff before so there may be holes I didn't catch. did the sprintf stuff to get the hex stream rather than unprintable garbage. May not work the way I think it does. :) `+ use strict; my $data; my $fd; my $buf; use POSIX; $fd = POSIX::open("/dev/mouse", &POSIX::O_RDONLY) \|\| die ($!); while($data = POSIX::read($fd, $buf, 1)) { print "READ $data Bytes: " . join("", map(sprintf("%x",ord($_)), sp +lit(//, $buf))) . "\n"; }` [download]	[reply] [d/l]
Re: Linux device filehandle roadblock by pbeckingham (Parson) on Jun 11, 2004 at 18:09 UTC
You need a numeric equivalent to `O_RDONLY`, from the `<sys/fcntl.h>`, which has the value 0 on my sytem.	[reply] [d/l] [select]
Re^2: Linux device filehandle roadblock by Fletch (Bishop) on Jun 11, 2004 at 18:17 UTC
Or you could do the correct thing and use the value provided by Fcntl and be right on anybody's system.	[reply]
Re^2: Linux device filehandle roadblock by happi (Initiate) on Jun 15, 2004 at 19:40 UTC
Thanks, I'll investigate this.. :)	[reply]
Re: Linux device filehandle roadblock by zentara (Cardinal) on Jun 12, 2004 at 15:24 UTC
This is just a WAG, but I remember playing around with something similar awile back, and had some luck by rewinding the file handle after every read. Something like (totally pseudo-code): `sysopen FH, /dev/fd0, O_RDWR\|O_CREAT,0666 or die "Cannot open : $!"; while(1){ seek FH, 0, SEEK_SET or die "Cannot rewind $file: $!"; my $data = <FH>; #truncate FH, 0 print "$data\n"; }` [download] The error "error that says "item O_RDONLY isn't numeric at line 3" may be caused by not having "use Fnctl;" at the top. I'm not really a human, but I play one on earth. flash japh	[reply] [d/l]