On Thu, May 7, 2009 at 05:45, Raymond Wan <[email protected]> wrote:
> Hi all,
>
> I would like to write binary values to disk (as well as read them) but don't
> know how to do it. In C-speak, something like this:
>
> unsigned int foo = 42;
> fwrite (&foo, sizeof (unsigned int), 1, stdout);
>
> I think the answer involves something with pack and unpack, but I'm
> completely lost as I have no experience with either. The closest I got was
>
> my $decimal_number = 42;
> my $binary_number = unpack("B32", pack("N", $decimal_number));
> print "Decimal number " . $decimal_number . " is " . $binary_number .
> " in binary.\n\n";
>
> which I've taken from
> http://www.linuxconfig.org/Perl_Programming_Tutorial. This doesn't
> work since it's printing the number "42" in text binary; but
> I think it is close... Or I might be barking up the wrong tree and I should
> be thinking about "fprintf" somehow...
>
> Does anyone have any ideas? Thank you!
>
> Ray
>
You use pack to create a binary value and unpack to read a binary
value. So, to write a file containing three 32 bit integers you say
#!/usr/bin/perl
use strict;
use warnings;
my $file = "/tmp/3ints";
#the :raw here tells Perl that the file will
#be binary
open my $fh, ">:raw", $file
or die "could not open $file: $!\n";
print $fh pack "lll", 1230, -897, 20;
close $fh or die "could not close $file: $!\n";
my $size = (stat $file)[7];
print "$file contains $size bytes\n";
And then to read the file, you say:
#!/usr/bin/perl
use strict;
use warnings;
my $file = "/tmp/3ints";
my $size = 12;
#the :raw here tells Perl that the file will
#be binary
open my $fh, "<:raw", $file
or die "could not open $file: $!\n";
sysread($fh, my $buffer, $size) == $size
or die "did not read $size bytes from $file\n";
close $fh or die "could not close $file: $!\n";
print "the ints are ", join(", ", unpack "lll", $buffer), "\n";
--
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.
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