Passing Perl pointers via PerlObjCBridge

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James Reynolds - 05 Jul 2005 05:56 GMT

I've tried passing perl pointers to Cocoa methods using the
PerlObjCBridge. I can't get it to work, probably because Perl
pointers "don't let you peek and poke at raw memory locations" (Adv
Perl Programming). Does anyone know of a way to get this to work?

For example, I'm trying to pass an int by reference:

#!/usr/bin/perl

use Foundation;

my $value;
$rawdata = NSString->stringWithFormat_("12358D");
$scanner = NSScanner->scannerWithString_( $rawdata );
$scanner->scanHexInt_( \$value );
print $value;

Nothing prints. Here is the Cocoa code, which works.

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

int val;
NSString *rawdata = [NSString stringWithFormat:@"12358D"];
NSScanner *scanner = [NSScanner scannerWithString:rawdata];
[scanner scanHexInt:&val];
NSLog (@"%d", val);

[pool release];
return 0;
}

Another example is this, which tries to pass a pointer to raw bytes.
However, I may not have a full understanding of this code, as I don't
really work with raw data much (pack, and dataWithBytes expects a
"const void *"). (I've tried many different forms of the following
code, and I haven't gotten anything to work correctly).

#!/usr/bin/perl

use Foundation;

$bytes = pack ("H*", "12358D" );
$data = NSData->dataWithBytes_length_( \$bytes, length $bytes );
print $data->description->cString();

--

James Reynolds
http://james.magnusviri.com
james@magnusviri.com - james@scl.utah.edu

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Sherm Pendley - 05 Jul 2005 07:18 GMT

> I've tried passing perl pointers to Cocoa methods using the
> PerlObjCBridge.

Perl doesn't have pointers, Perl has references. They're
superficially similar, but under the hood they're completely
different beasts.

> For example, I'm trying to pass an int by reference:

... Broken example snipped ...

> Nothing prints. Here is the Cocoa code, which works.
>
[quoted text clipped - 12 lines]
> return 0;
> }

Here's a Perl version that works in both PerlObjCBridge and CamelBones:

#!/usr/bin/perl

use strict;
use warnings;

use Foundation;

# Or, use CamelBones instead... :-)
# use CamelBones qw(:Foundation);

# Allocate enough space to hold a 4-byte int
my $value = "\0" x 4;

# Create a scanner
my $scanner = NSScanner->scannerWithString_('12358D');

# Get a pointer to $value, with pack(). That pointer is returned
# in a 4-byte string scalar, *not* as a numeric value - to
"promote"
# those 4 bytes to a numeric value, we unpack() them:
my $valuePointer = unpack("L", pack("p", $value));

# Scan
$scanner->scanHexInt_($valuePointer);

# Now, use unpack() to interpret the four bytes that were stored in
# $value as an integer
my ($intValue) = unpack("I", $value);

print "Int value: $intValue\n";

sherm--

Cocoa programming in Perl: http://camelbones.sourceforge.net
Hire me! My resume: http://www.dot-app.org

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Mac Forum / Programming / Perl / July 2005

Passing Perl pointers via PerlObjCBridge