Chapter 14: Callbacks, Context, & Closures

In Chapter 12, we passed a simple callback. But real-world C libraries often take a callback and an opaque pointer (usually something like void * user_data) which allows you to store context data. However, if the library's author decided not to provide that, Affix allows you to bake the context into the callback itself using closures.

The Recipe

We will wrap a hypothetical iteration function that takes a callback but no data argument.

use v5.40;
use feature 'class';
no warnings 'experimental::class';
use Affix qw[:all];
use Affix::Build;

# 1. C Library
my $c = Affix::Build->new();
$c->add( \<<~'END', lang => 'c' );
    void iterate_3( void (*cb)(int) ) {
        cb(1); cb(2); cb(3);
    }
END
my $lib = $c->link;
affix $lib, 'iterate_3', [ Callback [ [Int] => Void ] ] => Void;

# 2. The Problem
# We want to sum these numbers into a Perl object.
# But the C function doesn't give us a slot to hand it the object directly.
class Sum {
    field $total = 0;
    method add($n) { $total += $n; }
    method get()   { $total; }
}
my $obj = Sum->new;

# 3. The Closure Solution
# We define the callback inside the scope where $obj exists.
# Perl closes over $obj.
my $cb = sub ($n) { $obj->add($n); };

# 4. Execute
iterate_3($cb);
say 'Total: ' . $obj->get();    # 6

How It Works

• 1. Closures

  When you create sub { ... } in Perl, it captures the surrounding lexical variables ($obj). Event the new perlclass objects are just SV*s at their core.

• 2. The trampoline

  When you pass $cb to Affix, infix generates a reverse trampoline. This is a C function pointer that exists just to bounce arguments back to us. When C calls it, the trampoline invokes the specific Perl CODE reference (CV*), which restores the Perl environment and finds $obj.

  To the C library, it looks like a standard function pointer. To you, it looks like magic.