add greet exmaple: passing strings back and forth

examples/wasm/greet.pl

@@ -0,0 +1,7 @@
+use strict;
+use warnings;
+use Path::Tiny qw( path );
+use lib path(__FILE__)->parent->child('lib')->stringify;
+use Greet;
+
+print greet("Perl!"), "\n";

examples/wasm/lib/Greet.pm

@@ -0,0 +1,33 @@
+package Greet;
+
+use strict;
+use warnings;
+use FFI::Platypus;
+use FFI::Platypus::Memory qw( strcpy );
+use base qw( Exporter );
+use Wasm
+  -api => 0,
+  -self
+;
+
+our @EXPORT = qw( greet );
+
+sub greet
+{
+  my($subject) = @_;
+
+  my $input_size   = do { use bytes; length($subject)+1 };
+  my $input_offset = _allocate($input_size);
+  strcpy( $memory->address + $input_offset, $subject );
+
+  my $output_offset = _greet($input_offset);
+  my $greeting      = FFI::Platypus->new->cast('opaque', 'string', $memory->address + $output_offset);
+  my $output_size   = do { use bytes; length($greeting)+1 };
+
+  _deallocate($input_offset,  $input_size);
+  _deallocate($output_offset, $output_size);
+
+  return $greeting;
+}
+
+1;

examples/wasm/lib/Greet.rs

@@ -0,0 +1,34 @@
+/*
+ * rustc --target wasm32-unknown-unknown -O --crate-type=cdylib Greet.rs -o Greet.wasm
+ * chmod -x Greet.wasm
+ */
+
+use std::ffi::{CStr, CString};
+use std::mem;
+use std::os::raw::{c_char, c_void};
+
+#[no_mangle]
+pub extern fn _allocate(size: usize) -> *mut c_void {
+    let mut buffer = Vec::with_capacity(size);
+    let pointer = buffer.as_mut_ptr();
+    mem::forget(buffer);
+
+    pointer as *mut c_void
+}
+
+#[no_mangle]
+pub extern fn _deallocate(pointer: *mut c_void, capacity: usize) {
+    unsafe {
+        let _ = Vec::from_raw_parts(pointer, 0, capacity);
+    }
+}
+
+#[no_mangle]
+pub extern fn _greet(subject: *mut c_char) -> *mut c_char {
+    let subject = unsafe { CStr::from_ptr(subject).to_bytes().to_vec() };
+    let mut output = b"Hello, ".to_vec();
+    output.extend(&subject);
+    output.extend(&[b'!']);
+
+    unsafe { CString::from_vec_unchecked(output) }.into_raw()
+}