perl - Handling wide char values returned by Win32::API

Question

The answer provided in wide char and win32::api works for passing utf-16 to the Win API. But how do I convert utf16 strings returned by the Win API? (I am trying to use GetCommandLineW).

I have tried both Unicode::String and Encode::decode without success. I'm guessing that perhaps the data needs to be packed or unpacked first, but how?

After that, the next problem is how to deal with a pointer-to-pointer-to-utf16 like the one returned by CommandLineToArgvW.

Thanks for any help.

Answer 1

When you specify the return value is a string, Win32::API assumes it's a terminated by a byte with value 0, but bytes with that value are common in UTF-16le text.

As Win32::API suggests, you should use the N type (or Q on 64-bit builds) to get the pointer as a number, then read the pointed memory yourself. Win32::API's provides ReadMemory to read memory, but it requires knowing how much memory to read. That's not useful for NUL-terminated strings and wide NUL-terminated strings.

For wide NUL-terminated strings, Win32::API provides SafeReadWideCString. But SafeReadWideCString can return a string unrelated to the input on error, so I use my own decode_LPCWSTR instead.

use strict;
use warnings;
use feature qw( say state );

use open ':std', ':encoding('.do { require Win32; "cp".Win32::GetConsoleOutputCP() }.')';

use Config     qw( %Config );
use Encode     qw( decode encode );
use Win32::API qw( ReadMemory );

use constant PTR_SIZE => $Config{ptrsize};

use constant PTR_PACK_FORMAT =>
     PTR_SIZE == 8 ? 'Q'
   : PTR_SIZE == 4 ? 'L'
   : die("Unrecognized ptrsize
");

use constant PTR_WIN32API_TYPE =>
     PTR_SIZE == 8 ? 'Q'
   : PTR_SIZE == 4 ? 'N'
   : die("Unrecognized ptrsize
");

    
sub lstrlenW {
   my ($ptr) = @_;

   state $lstrlenW = Win32::API->new('kernel32', 'lstrlenW', PTR_WIN32API_TYPE, 'i')
      or die($^E);

   return $lstrlenW->Call($ptr);
}


sub decode_LPCWSTR {
   my ($ptr) = @_;
   return undef if !$ptr;

   my $num_chars = lstrlenW($ptr)
      or return '';

   return decode('UTF-16le', ReadMemory($ptr, $num_chars * 2));
}


# Returns true on success. Returns false and sets $^E on error.
sub LocalFree {
   my ($ptr) = @_;

   state $LocalFree = Win32::API->new('kernel32', 'LocalFree', PTR_WIN32API_TYPE, PTR_WIN32API_TYPE)
      or die($^E);

   return $LocalFree->Call($ptr) == 0;
}


sub GetCommandLine {
   state $GetCommandLine = Win32::API->new('kernel32', 'GetCommandLineW', '', PTR_WIN32API_TYPE)
      or die($^E);

   return decode_LPCWSTR($GetCommandLine->Call());
}


# Returns a reference to an array on success. Returns undef and sets $^E on error.
sub CommandLineToArgv {
   my ($cmd_line) = @_;

   state $CommandLineToArgv = Win32::API->new('shell32', 'CommandLineToArgvW', 'PP', PTR_WIN32API_TYPE)
      or die($^E);

   my $cmd_line_encoded = encode('UTF-16le', $cmd_line."");
   my $num_args_buf = pack('i', 0);  # Allocate space for an "int".

   my $arg_ptrs_ptr = $CommandLineToArgv->Call($cmd_line_encoded, $num_args_buf)
      or return undef;

   my $num_args = unpack('i', $num_args_buf);
   my @args =
      map { decode_LPCWSTR($_) }
         unpack PTR_PACK_FORMAT.'*',
            ReadMemory($arg_ptrs_ptr, PTR_SIZE * $num_args);

   LocalFree($arg_ptrs_ptr);
   return @args;
}


{
   my $cmd_line = GetCommandLine();

   say $cmd_line;

   my $args = CommandLineToArgv($cmd_line)
      or die("CommandLineToArgv: $^E
");

   for my $arg (@$args) {
      say "<$arg>";
   }
}