Pass a complex struct to the Windows API

Question

I am trying to use the GetConsoleScreenBufferInfo(HANDLE, PCONSOLE_SCREEN_BUFFER_INFO) function from the Windows API using Perl 6 and (of course) NativeCall.

I think I have set up the CONSOLE_SCREEN_BUFFER_INFO struct the function needs correctly, but the code crashes after the call when I try to dump its content.

This is the shortest (not quite but close) way to demonstrate the problem:

use NativeCall;

constant \HANDLE            := Pointer[void];
constant \SHORT             := int16;
constant \USHORT            := uint16;
constant \WORD              := uint16;
constant \DWORD             := uint32;
constant \BOOL              := int32;
constant \STD_OUTPUT_HANDLE := -11;
constant \STD_INPUT_HANDLE  := -10;

class COORD is repr('CStruct')            {
  has SHORT $.X;
  has SHORT $.Y;
}

class SMALL_RECT is repr("CStruct")            {
  has SHORT $.Left;
  has SHORT $.Top;
  has SHORT $.Right;
  has SHORT $.Bottom;
};

class CONSOLE_SCREEN_BUFFER_INFO is repr("CStruct")            {
  has COORD $.dwSize;
  has COORD $.dwCursorPosition;
  has WORD $.wAttributes;
  has SMALL_RECT $.srWindow;
  has COORD $.dwMaximumWindowSize;

  submethod TWEAK {
    $!dwSize := COORD.new;
    $!dwCursorPosition := COORD.new;
    $!srWindow := SMALL_RECT.new;
    $!dwMaximumWindowSize := COORD.new;
  }
}

# C: BOOL WINAPI GetConsoleScreenBufferInfo(_In_  HANDLE hConsoleOutput, _Out_ PCONSOLE_SCREEN_BUFFER_INFO lpConsoleScreenBufferInfo);
sub GetConsoleScreenBufferInfo(HANDLE, CONSOLE_SCREEN_BUFFER_INFO is rw) is native("Kernel32.dll") returns BOOL { * };
sub GetStdHandle(DWORD) is native('Kernel32') returns Pointer[void]  { * };

my CONSOLE_SCREEN_BUFFER_INFO
  $info = CONSOLE_SCREEN_BUFFER_INFO.new;

my HANDLE
  $handle-o = GetStdHandle( STD_OUTPUT_HANDLE );

dd $info;
say "GetConsoleScreenBufferInfo ", GetConsoleScreenBufferInfo( $handle-o, $info );
say "Will I live?";
dd $info; #crashes without notice

Any hints as to why the crash occurs and how to fix it are very welcome.

Answer 1

You need to use HAS instead of has for the members of CONSOLE_SCREEN_BUFFER_INFO that are structures as these are embedded instead of referenced by pointer (which is the Perl6 default).

Once you do that, you can drop the TWEAK as well so the code will read

class CONSOLE_SCREEN_BUFFER_INFO is repr("CStruct") {
  HAS COORD $.dwSize;
  HAS COORD $.dwCursorPosition;
  has WORD $.wAttributes;
  HAS SMALL_RECT $.srWindow;
  HAS COORD $.dwMaximumWindowSize;
}