Bug 1805 - P/Invoke returning native ptr to struct with one IntPtr field
Summary: P/Invoke returning native ptr to struct with one IntPtr field
Alias: None
Product: Runtime
Classification: Mono
Component: Interop ()
Version: unspecified
Hardware: PC Linux
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2011-11-01 06:58 UTC by Dmitry Azaraev
Modified: 2011-11-02 09:48 UTC (History)
3 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Dmitry Azaraev 2011-11-01 06:58:34 UTC
I'm extending mono/tests with next:


// simple return some pointer (from some allocator)
mono_return_custom_ptr ()
    return (void*)0x12345678;


// define returning struct
public struct custom_ptr
    public IntPtr value;

and in Main method adding follow:
    custom_ptr cp = mono_return_custom_ptr();
    Console.WriteLine("{0:X8}", (int)cp.value);

make it...

Running pinvoke11.exe with any of this Mono Runtimes on Ubuntu 11.10:
Mono 2.10.5 
Mono 2.10.6 (built from sources)
Mono latest (built from sources)

got invalid result:

$ ./pinvoke11.exe
$ ./pinvoke11.exe
$ ./pinvoke11.exe
Note, that this technique works fine with Mono 2.10.6 on Windows.
Comment 1 Dmitry Azaraev 2011-11-02 04:55:23 UTC
Sorry, i got this behaviour on X86 platform.
Same test works fine with .NET 4.
Comment 2 Paolo Molaro 2011-11-02 09:34:31 UTC
Your p/invoke binding is incorrect: if the native code returns a pointer, the managed method signature must return a pointer as well. In a similar case, if the native code returned a structure containing a pointer, the managed signature would need to return a struct with a pointer.

The incorrect code happens to work on windows because the ABI there says that small structures get returned in a register (which happens to be the same register where a pointer would be returned), while on Linux they are returned with a different mechanism.
Comment 3 Dmitry Azaraev 2011-11-02 09:48:18 UTC
Thanks for clear explanation!