Bug 6657 - P/Invoke mess when returning a struct (x86 only)
Summary: P/Invoke mess when returning a struct (x86 only)
Alias: None
Product: Runtime
Classification: Mono
Component: Interop ()
Version: unspecified
Hardware: PC Linux
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2012-08-23 03:15 UTC by Florian Kummer
Modified: 2012-08-24 09:44 UTC (History)
3 users (show)

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

Sorce (Native, C#) and output listing and compile script (1.57 KB, application/x-zip-compressed)
2012-08-23 03:15 UTC, Florian Kummer

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Description Florian Kummer 2012-08-23 03:15:57 UTC
Created attachment 2396 [details]
Sorce (Native, C#) and output listing and compile script

Note: seems to be similar to Bug 4678: 'P/Invoke with struct return values crashes on x86', but affects different Platform; I think my example is more elementary.

I have a native library (libfakedl.so), containing the function:

  void* my_dlopen(const char* file, int flag) {
    printf("Filename = %s \n", file);
    printf("flag = %d \n", flag);
    return (void*)(666);

I compare two different P/Invoke defintitions; the frist one is just:

  [DllImport("fakedl", EntryPoint="my_dlopen", CharSet = CharSet.Ansi)]
  static extern IntPtr dlopen_working(string filename, int flag);

But for type saefty reasons, I usually declare my own return type:

  public struct DynLibHandle {
     public IntPtr val;

  [DllImport("fakedl", EntryPoint="my_dlopen", CharSet = CharSet.Ansi)]
  static extern DynLibHandle dlopen_failing(string filename, int flag);

The later declaration works fine on a x64-mono, but not on a x86 mono (a very old machine).
Using the first declaration, everything is fine, the C-Routine prints
exactly the values I have passed, also the return value is correct.
But the second declaration seems to be messy: the C-function prints

Filename = 8�п���ة�     ,�п�b�  @�V�▒ԛ   
flag = 161444176 

and the return value is wrong.

This error occurs only on a x86-machine (on two linux variants, OpenSUSE and Debian), on a x64-Debian-system everything is fine.
Comment 1 Zoltan Varga 2012-08-23 06:03:30 UTC
The second declaration is not correct, the C routine returns a pointer, not a struct containing one pointer. The two might be returned the same way on some architectures, like x86-64, but returned differently on x86.
Comment 2 Florian Kummer 2012-08-24 09:44:59 UTC
Thanks for the quick reply;
I learned somethinng today.