[Mono-devel-list] Marshalling char**

Hisham Mardam Bey hisham.mardambey at gmail.com
Sun May 29 16:52:01 EDT 2005

Hey Jonathan,

When this line runs:

IntPtr s = Marshal.ReadIntPtr (stringArray, i * IntPtr.Size);

I get:

** ERROR **: Structure field of type String[] can't be marshalled as LPArray

This is the same error I get with the rest of the calls I tried. (I
even implemented PtrToStringArray in my own code before looking at
your reply and it didnt work).

I'm not using the null terminated version of it as I know how large
the string is going to be (its an int field in the struct). Any other
tips or things I need to do to figure out this problem?

Thank you.
Best Regards,

On 5/29/05, Jonathan Pryor <jonpryor at vt.edu> wrote:
> On Sun, 2005-05-29 at 02:48 +0300, Hisham Mardam Bey wrote:
> > I wanna try and marshal a char** that is returned in a struct. You can
> > see the code here:
> >
> > http://pastebin.com/291223
> >
> > I've tried using Copy and ReadIntPtr to no avail. I might be doing
> > something wrong.
> I do not believe that char** can be marshaled without additional
> assistance; MarshalAsAttribute.SizeParamIndex *might* work for structs,
> but I don't think so, and that's the type of support the runtime
> marshaler would need to avoid manual marshaling.
> Fortunately, it's fairly straightforward to do it manually.  Mono.Unix
> marshals char** for `struct group`, so you might want to look at:
> http://mono.myrealbox.com/source/trunk/mcs/class/Mono.Posix/Mono.Unix/Syscall.cs
> Syscall.CopyGroup() converts and unmanaged representation into the
> Managed representation;
> http://mono.myrealbox.com/source/trunk/mcs/class/Mono.Posix/Mono.Unix/UnixMarshal.cs
> UnixMarshal.PtrToStringArray(), which converts an IntPtr (char**) into a
> string[], and
> http://mono.myrealbox.com/source/trunk/mono/support/grp.c
> copy_group() which converts a Unix `struct group` into the
> representation shared between Mono.Posix.dll and MonoPosixHelper.  You
> probably won't need to go through this step, since your structure isn't
> going to vary between differing Unix platforms.
>  - Jon

Hisham Mardam Bey
MSc (Computer Science)
Codito Ergo Sum (I Code Therefore I Am)

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