[Mono-dev] Embedding Mono + SWIG string return values leads to bad free

Jonathan Chambers joncham at gmail.com
Thu May 15 09:52:13 EDT 2008

Looking at the marshal.c code, this may be a problem in the runtime. We
allocate return strings using mono_marshal_alloc (evaluates to
CoTaskMemAlloc on Windows). However, we always free returned strings using
g_free on all platforms. I suppose we should use mono_marshal_free instead?
Please review attached patch. Not sure if this will fix your issue, but it

Code is contributed under MIT/X11 license.


On Fri, Apr 25, 2008 at 7:16 PM, Miguel de Icaza <miguel at ximian.com> wrote:

> [resending, email server not working]
> On Thu, 2008-04-24 at 18:33 -0400, Miguel de Icaza wrote:
> > Hello,
> >
> > > The returned string is passed back to the SWIG C# class and used
> > > (successfully) and then later the GC tries to collect it and fails.
> > > The C# delegate is just something like
> >
> > The GC does not collect strings returned from P/Invoke, but it will free
> > return values.
> >
> > If you are returning strings that are now malloc()ed (and ownership is
> > transferred to the caller), you may want to change the P/Invoke
> > signature to return an IntPtr, and then use Marshal.PtrToStringAuto
> >
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