[Mono-dev] SIGSEGV in Cairo.CairoAPI:cairo_get/set_matrix()

Jonathan Pryor jonpryor at vt.edu
Mon Oct 31 21:37:39 EST 2005

On Mon, 2005-10-31 at 18:54 -0700, Peter Dennis Bartok wrote:
> >Regarding use of ref/out/neither: I think use of ref or out is
> >*required*. It seems that (most) cairo calls involving matrices expect a
> >cairo_matrix_t* argument. As I understand it, this means that these
> That is correct, cairo expects a pointer (aka reference), not the actual 
> value, therefore ref (or [in, out]) is required. If you drop the ref you 
> will be passing by value (ie putting the whole struct on the stack, instead 
> of just the pointer)

Just to mention (for completeness), Matrix_T could instead be a class
with a [StructLayout (LayoutKind.Sequential)] attribute applied to the
class.  This would marshal as a pointer, meaning:

 1. You wouldn't need `ref' or `out' anymore
 2. You can't pass any instances by value

Given that you're always passing parameters by pointer, this may be a
desirable feature.  Beware class types as DllImport return values
though, since the runtime expects to free the memory returned from the
DllImported function using platform-specific mechanisms.

See also:


(With a cautionary note that the above is probably partially wrong --
Marshal.PtrToStructure() probably works with classes as well as structs.
I have to test that.)

 - Jon

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