Can XPUSHs be used in C callbacks?

Hello,

I am new to XS and started with an existing project. From what I read and
understood in tutorials and perlapi so far, XPUSHs is a short way to put
an SV on the stack and to make sure there is enough place for it - an
alternative to use EXTEND and to assign the value directly to a stack
position.

So, when I found code like the following in a C function called as a
callback by another C function which is invoked by an xsub:

    dSP;
    EXTEND (sp, result_nr);
    sp[sp_pos] = sv_2mortal (sv);

I thought it could be shortened to

    dSP;
    XPUSHs (sv_2mortal (sv));

but was wrong.

In the original code, sp is passed through to the callback and
initialized as PL_stack_base + ax in the xsub. result_nr is set to 0 in
the xsub, passed through to the callback and incremented with each callback
invocation, being 1, 2, 3, ... in subsequent calls. Likewise, sp_pos is
starting with a value of 0 and incremented each time. The idea is to place
results on the stack subsequently, like so:

 1st call:

    dSP;
    EXTEND (sp, 1);
    sp[0] = ...;

 2nd call:

    dSP;
    EXTEND (sp, 2);
    sp[1] = ...;

 3rd call:

    dSP;
    EXTEND (sp, 3);
    sp[2] = ...;

and so on.

When I replaced the original code fragment by

    dSP;
    XPUSHs (sv_2mortal (sv));

in order to have stack extension and result positioning handled behind
the scenes, calls to the xsub failed with the error message:

    "Bizarre copy of ARRAY in aassign [...]"

Now, it is no problem to live with the original code, but I'd like to
understand what I did wrong. Can XPUSHs be used in a callback, and will
it handle stack extension and result positioning as I expected? If XPUSHs
use is possible in general, what did I wrong so that the shorter code did
not work?

Thank you for any advice in advance!

            Jochen Stenzel