[Plplot-devel] Can statically allocated z arrays be used as the z argument for plshade (and plshades, plcont, plimage, and plvect)?

[Alan W. I. <ir...@be...>]

This general question has come up because plshade1 only differs from
plshade in how the z 2D matrix argument is typed and handled inside
the routine, but there is no similar plshades1, plcont1, plimage1, and
plvect1 equivalents to plshades, plcont, plimage, and plvect.  So for
consistency with the rest, I would like to deprecate plshade1 and
encourage the use of plshade instead.  But part of that deprecation is
explaining to users how to use plshade, but I need help with that.

I could recommend to users to call plshade with a z argument that
follows how the z 2D matrix argument of plcont is typed and defined.
For example, in example 15 we currently use the statically defined z
matrix

PLFLT z[35][45];

z[i][j] = ....;

plshade1( &z[0][0], ...);

And I could replace that logic with logic where z is
defined as in example 9 with plcont, i.e., as a 2D Iliffe vector
(a vector of pointers to vectors of data, see
<https://en.wikipedia.org/wiki/Iliffe_vector>).  So in this
case examples/c/x15c.c would look like the following:

PLFLT **z;

plAlloc2dGrid( &z, 35, 45);

z[i][j] = ....;

plshade( (const PLFLT * const *) z, ...);

plFree2dGrid( z, 35, 45);

Because of the similarity with how the z argument of plcont is
handled, I am positive the above method would work, but I haven't
implemented that yet because I think there _might_ be a simpler
alternative where we use a combination of

PLFLT z[35][45];

z[i][j] = ....;

and a call to

plshade( (const PLFLT * const *) z, ...);

The assumption here is that this statically allocated z is a special case
of an Iliffe vector and should "just work".  But perhaps that assumption
is wrong?

The reason I am uncertain about that assumption, is I have already
tried the above combination of statically allocated z array and that
call to plshade, and it compiles without any warnings which is
encouraging. However, the result segfaults at run time.  So the
question is whether there is something extra I have to do to get the
above combination to work (and similarly for plshades, plcont,
plimage, and plvect) or do users _always_ have to call those routines
with z dynamically allocated with plAlloc2dGrid?

Help with this question from the C experts here would be much appreciated.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
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