Re: [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...>]

Hi Phil:

I am not quite sure how I could absolutely prove yesterday's tentative
hypothesis that statically allocated 2D arrays reference a single
block of memory with no storage of pointers to each of the row vectors
because gdb and C compilers make certain hidden assumptions that only
those with complete knowledge of the C standards could figure out. But
having slept on it, I do think that tentative hypothesis is correct.

Therefore on that assumption here is a revision of the C example which
demonstrates how easy it is to make an Iliffe vector (which does
store those pointers) from a static 2D
array.

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// C programme to test casts of a statically allocated array

#include<stdio.h>
void main(void)
{
   int zStatic[2][3];
   int* zCast = (int*)zStatic;
   int (*zCast2)[3] = zStatic;
   int * zIliffe[2];
   zStatic[0][0] = 0;
   zStatic[0][1] = 1;
   zStatic[0][2] = 2;
   zStatic[1][0] = 3;
   zStatic[1][1] = 4;
   zStatic[1][2] = 5;
   zIliffe[0] = zStatic[0];
   zIliffe[1] = zStatic[1];

   printf("%d, %d, %d, %d, %d, %d\n", zStatic[0][0], zStatic[0][1], zStatic[0][2], zStatic[1][0], zStatic[1][1], zStatic[1][2]);
   printf("%d, %d, %d, %d, %d, %d\n", zCast[0], zCast[1], zCast[2], zCast[3], zCast[4], zCast[5]);
   printf("%d, %d, %d, %d, %d, %d\n", zCast2[0][0], zCast2[0][1], zCast2[0][2], zCast2[1][0], zCast2[1][1], zCast2[1][2]);
   printf("%d, %d, %d, %d, %d, %d\n", zIliffe[0][0], zIliffe[0][1], zIliffe[0][2], zIliffe[1][0], zIliffe[1][1], zIliffe[1][2]);
}
____________________________

The (perfect) valgrind results are

irwin@raven> gcc -g test_2d_static.c 
irwin@raven> valgrind ./a.out
==18848== Memcheck, a memory error detector
==18848== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et
al.
==18848== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info
==18848== Command: ./a.out
==18848== 
0, 1, 2, 3, 4, 5
0, 1, 2, 3, 4, 5
0, 1, 2, 3, 4, 5
0, 1, 2, 3, 4, 5
==18848== 
==18848== HEAP SUMMARY:
==18848==     in use at exit: 0 bytes in 0 blocks
==18848==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==18848== 
==18848== All heap blocks were freed -- no leaks are possible
==18848== 
==18848== For counts of detected and suppressed errors, rerun with: -v
==18848== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

That conversion from static 2D array to Iliffe vector is so
straightforward, I plan to allow all our C (and C++) users to call
plshade, plshades, plcont, plimage, or plvect with an Iliffe vector
that is conveniently determined (like above) with one call to an
extremely simple routine (still to be implemented, but let's
tentatively call it plStatic2dGrid in analogy with plAlloc2dGrid) that
generates that Iliffe vector from a corresponding statically allocated
z array. And also following the plAlloc2dGrid analogy, wrap that
routine as Static2dGrid for the c++ binding, but do not propagate it
to any other binding. Then follow up by using plStatic2dGrid in
examples/c/x15c and Static2dGrid in examples/c++/x15.cc and also by
documenting these 2D array nuances for C, C++, and the rest of our
different bindings.

Development of (pl)Static2dGrid, of course, means there is little
purpose to plshade1 anymore even as a demonstration routine. 
Therefore, I have changed my plans for plshade1 back from converting
it to a C-only demonstration routine to deprecating it (and eventually
eliminating it) again.

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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