I ran in to a problem with plcont while translating example 16 to
OCaml. While writing the OCaml PLplot bindings I mistakenly assumed
that the plcont function would accept NULL for its pltr and pltr_data
arguments and fall back on a default transform of some sort. However,
setting pltr to NULL gave me segmentation faults as plcont uses the
pltr callback function regardless of its value.
I have attached three small patches to this email in the hope that one
of them will be acceptable to the core developers:
plcont_doc_pltr.diff - This patch adds a note to the plcont
documentation (api.xml) stating that behavior is undefined when pltr
is NULL.
plcont_default_pltr.diff - This patch changes plcont to use pltr0 as a
callback when the pltr argument is NULL.
plcont_pltr_null_abort.diff - This patch changes plcont to abort using
plabort with an error if the pltr argument is NULL. This is my
personal preference for a solution.
While the documentation does not say that the pltr callback can be set
to NULL for plcont, that is how (I think) all of the other plotting
functions which accept a transformation callback work. Any of the
above 3 patches will do something to remedy this problem. I
personally prefer the 3rd patch (plcont_pltr_null_abort.diff) because
it is less likely to surprise users with a potentially strange default
transform and it removes a source of potential segmentation faults or
other crashes.
Sincerely,
Hez
--
Hezekiah M. Carty
Graduate Research Assistant
University of Maryland
Department of Atmospheric and Oceanic Science
diff --git a/doc/docbook/src/api.xml b/doc/docbook/src/api.xml
index 517e0a5..3815f9b 100644
--- a/doc/docbook/src/api.xml
+++ b/doc/docbook/src/api.xml
@@ -1796,6 +1796,13 @@ from relative device coordinates,
<xref linkend="contour-plots-c"/>.
The transformation function should
have the form given by any of &pltr0;, &pltr1;, or &pltr2;.
+ Note that unlike &plshades; and similar PLplot functions
+ which have a <literal><parameter>pltr</parameter></literal>
+ argument, plcont requires that a transformation function be
+ provided in the C interface. There is no fallback if
+ <literal><parameter>pltr</parameter></literal> is NULL. Leaving
+ <literal><parameter>pltr</parameter></literal> as a NULL will
+ lead to undefined behavior.
</para>
</listitem>
</varlistentry>
diff --git a/src/plcont.c b/src/plcont.c
index 02b574e..346ed1d 100644
--- a/src/plcont.c
+++ b/src/plcont.c
@@ -485,9 +485,17 @@ c_plcont(PLFLT **f, PLINT nx, PLINT ny, PLINT kx, PLINT lx,
PLfGrid2 grid;
grid.f = f;
- plfcont(plf2eval2, (PLPointer) &grid,
- nx, ny, kx, lx, ky, ly, clevel, nlevel,
- pltr, pltr_data);
+ if (pltr == NULL) {
+ // If pltr is undefined, use a simple identity transformation
+ plfcont(plf2eval2, (PLPointer) &grid,
+ nx, ny, kx, lx, ky, ly, clevel, nlevel,
+ pltr0, pltr_data);
+ }
+ else {
+ plfcont(plf2eval2, (PLPointer) &grid,
+ nx, ny, kx, lx, ky, ly, clevel, nlevel,
+ pltr, pltr_data);
+ }
}
/*--------------------------------------------------------------------------*\
diff --git a/src/plcont.c b/src/plcont.c
index 02b574e..c217314 100644
--- a/src/plcont.c
+++ b/src/plcont.c
@@ -485,9 +485,15 @@ c_plcont(PLFLT **f, PLINT nx, PLINT ny, PLINT kx, PLINT lx,
PLfGrid2 grid;
grid.f = f;
- plfcont(plf2eval2, (PLPointer) &grid,
- nx, ny, kx, lx, ky, ly, clevel, nlevel,
- pltr, pltr_data);
+ if (pltr == NULL) {
+ // If pltr is undefined, abort with an error.
+ plabort("plcont: The pltr callback must be defined");
+ }
+ else {
+ plfcont(plf2eval2, (PLPointer) &grid,
+ nx, ny, kx, lx, ky, ly, clevel, nlevel,
+ pltr, pltr_data);
+ }
}
/*--------------------------------------------------------------------------*\
-------------------------------------------------------------------------
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