smyrna Init: squash -Wincompatible-pointer-types

OpenGL has an unorthodox API wherein the `gluTessCallback` function’s prototype
indicates it takes a `void(*)(void)`, but its docs¹ explain that the type
actually varies depending on the second argument. As a result, the compiler
(correctly) warns that some of these `gluTessCallback` calls are passing
function pointers that do not have the same ABI. Presumably this works out
because at the end of the day a function pointer is just some bits in C and the
OpenGL implementation branches on the `which` argument and invokes the pointer
correctly. But if OpenGL really wanted to discard type safety this way, it is
not clear to me why they did not make the function pointer argument a `void*`.
Anyway, this commit squashes the compiler warnings which emerge when enabling
this in the CMake build system, failing the build.

Gitlab: related to #1836

¹ This is not the authoritative source, but Microsoft’s docs for their
  implementation provide a good explanation.
  https://docs.microsoft.com/en-us/windows/win32/opengl/glutess

diff --git a/cmd/smyrna/polytess.c b/cmd/smyrna/polytess.c
index b31a674f790af32a42151091ed69f95b96794aa5..ee94fa324e65c07451ea2facb550748b820df546 100644 (file)
--- a/cmd/smyrna/polytess.c
+++ b/cmd/smyrna/polytess.c
@@ -42,15 +42,30 @@ static void CALLBACK vertexCallback(GLvoid *vertex)
 
 }
 
+// OpenGL’s `gluTessCallback` function has a prototype indicating it takes a
+// `void(*)(void)`. But its documentation describes passing in various function
+// pointers with differing calling conventions. To use this API while also
+// pacifying all the various build environments, we need this rather silly
+// wrapper.
+#ifdef _MSC_VER
+// MSVC is of the (correct) opinion that casting between function pointers of
+// incompatible calling conventions is unacceptable behavior…
+#define MAKE_GLU_CALLBACK(f) f
+#else
+// …nevertheless other compilers insist we cast or they believe we have made a
+// typo
+#define MAKE_GLU_CALLBACK(f) ((void (*)(void))(f))
+#endif
+
 static GLUtesselator* Init(void)
 {
     // Create a new tessellation object 
     GLUtesselator* tobj = gluNewTess(); 
     // Set callback functions
-    gluTessCallback(tobj, GLU_TESS_VERTEX, &vertexCallback);
-    gluTessCallback(tobj, GLU_TESS_BEGIN, &glBegin);
-    gluTessCallback(tobj, GLU_TESS_END, &glEnd);
-    gluTessCallback(tobj, GLU_TESS_COMBINE,&combineCallback);
+    gluTessCallback(tobj, GLU_TESS_VERTEX, MAKE_GLU_CALLBACK(vertexCallback));
+    gluTessCallback(tobj, GLU_TESS_BEGIN, MAKE_GLU_CALLBACK(glBegin));
+    gluTessCallback(tobj, GLU_TESS_END, MAKE_GLU_CALLBACK(glEnd));
+    gluTessCallback(tobj, GLU_TESS_COMBINE, MAKE_GLU_CALLBACK(combineCallback));
     return tobj;
 }