Basic MathGL example in c does not compile - c

Question

Here is the basic c example from the documentation:
#include <mgl2/mgl_cf.h>
int sample(HMGL gr, void *)
{
mgl_rotate(gr,60,40,0);
mgl_box(gr);
}
int main(int argc,char **argv)
{
HMGL gr;
gr = mgl_create_graph_qt(sample,"MathGL examples",0,0);
return mgl_qt_run();
/* generally I should call mgl_delete_graph() here,
* but I omit it in main() function. */
}
Here is the start of compilation output:
$ gcc test.c -lmgl-qt5 -lmgl
In file included from /usr/include/mgl2/mgl_cf.h:29,
from test.c:1:
/usr/include/mgl2/data_cf.h:527:17: error: expected ‘,’ or ‘;’ before ‘mgl_find_roots’
527 | bool MGL_EXPORT mgl_find_roots(size_t n, void (*func)(const mreal *x, mreal *f, void *par), mreal *x0, void *par);
| ^~~~~~~~~~~~~~
test.c: In function ‘sample’:
test.c:2:21: error: parameter name omitted
2 | int sample(HMGL gr, void *)
| ^~~~~~
It seems clear to me that the example is not even valid c, missing a parameter (that is not actually used) to the sample() function. I have tried removing it but still get the first (internal mathgl) error.
Any ideas how to proceed?

Answer 1

It seems MathGL doesn't have their internal #include statements in order, and require you to be careful about what you #include and in what order. In particular, ensure you #include <mgl2/mgl.h> before any other MathGL header, and before that one ensure you #include <stdbool.h>. Futhermore, when you use for example Qt-related functions, ensure you #include <mgl2/qt.h>. This should work:
#include <stdbool.h>
#include <mgl2/mgl.h>
#include <mgl2/qt.h>
int sample(HMGL gr, void *ignored)
{
mgl_rotate(gr,60,40,0);
mgl_box(gr);
}
int main(int argc, char **argv)
{
HMGL gr = mgl_create_graph_qt(sample, "MathGL examples", 0, 0);
return mgl_qt_run();
}