I believe the following code is well-defined according to the C standard:

void f(void **);
void *p;
f((void **)&p);

But what if p isn’t a pointer to void, say:

void f(void **);
double *p;
f((void **)&p);

According to the C FAQ, it is not portable. However, it does not cause my compiler to emit any warnings, and searching well known C codebases yields many instances of such non-void *-casts, for example Linux, CPython, and COM all do it. The answers of the most closely related SO question that I could find make no mention of this being strange or nonstandard. So, is this usage of void ** really non-portable? And if so why does it seem to be common practice?