[josefx]

> For example, every example in Kernighan & Ritchie: "The C Programming Language (2nd Edition)" is also a C++ program. "

That is rather dated, they do things like explicitly cast the void* pointer returned by malloc, but point out in the appendix that ANSI C dropped the cast requirement for pointer conversions involving void, C++ does not allow implicit void conversions to this day.

[pjmlp]

> Well written C tends to be legal C++ also

The "well written" remark is relevant.

Many style guides will consider implicit void conversions not well written C.

Naturally we are now on C23, and almost every C developer considers language extensions as being C, so whatever.

[josefx]

> The "well written" remark is relevant.

So using casts that hid implicit int declarations for years is "well written"?

> Many style guides will consider implicit void conversions not well written C.

I could not find a "Many" guide, Linux Kernel and ffmpeg seem to advocate against pointless casts.

> Naturally we are now on C23,

So irrelevant to the creation time of ffmpeg and only applicable to intentionally non portable libraries.

[uecker]

Well written idiomatic C is certainly not valid C++.

[pjmlp]

Depends on the beholder, however it hardly matters on the days of C23, as mentioned.