"Compatibility" has a specific meaning in C which isn't the same as what you're describing here. What you're talking about is implicit conversions without narrowing.
The C standard is quite loose about pointer requirements. Conversions between data pointers or between function pointers are allowed, but conversions between each other are not guaranteed. You can read the nitty gritty here: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
POSIX defines stricter requirements for pointers than the C standard. All pointers are compatible, so they have the same size and representation, and conversions between function pointers and void* are explicitly allowed.
Comments
"Compatibility" has a specific meaning in C which isn't the same as what you're describing here. What you're talking about is implicit conversions without narrowing.
Compatibility is essentially about ABI: https://en.cppreference.com/w/c/language/type#Compatible_typ...
The C standard is quite loose about pointer requirements. Conversions between data pointers or between function pointers are allowed, but conversions between each other are not guaranteed. You can read the nitty gritty here: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
POSIX defines stricter requirements for pointers than the C standard. All pointers are compatible, so they have the same size and representation, and conversions between function pointers and void* are explicitly allowed.