[uecker]

In C programs only the external definition of an interface goes into the header of a library but not implementation details (there could be headers intentionally exposing details for internal use, of course).

The problem is real for C++.

Optimizers can look across translation units nowadays (link-time optimization), so there is no reason to expose internal details in a header for this. For dynamic libraries this does not work of course, but it also shouldn't.

[johannes1234321]

Even for C it's normal ways true: When size of structures have to be known to the user (if they are supposed to keep them on stack or mallox themselves or whatever) the "private" structure often ends up in a "private" header. (There are ways to still hide it, but they cause work to keep things proper)

And then there are cases where (often die to performance) you want inlining of some operations without relying on Link time optimisation, then implementation has to go to headers, too.