Hello.

So far I have successfully built Dovecot until v2.3.14 on Centos 4-6 (yes I know, they are old, but as they are remote servers, distros can't upgraded), and sometimes I have also met compilation issues, but after highlighting them, Timo have fixed the code.

Found from similar thread (Dovecot 2.3.15 compilation fails https://dovecot.org/list/dovecot/2021-June/122412.html ) that new GCC is needed. However I can't find such requirement in v2.3.15 change log.

Looks like we forgot to mention that.

As seems that I'm not the only one with compilation issues, then @Timo - can You please fix the code so that also v2.3.15 can compiled on older distros?

Sorry, it's no longer possible. The buffer.h API had to be changed to fix some real bugs: https://github.com/dovecot/core/commit/1d9b4e14008b15b7a34b5c633b09b9670e866256

The original way I changed it would have made the code more complex and it was also a huge change. The final method was much simpler, but it requires now compiler support for anonymous unions and anonymous structs, i.e. C11. Although some older GCCs should also support it, but I guess you've a too old version.