> From: Michael Richardson <mcr@xxxxxxxxxxxx> > Sent: Thursday, 31 March, 2022 14:18 > > Michael Wojcik <Michael.Wojcik@xxxxxxxxxxxxxx> wrote: > > #if defined OPENSSL_SYS_WINDOWS > > # include <shared/netiodef.h> > > #else > > # include <netinet/ip6.h> > > #endif > > But, don't all the OPENSSL_* macros expand to 0/1, anyway, so we actually > just want #if OPENSSL_SYS_WINDOWS? I did a quick grep through the source for 1.1.1k (just because that's what I had to hand; we've actually just finished updating my portfolio to 1.1.1n), and there's a mix of #if OPENSSL_SYS_WINDOWS and #if defined(OPENSSL_SYS_WINDOWS). apps/s_client.c uses the latter, for example. Actually, in the context of #if expressions, unrecognized tokens expand to 0 anyway: After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers are replaced with the pp-number 0... (ISO 9899:1999 6.10.1 #3) So defining a macro used for conditional inclusion to the value 0 is kind of a bad idea, since that means there's different behavior between #if FOO and #if defined FOO. Much better to not define it and get the default value of 0 if you want to turn it off. But that said, #if OPENSSL_SYS_WINDOWS is safer for the same reason: it doesn't matter whether it's defined as 0, or not defined at all. The "defined" operator is overused in C source generally. It's good for things like header inclusion guards. It's not really a good choice for most other cases of conditional inclusion. -- Michael Wojcik
