I was reading through some of the best practices for the GNOME project, and one thing that they kept stressing was avoiding C99 features, as support was still not widespread. Some of the things they mentioned were features like single-line comments and declaring variables in the middle of a block. It made me wonder, is C99 support, even for basic features like // comments, really still not widespread? It's been almost 15 years now since the standard was adopted, and we've even had a new standard come out in that time. Is this really still an issue?
http://en.wikipedia.org/wiki/C99#Implementations
Seems like it still is a bit of an issue, even in GCC, though it looks like some commercial compilers have implemented the entire standard.
Microsoft have said repeatedly there are no plans to implement the C99 standard at any point in the future which is a shame for C programming on Windows.