How universally is C99 supported?

How universally is the C99 standard supported in today's compilers? I understand that not even GCC fully supports it. Is this right?

Which features of C99 are supported more than others, i.e. which can I use to be quite sure that most compilers will understand me?

If you want to write portable C code, then I'd suggest you to write in C89 (old ANSI C standard). This standard is supported by most compilers.

The Intel C Compiler has very good C99 support and it produces fast binaries. (Thanks 0x69!)

MSVC supports some new features and Microsoft plan to broaden support in future versions.

GCC supports some new things of C99. They created a table about the status of C99 features. Probably the most usable feature of C99 is the variable length array, and GCC supports it now. Clang (LLVM's C fronted) supports most features except floating-point pragmas.

Wikipedia seems to have a nice summary of C99 support of the compilers.

Someone mentioned the Intel compiler has C99 support. There is also the Comeau C/C++ compiler which fully supports C99. These are the only ones I'm aware of.

C99 features that I do not use because they are not well supported include:

• variable length arrays
• macros with variable number of parameters.

C99 features that I regularly use that seem to be pretty well supported (except by Microsoft):

• stdint.h
• snprintf() - MS has a non-standard _snprintf() that has serious limitations of not always null terminating the buffer and not indicating how big the buffer should be

To work around Microsoft's non-support, I use a public domain stdint.h from MinGW (that I modified to also work on VC6) and a nearly public domain snprintf() from Holger Weiss

Items that are not supported by Microsoft, but will still use on other compilers depending on the project include:

• mixed declarations and code
• inline functions
• _Pragma() - this makes pragmas much more usable