What's the difference between GNU99 and C99 (Clang)?

Differences between various standard modes

clang supports the -std option, which changes what language mode clang uses. The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases for those modes. If no -std option is specified, clang defaults to gnu99 mode.

Differences between all c* and gnu* modes:

• c* modes define __STRICT_ANSI__.
• Target-specific defines not prefixed by underscores, like "linux", are defined in gnu* modes.
• Trigraphs default to being off in gnu* modes; they can be enabled by the -trigraphs option.
• The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the variants __asm__ and __typeof__ are recognized in all modes.
• The Apple "blocks" extension is recognized by default in gnu* modes on some platforms; it can be enabled in any mode with the -fblocks option.

More links

• Options controlling C dialect for GCC
• Extensions to the C Language Family
• Clang Language Extensions
• Useful GCC flags for C

C99 is straight C99, GNU99 is C99 with gnu extensions. See the GCC manpage.

C99 is simply the version of the C standard as of 1999 as we all know it. In GCC it is not fully supported.

GNU99 is an extension to C99, just like GNU98 is an extension of C98. From the docs:

ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.

Clang supports these extensions also.