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

Question

I have saw the compiler option GNU99 and C99. What's the difference of them? Any detail documentation? (Clang, Xcode, Mac OS X)

Answer 1

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