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

c compiler-construction option clang
eonil picture eonil · Mar 15, 2011 · Viewed 38k times · Source

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

Answer

Matt Joiner picture Matt Joiner · Mar 16, 2011

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.

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