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

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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

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