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GCC Compiler options -wno-four-char-constants and -wno-multichar

Couldn't find any documentation on -Wno-four-char-constants, however I suspect that it is similar to -Wno-multichar. Am I correct?

Solution

  • They're related but not the same thing.

    Compiling with the -Wall --pedantic flags, the assignment:

    int i = 'abc';

    produces:

    warning: multi-character character constant [-Wmultichar]

    with both GCC and CLANG, while:

     int i = 'abcd';

    produces:

    GCC warning: multi-character character constant [-Wmultichar]

    CLANG warning: multi-character character constant [-Wfour-char-constants]

    The standard (C99 standard with corrigenda TC1, TC2 and TC3 included, subsection 6.4.4.4 - character constants) states that:

    The value of an integer character constant containing more than one character (e.g., 'ab'), [...] is implementation-defined.

    A multi-char always resolves to an int but, since the order in which the characters are packed into one int is not specified, portable use of multi-character constants is difficult (the exact value is implementation-dependent).

    Also compilers differ in how they handle incomplete multi-chars (such as 'abc').

    Some compilers pad on the left, some on the right, regardless of endian-ness (some compilers may not pad at all).

    Someone who can accept the portability problems of a complete multi-char may anyway want a warning for an incomplete one (-Wmultichar -Wno-four-char-constants).