The C Preprocessor: __has_attribute

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#### 4.2.6 `__has_attribute`

The special operator `__has_attribute (operand)` may be used in ‘`#if`’ and ‘`#elif`’ expressions to test whether the attribute referenced by its operand is recognized by GCC. Using the operator in other contexts is not valid. In C code, if compiling for strict conformance to standards before C2x, operand must be a valid identifier. Otherwise, operand may be optionally introduced by the `attribute-scope::` prefix. The attribute-scope prefix identifies the “namespace” within which the attribute is recognized. The scope of GCC attributes is ‘`gnu`’ or ‘`__gnu__`’. The `__has_attribute` operator by itself, without any operand or parentheses, acts as a predefined macro so that support for it can be tested in portable code. Thus, the recommended use of the operator is as follows:

    #if defined __has_attribute
    #  if __has_attribute (nonnull)
    #    define ATTR_NONNULL __attribute__ ((nonnull))
    #  endif
    #endif

The first ‘`#if`’ test succeeds only when the operator is supported by the version of GCC (or another compiler) being used. Only when that test succeeds is it valid to use `__has_attribute` as a preprocessor operator. As a result, combining the two tests into a single expression as shown below would only be valid with a compiler that supports the operator but not with others that don’t.

    #if defined __has_attribute && __has_attribute (nonnull)   /* not portable */
    …
    #endif