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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

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 dont.

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