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
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