__has_builtin ¶The special operator __has_builtin (operand) may be used in
constant integer contexts and in preprocessor ‘#if’ and ‘#elif’
expressions to test whether the symbol named by its operand is
recognized as a built-in function by GCC in the current language and
conformance mode. It evaluates to a constant integer with a nonzero
value if the argument refers to such a function, and to zero otherwise.
The operator may also be used in preprocessor ‘#if’ and ‘#elif’
expressions. The __has_builtin 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_builtin # if __has_builtin (__builtin_object_size) # define builtin_object_size(ptr) __builtin_object_size (ptr, 2) # endif #endif #ifndef builtin_object_size # define builtin_object_size(ptr) ((size_t)-1) #endif