4.2.8 __has_include

The special operator __has_include ( operand ) may be used in ‘ #if ’ and ‘ #elif ’ expressions to test whether the header referenced by its operand can be included using the ‘ #include ’ directive. Using the operator in other contexts is not valid. The operand takes the same form as the file in the ‘ #include ’ directive (see Include Syntax ) and evaluates to a nonzero value if the header can be included and to zero otherwise. Note that that the ability to include a header doesn't imply that the header doesn't contain invalid constructs or ‘ #error ’ directives that would cause the preprocessor to fail.

The __has_include 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_include
     #  if __has_include (<stdatomic.h>)
     #    include <stdatomic.h>
     #  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_include 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_include && __has_include ("header.h")   /* not portable */
     ...
     #endif