A current list of predefinitions is given in the cc(1) man page.
The __STDC__ identifier is predefined as an object-like macro as shown in the following table.
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On Oracle Linux the compiler predefines __STDC__ to expand to 1.
The compiler issues a warning if __STDC__ is undefined (#undef __STDC__). __STDC__ is not defined in –Xs mode.