It is a macro definition of type int that expands into a negative integral constant expression (generally, -1).
It is used as the value returned by several functions in header <cstdio> to indicate that the End-of-File has been reached or to signal some other failure conditions.
It is also used as the value to represent an invalid character.
In C++, this macro corresponds to the value of
- Check end-of-file indicator (function)
- Check error indicator (function)