#include <wchar.h> int fwide(FILE *stream, int mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Once a stream has an orientation, it cannot be changed and persists until the stream is closed.
When mode is nonzero, the fwide() function first attempts to set stream's orientation (to wide-character oriented if mode is greater than 0, or to byte oriented if mode is less than 0). It then returns a value denoting the current orientation, as above.
Char oriented output to a wide-character oriented stream can be performed through the fwprintf(3) function with the %c and %s directives.