int fwide(FILE *stream, int mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
_ISOC95_SOURCE /* Since glibc 2.12 */ ||
_POSIX_C_SOURCE >= 200112L;
or cc -std=c99
When mode is zero, the fwide() function determines the current orienta-
tion of stream. It returns a positive value if stream is wide-charac-
ter oriented, that is, if wide-character I/O is permitted but char I/O
is disallowed. It returns a negative value if stream is byte oriented,
i.e., if char I/O is permitted but wide-character I/O is disallowed.
It returns zero if stream has no orientation yet; in this case the next
I/O operation might change the orientation (to byte oriented if it is a
char I/O operation, or to wide-character oriented if it is a wide-char-
acter I/O operation).
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.
The fwide() function returns the stream's orientation, after possibly
changing it. A positive return value means wide-character oriented. A
negative return value means byte oriented. A return value of zero
Wide-character output to a byte oriented stream can be performed
through the fprintf(3) function with the %lc and %ls directives.
Char oriented output to a wide-character oriented stream can be per-
formed through the fwprintf(3) function with the %c and %s directives.
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