fwide - set and determine the orientation of a FILE stream
mode is zero, the fwide() function determines the current orientation of
stream. It returns a positive value if
stream is wide-character 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—that is, 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-character I/O operation).
Once a stream has an orientation, it cannot be changed and persists until the stream is closed.
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 means undecided.
POSIX.1-2001, POSIX.1-2008, C99.
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