FPUTWC(3) Linux Programmer's Manual FPUTWC(3)

fputwc, putwc - write a wide character to a FILE stream

#include <stdio.h>
#include <wchar.h>
wint_t fputwc(wchar_t wc, FILE *stream);
wint_t putwc(wchar_t wc, FILE *stream);

The fputwc() function is the wide-character equivalent of the fputc(3) function. It writes the wide character wc to stream. If ferror(stream) becomes true, it returns WEOF. If a wide-character conversion error occurs, it sets errno to EILSEQ and returns WEOF. Otherwise, it returns wc.
The putwc() function or macro functions identically to fputwc(). It may be implemented as a macro, and may evaluate its argument more than once. There is no reason ever to use it.
For nonlocking counterparts, see unlocked_stdio(3).

The fputwc() function returns wc if no error occurred, or WEOF to indicate an error. In the event of an error, errno is set to indicate the cause.

Apart from the usual ones, there is
Conversion of wc to the stream's encoding fails.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
fputwc (), putwc () Thread safety MT-Safe

POSIX.1-2001, POSIX.1-2008, C99.

The behavior of fputwc() depends on the LC_CTYPE category of the current locale.
In the absence of additional information passed to the fopen(3) call, it is reasonable to expect that fputwc() will actually write the multibyte sequence corresponding to the wide character wc.

fgetwc(3), fputws(3), unlocked_stdio(3)
2017-09-15 GNU