Upon successful completion a pointer is returned; this pointer becomes invalid after the next I/O operation on stream (whether successful or not) or as soon as the stream is closed. Otherwise, NULL is returned. The fgetwln function does not distinguish between end-of-file and error; the routines feof(3) and ferror(3) must be used to determine which occurred. If an error occurs, the global variable errno is set to indicate the error. The end-of-file condition is remembered, even on a terminal, and all subsequent attempts to read will return NULL until the condition is cleared with clearerr(3).
The text to which the returned pointer points may be modified, provided that no changes are made beyond the returned size. These changes are lost as soon as the pointer becomes invalid.