When you are using wide-oriented output functions, wide characters are converted to multibyte characters. No validation is performed to ensure that records start or end in initial shift state. When the file is closed, any appends are completed with a shift-in character, if it is needed to end the stream in initial shift state. If you are updating a record when the stream is closed, the stream is flushed. See Flushing buffers for more information.
Byte-oriented output functions do not interpret binary data. If you use them for writing multibyte data, ensure that your data is correct and ends in initial shift state.
Updating a wide-oriented file or a file containing multibyte characters is strongly discouraged, because your update may overwrite part of a multibyte string or character, thereby invalidating subsequent data. For example, you could inadvertently add data that overwrites a shift-out. The data after the shift-out is meaningless when it is treated in initial shift state. Appending new data to the end of the file is safe for a wide-oriented file.
If you update a record after you call fgetpos(), the shift state may change. Using the fpos_t value with the fsetpos() function may cause the shift state to be set incorrectly.