When you use the fseek() or fsetpos() function to reposition within files, z/OS® XL C recalculates the shift state.
If you update a record after a successful call to the fseek() function or the fsetpos() function, a partial multibyte character can be overwritten. Calling a wide character function for data after the written character can result in undefined behavior.
Use the fseek() or fsetpos() functions to reposition only to the start of a multibyte character. If you reposition to the middle of a multibyte character, undefined behavior can occur.