Unicode Stream I/O in Text and Binary Modes


For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

When a Unicode stream I/O routine (such as fwprintf, fwscanf, fgetwc, fputwc, fgetws, or fputws) operates on a file that is open in text mode (the default), two kinds of character conversions take place:

  • Unicode-to-MBCS or MBCS-to-Unicode conversion. When a Unicode stream-I/O function operates in text mode, the source or destination stream is assumed to be a sequence of multibyte characters. Therefore, the Unicode stream-input functions convert multibyte characters to wide characters (as if by a call to the mbtowc function). For the same reason, the Unicode stream-output functions convert wide characters to multibyte characters (as if by a call to the wctomb function).

  • Carriage return – linefeed (CR-LF) translation. This translation occurs before the MBCS – Unicode conversion (for Unicode stream input functions) and after the Unicode – MBCS conversion (for Unicode stream output functions). During input, each carriage return – linefeed combination is translated into a single linefeed character. During output, each linefeed character is translated into a carriage return – linefeed combination.

However, when a Unicode stream-I/O function operates in binary mode, the file is assumed to be Unicode, and no CR-LF translation or character conversion occurs during input or output. Use the _setmode( _fileno( stdin ), _O_BINARY ); instruction in order to correctly use wcin on a UNICODE text file.

Run-Time Routines by Category
Input and Output