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Sets the file translation mode.


int _setmode (
   int fd,
   int mode


File descriptor.

New translation mode.

Return value

If successful, returns the previous translation mode.

If invalid parameters are passed to this function, the invalid-parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, this function returns -1 and sets errno to either EBADF, which indicates an invalid file descriptor, or EINVAL, which indicates an invalid mode argument.

For more information about these and other return codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.


The _setmode function sets to mode the translation mode of the file given by fd. Passing _O_TEXT as mode sets ANSI text (that is, translated) mode. Carriage return-line feed (CR-LF) combinations are translated into a single line feed character on input. Line feed characters are translated into CR-LF combinations on output. Passing _O_BINARY sets binary (untranslated) mode, in which these translations are suppressed.

You can also pass _O_U16TEXT, _O_U8TEXT, or _O_WTEXT to enable Unicode mode, as demonstrated in the second example later in this document.


Unicode mode is for wide print functions (for example, wprintf) and is not supported for narrow print functions. Use of a narrow print function on a Unicode mode stream triggers an assert.

_setmode is typically used to modify the default translation mode of stdin and stdout, but you can use it on any file. If you apply _setmode to the file descriptor for a stream, call _setmode before you perform any input or output operations on the stream.


If you write data to a file stream, explicitly flush the code by using fflush before you use _setmode to change the mode. If you do not flush the code, you might get unexpected behavior. If you have not written data to the stream, you do not have to flush the code.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.


Routine Required header Optional Headers
_setmode <io.h> <fcntl.h>

For more compatibility information, see Compatibility.

Example: Use _setmode to change stdin

// crt_setmode.c
// This program uses _setmode to change
// stdin from text mode to binary mode.

#include <stdio.h>
#include <fcntl.h>
#include <io.h>

int main( void )
   int result;

   // Set "stdin" to have binary mode:
   result = _setmode( _fileno( stdin ), _O_BINARY );
   if( result == -1 )
      perror( "Cannot set mode" );
      printf( "'stdin' successfully changed to binary mode\n" );
'stdin' successfully changed to binary mode

Example: Use _setmode to change stdout

// crt_setmodeunicode.c
// This program uses _setmode to change
// stdout to Unicode. Cyrillic and Ideographic
// characters will appear on the console (if
// your console font supports those character sets).

#include <fcntl.h>
#include <io.h>
#include <stdio.h>

int main(void) {
    _setmode(_fileno(stdout), _O_U16TEXT);
    wprintf(L"\x043a\x043e\x0448\x043a\x0430 \x65e5\x672c\x56fd\n");
    return 0;

See also

File handling
_creat, _wcreat
fopen, _wfopen
_open, _wopen