_fdopen, _wfdopen

Visual Studio 6.0

Associate a stream with a file that was previously opened for low-level I/O.

FILE *_fdopen( int handle, const char *mode );

FILE *_wfdopen( int handle, const wchar_t *mode );

Function Required Header Compatibility
_fdopen <stdio.h> Win 95, Win NT
_wfdopen <stdio.h> or <wchar.h> Win 95, Win NT

For additional compatibility information, see Compatibility in the Introduction.


LIBC.LIB Single thread static library, retail version
LIBCMT.LIB Multithread static library, retail version
MSVCRT.LIB Import library for MSVCRT.DLL, retail version

Return Value

Each of these functions returns a pointer to the open stream. A null pointer value indicates an error.



Handle to open file


  Type of file access


The _fdopen function associates an I/O stream with the file identified by handle, thus allowing a file opened for low-level I/O to be buffered and formatted. _wfdopen is a wide-character version of _fdopen; the mode argument to _wfdopen is a wide-character string. _wfdopen and _fdopen behave identically otherwise.

Generic-Text Routine Mappings

TCHAR.H Routine _UNICODE & _MBCS Not Defined _MBCS Defined _UNICODE Defined
_tfdopen _fdopen _fdopen _wfdopen

The mode character string specifies the type of file and file access.

The character string mode specifies the type of access requested for the file, as follows:


Opens for reading. If the file does not exist or cannot be found, the fopen call fails.


Opens an empty file for writing. If the given file exists, its contents are destroyed.


Opens for writing at the end of the file (appending); creates the file first if it doesn’t exist.


Opens for both reading and writing. (The file must exist.)


Opens an empty file for both reading and writing. If the given file exists, its contents are destroyed.


Opens for reading and appending; creates the file first if it doesn’t exist.

When a file is opened with the "a" or "a+" access type, all write operations occur at the end of the file. The file pointer can be repositioned using fseek or rewind, but is always moved back to the end of the file before any write operation is carried out. Thus, existing data cannot be overwritten. When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for “update”). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.

In addition to the above values, the following characters can be included in mode to specify the translation mode for newline characters:


Open in text (translated) mode. In this mode, carriage return–linefeed (CR-LF) combinations are translated into single linefeeds (LF) on input, and LF characters are translated to CR-LF combinations on output. Also, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading/writing, fopen checks for a CTRL+Z at the end of the file and removes it, if possible. This is done because using the fseek and ftell functions to move within a file that ends with a CTRL+Z may cause fseek to behave improperly near the end of the file.


Open in binary (untranslated) mode; the above translations are suppressed.


Enable the commit flag for the associated filename so that the contents of the file buffer are written directly to disk if either fflush or _flushall is called.


Reset the commit flag for the associated filename to “no-commit.” This is the default. It also overrides the global commit flag if you link your program with COMMODE.OBJ. The global commit flag default is “no-commit” unless you explicitly link your program with COMMODE.OBJ.

The t, c, and n mode options are Microsoft extensions for fopen and _fdopen and should not be used where ANSI portability is desired.

If t or b is not given in mode, the default translation mode is defined by the global variable _fmode. If t or b is prefixed to the argument, the function fails and returns NULL. For a discussion of text and binary modes, see Text and Binary Mode File I/O.

Valid characters for the mode string used in fopen and _fdopen correspond to oflag arguments used in _open and _sopen, as follows.

Characters in mode String
Equivalent oflag Value for _open/_sopen
a+ _O_RDWR | _O_APPEND (usually _O_RDWR | _O_APPEND | _O_CREAT )
r+ _O_RDWR
w _O_WRONLY (usually _O_WRONLY | _O_CREAT | _O_TRUNC)
w+ _O_RDWR (usually _O_RDWR | _O_CREAT | _O_TRUNC)
c None
n None


/* _FDOPEN.C: This program opens a file using low-
 * level I/O, then uses _fdopen to switch to stream
 * access. It counts the lines in the file.

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

void main( void )
   FILE *stream;
   int  fh, count = 0;
   char inbuf[128];

   /* Open a file handle. */
   if( (fh = _open( "_fdopen.c", _O_RDONLY )) == -1 )
      exit( 1 );

   /* Change handle access to stream access. */
   if( (stream = _fdopen( fh, "r" )) == NULL )
      exit( 1 );

   while( fgets( inbuf, 128, stream ) != NULL )

   /* After _fdopen, close with fclose, not _close. */
   fclose( stream );
   printf( "Lines in file: %d\n", count );


Lines in file: 32

Stream I/O Routines

See Also   _dup, fclose, fopen, freopen, _open