_fdopen, _wfdopen

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


FILE *_fdopen(
   int fd,
   const char *mode
FILE *_wfdopen(
   int fd,
   const wchar_t *mode


File descriptor of the open file.

Type of file access.

Return value

Each of these functions returns a pointer to the open stream. A null pointer value indicates an error. When an error occurs, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, errno is set either to EBADF, which indicates a bad file descriptor, or EINVAL, which indicates that mode was a null pointer.

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


The _fdopen function associates an I/O stream with the file that is identified by fd, and thus allows a file that is 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 otherwise behave identically.

File descriptors passed into _fdopen are owned by the returned FILE * stream. If _fdopen is successful, don't call _close on the file descriptor. Calling fclose on the returned FILE * also closes the file descriptor.

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

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

mode Access
"r" Opens for reading. If the file doesn't exist or can't be found, the fopen call fails.
"w" Opens an empty file for writing. If the given file exists, its contents are destroyed.
"a" Opens for writing at the end of the file (appending). Creates the file if it doesn't exist.
"r+" Opens for both reading and writing. The file must exist.
"w+" Opens an empty file for both reading and writing. If the file exists, its contents are destroyed.
"a+" Opens for reading and appending. Creates the file 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 by using fseek or rewind, but it's always moved back to the end of the file before any write operation is carried out. Thus, existing data can't 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. You can specify the current position for the fsetpos or fseek operation, if you want to.

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

mode modifier Behavior
t Open in text (translated) mode. In this mode, carriage return-line feed (CR-LF) combinations are translated into one-line feeds (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.
b Open in binary (untranslated) mode. Any translations from t mode are suppressed.
c 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.
n Reset the commit flag for the associated filename to "no-commit." This flag 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. Don't use them if you want to preserve ANSI portability.

If t or b isn't 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 shown in this table:

Characters in mode string Equivalent oflag value for _open and _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


Function Required header C++ header
_fdopen <stdio.h> <cstdio>
_wfdopen <stdio.h> or <wchar.h> <cstdio>

For more information on standards conformance and naming conventions in the C runtime library, see Compatibility.

Generic-text routine mappings

<tchar.h> routine _UNICODE and _MBCS not defined _MBCS defined _UNICODE defined
_tfdopen _fdopen _fdopen _wfdopen


// crt_fdopen.c
// This program opens a file by 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>
#include <share.h>

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

   // Open a file.
   if( _sopen_s( &fd, "crt_fdopen.txt", _O_RDONLY, _SH_DENYNO, 0 ) )
      exit( 1 );

   // Get stream from file descriptor.
   if( (stream = _fdopen( fd, "r" )) == NULL )
      exit( 1 );

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

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

Input: crt_fdopen.txt

Line one
Line two


Lines in file: 2

See also

Stream I/O
_dup, _dup2
fclose, _fcloseall
fopen, _wfopen
freopen, _wfreopen
_open, _wopen