_fdopen, _wfdopen

 

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

For the latest documentation on Visual Studio 2017, see _fdopen, _wfdopen on docs.microsoft.com. 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   
);  

Parameters

fd
File descriptor of the open file.

mode
Type of file access.

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 _doserrno, errno, _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.

Generic-Text Routine Mappings

Tchar.h routine_UNICODE and _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 shown in the following table.

"r"
Opens for reading. If the file does not exist or cannot 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 does not exist.

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

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

"a+"
Opens for reading and appending. Creates the file if it does not 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 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. 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.

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. 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 might cause fseek to behave incorrectly near the end of the file.

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 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. Do not use them if you want to preserve ANSI portability.

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 stringEquivalent oflagvalue for _open/_sopen
a_O_WRONLY | _O_APPEND (usually _O_WRONLY | _O_CREAT | _O_APPEND)
a+_O_RDWR | _O_APPEND (usually _O_RDWR | _O_APPEND | _O_CREAT )
r_O_RDONLY
r+_O_RDWR
w_O_WRONLY (usually _O_WRONLY | _O_CREAT | _O_TRUNC)
w+_O_RDWR (usually _O_RDWR | _O_CREAT | _O_TRUNC)
b_O_BINARY
t_O_TEXT
cNone
nNone
FunctionRequired header
_fdopen<stdio.h>
_wfdopen<stdio.h> or <wchar.h>

For more compatibility information, see Compatibility.

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

Line one  
Line two  

Output

Lines in file: 2  

FileStream

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

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