_open, _wopen

Opens a file. These functions are deprecated because more secure versions are available; see _sopen_s, _wsopen_s.

int _open(
   const char *filename,
   int oflag [,
   int pmode] 
);
int _wopen(
   const wchar_t *filename,
   int oflag [,
   int pmode] 
);

Parameters

• filename
  File name.

• oflag
  Type of operations allowed.

• pmode
  Permission mode.

Return Value

Each of these functions returns a file descriptor for the opened file. A return value of -1 indicates an error, in which case errno is set to one of the following values.

• EACCES
  Tried to open read-only file for writing, file's sharing mode does not allow specified operations, or given path is directory.

• EEXIST
  _O_CREAT and _O_EXCL flags specified, but filename already exists.

• EINVAL
  Invalid oflag or pmode argument.

• EMFILE
  No more file descriptors available (too many open files).

• ENOENT
  File or path not found.

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

Remarks

The _open function opens the file specified by filename and prepares the file for reading or writing, as specified by oflag. _wopen is a wide-character version of _open; the filename argument to _wopen is a wide-character string. _wopen and _open behave identically otherwise.

Generic-Text Routine Mappings

Tchar.h routine	_UNICODE and _MBCS not defined	_MBCS defined	_UNICODE defined
_topen	_open	_open	_wopen

oflag is an integer expression formed from one or more of the following manifest constants or constant combinations defined in Fcntl.h.

• _O_APPEND
  Moves file pointer to end of file before every write operation.

• _O_BINARY
  Opens file in binary (untranslated) mode. (See fopen for a description of binary mode.)

• _O_CREAT
  Creates and opens a new file for writing. Has no effect if the file specified by filename exists. pmode argument is required when _O_CREAT is specified.

• _O_CREAT| _O_SHORT_LIVED
  Create a file as temporary and if possible do not flush to disk. pmode argument is required when _O_CREAT is specified.

• _O_CREAT| _O_TEMPORARY
  Create a file as temporary; the file is deleted when the last file descriptor is closed. pmode argument is required when _O_CREAT is specified.

• _O_CREAT| _O_EXCL
  Returns an error value if the file specified by filename exists. Applies only when used with _O_CREAT.

• _O_NOINHERIT
  Prevents creation of a shared file descriptor.

• _O_RANDOM
  Specifies that caching is optimized for, but not restricted to, random access from disk.

• _O_RDONLY
  Opens a file for reading only; cannot be specified with _O_RDWR or _O_WRONLY.

• _O_RDWR
  Opens file for both reading and writing; you cannot specify this flag with _O_RDONLY or _O_WRONLY.

• _O_SEQUENTIAL
  Specifies that caching is optimized for, but not restricted to, sequential access from disk.

• _O_TEXT
  Opens a file in text (translated) mode. (For more information, see Text and Binary Mode File I/O and fopen.)

• _O_TRUNC
  Opens a file and truncates it to zero length; the file must have write permission. You cannot specify this flag with _O_RDONLY. _O_TRUNC used with _O_CREAT opens an existing file or creates a new file.

  Note

  The _O_TRUNC flag destroys the contents of the specified file.

• _O_WRONLY
  Opens the file for writing only; cannot be specified with _O_RDONLY or _O_RDWR.

• _O_U16TEXT
  Open the file in Unicode UTF-16 mode.

• _O_U8TEXT
  Open the file in Unicode UTF-8 mode.

• _O_WTEXT
  Open the file in Unicode mode.

To specify the file access mode, you must specify either _O_RDONLY, _O_RDWR, or _O_WRONLY. There is no default value for the access mode.

If _O_WTEXT is used to open a file for reading, _open reads the beginning of the file and check for a byte order mark (BOM). If there is a BOM, the file is treated as UTF-8 or UTF-16LE depending on the BOM. If no BOM is present, the file is treated as ANSI. When a file is opened for writing using _O_WTEXT, UTF-16 is used. If _O_UTF8 is used, the file is always opened as UTF-8 and if _O_UTF16 is used, the file is always opened as UTF-16 regardless of any previous setting or byte order mark.

If _open is called with _O_WRONLY|_O_APPEND (append mode) and _O_WTEXT, _O_U16TEXT, or _O_U8TEXT, it will first try to open the file for reading and writing, read the BOM, then reopen it for writing only. If opening the file for reading and writing fails, it will open the file for writing only and use the default value for the Unicode mode setting.

When two or more manifest constants are used to form the oflag argument, the constants are combined with the bitwise-OR operator ( | ). For a discussion of binary and text modes, see Text and Binary Mode File I/O.

The pmode argument is required only when _O_CREAT is specified. If the file already exists, pmode is ignored. Otherwise, pmode specifies the file permission settings, which are set when the new file is closed the first time. _open applies the current file-permission mask to pmode before setting the permissions (for more information, see _umask). pmode is an integer expression containing one or both of the following manifest constants, defined in SYS\Stat.h.

• _S_IREAD
  Reading only permitted.

• _S_IWRITE
  Writing permitted (effectively permits reading and writing).

• _S_IREAD | _S_IWRITE
  Reading and writing permitted.

When both constants are given, they are joined with the bitwise-OR operator ( | ). In Windows NT, all files are readable, so write-only permission is not available; thus the modes _S_IWRITE and _S_IREAD | _S_IWRITE are equivalent.

If a value other than the above is specified for pmode (even if it would specify a valid pmode in another operating system) or any value other than the allowed oflag values is specified, the function generates an assertion in Debug mode and invokes the invalid parameter handler as described in Parameter Validation. If execution is allowed to continue, the function returns -1 and sets errno to EINVAL.

Requirements

Routine	Required header	Optional header
_open	<io.h>	<fcntl.h>, <sys/types.h>, <sys/stat.h>
_wopen	<io.h> or <wchar.h>	<fcntl.h>, <sys/types.h>, <sys/stat.h>

For more compatibility information, see Compatibility in the Introduction.

Libraries

All versions of the C run-time libraries.

Example

// crt_open.c
// compile with: /W3
/* This program uses _open to open a file
 * named CRT_OPEN.C for input and a file named CRT_OPEN.OUT
 * for output. The files are then closed.
 */
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <io.h>
#include <stdio.h>

int main( void )
{
   int fh1, fh2;

   fh1 = _open( "CRT_OPEN.C", _O_RDONLY ); // C4996
   // Note: _open is deprecated; consider using _sopen_s instead
   if( fh1 == -1 )
      perror( "Open failed on input file" );
   else
   {
      printf( "Open succeeded on input file\n" );
      _close( fh1 );
   }

   fh2 = _open( "CRT_OPEN.OUT", _O_WRONLY | _O_CREAT, _S_IREAD | 
                            _S_IWRITE ); // C4996
   if( fh2 == -1 )
      perror( "Open failed on output file" );
   else
   {
      printf( "Open succeeded on output file\n" );
      _close( fh2 );
   }
}

Output

Open succeeded on input file
Open succeeded on output file

.NET Framework Equivalent

• System::IO::File::Open

• System::IO::FileStream::FileStream

See Also

Reference

Low-Level I/O

_chmod, _wchmod

_close

_creat, _wcreat

_dup, _dup2

fopen, _wfopen

_sopen, _wsopen