_open, _wopen

Article
12/18/2017

The latest version of this topic can be found at _open, _wopen.

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

Syntax

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

Parameters

filename
File name.

oflag
The kind 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 that case errno is set to one of the following values.

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

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

EINVAL
Invalid oflag or pmode argument.

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

ENOENT
File or path not found.

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

Remarks

The _open function opens the file specified by filename and prepares it 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, which are defined in <fcntl.h>.

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

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

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

_O_CREAT | _O_SHORT_LIVED
Creates a file as temporary and if possible does not flush to disk. The pmode argument is required when _O_CREAT is specified.

_O_CREAT | _O_TEMPORARY
Creates a file as temporary; the file is deleted when the last file descriptor is closed. The 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. Cannot be specified 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. Cannot be specified with _O_RDONLY. _O_TRUNC used with _O_CREAT opens an existing file or creates a 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
Opens the file in Unicode UTF-16 mode.

_O_U8TEXT
Opens the file in Unicode UTF-8 mode.

_O_WTEXT
Opens 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 checks 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 by using _O_WTEXT, UTF-16 is used. Regardless of any previous setting or byte order mark, if _O_U8TEXT is used, the file is always opened as UTF-8; if _O_U16TEXT is used, the file is always opened as UTF-16.

When a file is opened in Unicode mode by using _O_WTEXT, _O_U8TEXT, or _O_U16TEXT, input functions translate the data that's read from the file into UTF-16 data stored as type wchar_t. Functions that write to a file opened in Unicode mode expect buffers that contain UTF-16 data stored as type wchar_t. If the file is encoded as UTF-8, then UTF-16 data is translated into UTF-8 when it is written, and the file's UTF-8-encoded content is translated into UTF-16 when it is read. An attempt to read or write an odd number of bytes in Unicode mode causes a parameter validation error. To read or write data that's stored in your program as UTF-8, use a text or binary file mode instead of a Unicode mode. You are responsible for any required encoding translation.

If _open is called with _O_WRONLY|_O_APPEND (append mode) and _O_WTEXT, _O_U16TEXT, or _O_U8TEXT, it first tries 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 opens the file for writing only and uses 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 the permissions are set. (For more information, see _umask.) pmode is an integer expression that contains one or both of the following manifest constants, which are defined in <sys\stat.h>.

_S_IREAD
Only reading permitted.

_S_IWRITE
Writing permitted. (In effect, 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, all files are readable; write-only permission is not available. Therefore, the modes _S_IWRITE and _S_IREAD | _S_IWRITE are equivalent.

If a value other than some combination of _S_IREAD and _S_IWRITE is specified for pmode—even if it would specify a valid pmode in another operating system—or if 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>

_open and _wopen are Microsoft extensions. For more compatibility information, see Compatibility.

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

Share via