_open, _wopen

Visual Studio 6.0

Open a file.

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

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

Routine Required Header Optional Headers Compatibility
_open <io.h> <fcntl.h>, <sys/types.h>, <sys/stat.h> Win 95, Win NT
_wopen <io.h> or <wchar.h> <fcntl.h>, <sys/types.h>, <sys/stat.h> 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 file handle for the opened file. A return value of –1 indicates an error, in which case errno is set to one of the following values:


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


_O_CREAT and _O_EXCL flags specified, but filename already exists


Invalid oflag or pmode argument


No more file handles available (too many open files)


File or path not found





Type of operations allowed


Permission mode


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 & _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:


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


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


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


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


Create file as temporary; file is deleted when last file handle is closed. pmode argument is required when _O_CREAT is specified.


Returns error value if file specified by filename exists. Applies only when used with _O_CREAT.


Specifies primarily random access from disk


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


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


Specifies primarily sequential access from disk


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


Opens file and truncates it to zero length; 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.

Warning   The _O_TRUNC flag destroys the contents of the specified file.


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

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.

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

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:


Reading only permitted


Writing permitted (effectively permits reading and writing)


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.


/* OPEN.C: This program uses _open to open a file
 * named OPEN.C for input and a file named 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>

void main( void )
   int fh1, fh2;

   fh1 = _open( "OPEN.C", _O_RDONLY );
   if( fh1 == -1 )
      perror( "open failed on input file" );
      printf( "open succeeded on input file\n" );
      _close( fh1 );

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


Open succeeded on input file
Open succeeded on output file

Low-Level I/O Routines

See Also   _chmod, _close, _creat, _dup, fopen, _sopen