Sets the default file-permission mask. A version of _umask with security enhancements as described in Security features in the CRT.


errno_t _umask_s(
   int mode,
   int* pOldMode


Default permission setting.

The previous value of the permission setting.

Return value

Returns an error code if mode doesn't specify a valid mode or the pOldMode pointer is NULL.

Error conditions

mode pOldMode Return value Contents of pOldMode
any NULL EINVAL not modified
invalid mode any EINVAL not modified

If one of the above conditions occurs, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, _umask_s returns EINVAL and sets errno to EINVAL.


The _umask_s function sets the file-permission mask of the current process to the mode specified by mode. The file-permission mask modifies the permission setting of new files created by _creat, _open, or _sopen. If a bit in the mask is 1, the corresponding bit in the file's requested permission value is set to 0 (disallowed). If a bit in the mask is 0, the corresponding bit is left unchanged. The permission setting for a new file isn't set until the file is closed for the first time.

The integer expression mode contains one or both of the following manifest constants, defined in SYS\STAT.H:

mode Description
_S_IWRITE Writing permitted.
_S_IREAD Reading permitted.
_S_IREAD | _S_IWRITE Reading and writing permitted.

When both constants are given, they're joined with the bitwise-OR operator ( | ). If the mode argument is _S_IREAD, reading isn't allowed (the file is write-only). If the mode argument is _S_IWRITE, writing isn't allowed (the file is read-only). For example, if the write bit is set in the mask, any new files will be read-only. In MS-DOS and the Windows operating systems, all files are readable; it isn't possible to give write-only permission. Therefore, setting the read bit with _umask_s has no effect on the file's modes.

If mode isn't a combination of one of the manifest constants or incorporates an alternate set of constants, the function ignores them.

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


Function Required header
_umask_s <io.h> and <sys/stat.h> and <sys/types.h>

For more compatibility information, see Compatibility.


// crt_umask_s.c
/* This program uses _umask_s to set
* the file-permission mask so that all future
* files will be created as read-only files.
* It also displays the old mask.

#include <sys/stat.h>
#include <sys/types.h>
#include <io.h>
#include <stdio.h>

int main( void )
   int oldmask, err;

   /* Create read-only files: */
   err = _umask_s( _S_IWRITE, &oldmask );
   if (err)
      printf("Error setting the umask.\n");
   printf( "Oldmask = 0x%.4x\n", oldmask );
Oldmask = 0x0000

See also

File handling
Low-level I/O
_chmod, _wchmod
_creat, _wcreat
_mkdir, _wmkdir
_open, _wopen