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

[in] mode

Default permission setting.

[out] oldMode

The previous value of the permission setting.

Returns an error code if Mode does not specify a valid mode or the pOldMode pointer is NULL.

Error Conditions



Return Value

Contents of oldMode




not modified

invalid mode



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 is not set until the file is closed for the first time.

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


Writing permitted.


Reading permitted.


Reading and writing permitted.

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

If pmode is not a combination of one of the manifest constants or incorporates an alternate set of constants, the function will simply ignore those.


Required header


<io.h> and <sys/stat.h> and <sys/types.h>

For additional compatibility information, see Compatibility in the Introduction.

// 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