_mktemp_s, _wmktemp_s

Creates a unique file name. These functions are versions of _mktemp, _wmktemp with security enhancements as described in Security features in the CRT.


errno_t _mktemp_s(
   char *nameTemplate,
   size_t sizeInChars
errno_t _wmktemp_s(
   wchar_t *nameTemplate,
   size_t sizeInChars
template <size_t size>
errno_t _mktemp_s(
   char (&nameTemplate)[size]
); // C++ only
template <size_t size>
errno_t _wmktemp_s(
   wchar_t (&nameTemplate)[size]
); // C++ only


File name pattern.

Size of the buffer in single-byte characters in _mktemp_s; wide characters in _wmktemp_s, including the null terminator.

Return value

Both of these functions return zero on success; an error code on failure.

Error conditions

nameTemplate sizeInChars Return value New value in nameTemplate
Incorrect format (see Remarks section for correct format) any EINVAL empty string
any <= number of X characters EINVAL empty string

If any of the above error conditions occurs, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, errno is set to EINVAL and the functions returns EINVAL.


The _mktemp_s function creates a unique file name by modifying the nameTemplate argument, so that after the call, the nameTemplate pointer points to a string containing the new file name. _mktemp_s automatically handles multibyte-character string arguments as appropriate, recognizing multibyte-character sequences according to the multibyte code page currently in use by the run-time system. _wmktemp_s is a wide-character version of _mktemp_s; the argument of _wmktemp_s is a wide-character string. _wmktemp_s and _mktemp_s behave identically otherwise, except that _wmktemp_s doesn't handle multibyte-character strings.

The debug library versions of these functions first fill the buffer with 0xFE. To disable this behavior, use _CrtSetDebugFillThreshold.

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

Generic-text routine mappings

Tchar.h routine _UNICODE and _MBCS not defined _MBCS defined _UNICODE defined
_tmktemp_s _mktemp_s _mktemp_s _wmktemp_s

The nameTemplate argument has the form baseXXXXXX, where base is the part of the new file name that you supply and each X is a placeholder for a character supplied by _mktemp_s. Each placeholder character in nameTemplate must be an uppercase X. _mktemp_s preserves base and replaces the first trailing X with an alphabetic character. _mktemp_s replaces the X characters that follow with a five-digit value. This value is a unique number that identifies the calling process, or in multithreaded programs, the calling thread.

Each successful call to _mktemp_s modifies nameTemplate. In each subsequent call from the same process or thread with the same nameTemplate argument, _mktemp_s checks for file names that match names returned by _mktemp_s in previous calls. If no file exists for a given name, _mktemp_s returns that name. If files exist for all previously returned names, _mktemp_s creates a new name by replacing the alphabetic character it used in the previously returned name with the next available lowercase letter, in order, from 'a' through 'z'. For example, if base is:


and the five-digit value supplied by _mktemp_s is 12345, the first name returned is:


If this name is used to create file FNA12345 and this file still exists, the next name returned on a call from the same process or thread with the same base for nameTemplate is:


If FNA12345 doesn't exist, the next name returned is again:


_mktemp_s can create a maximum of 26 unique file names for any given combination of base and nameTemplate values. Therefore, FNZ12345 is the last unique file name _mktemp_s can create for the base and nameTemplate values used in this example.

In C++, using these functions is simplified by template overloads; the overloads can infer buffer length automatically (eliminating the need to specify a size argument) and they can automatically replace older, non-secure functions with their newer, secure counterparts. For more information, see Secure template overloads.


Routine Required header
_mktemp_s <io.h>
_wmktemp_s <io.h> or <wchar.h>

For more compatibility information, see Compatibility.


// crt_mktemp_s.cpp
/* The program uses _mktemp to create
* five unique filenames. It opens each filename
* to ensure that the next name is unique.

#include <io.h>
#include <string.h>
#include <stdio.h>

char *fnTemplate = "fnXXXXXX";
char names[5][9];

int main()
   int i, err, sizeInChars;
   FILE *fp;

   for( i = 0; i < 5; i++ )
      strcpy_s( names[i], sizeof(names[i]), fnTemplate );
      /* Get the size of the string and add one for the null terminator.*/
      sizeInChars = strnlen(names[i], 9) + 1;
      /* Attempt to find a unique filename: */
      err = _mktemp_s( names[i], sizeInChars );
      if( err != 0 )
         printf( "Problem creating the template" );
         if( fopen_s( &fp, names[i], "w" ) == 0 )
            printf( "Unique filename is %s\n", names[i] );
            printf( "Cannot open %s\n", names[i] );
         fclose( fp );

   return 0;

Sample output

Unique filename is fna03188
Unique filename is fnb03188
Unique filename is fnc03188
Unique filename is fnd03188
Unique filename is fne03188

See also

File handling
fopen, _wfopen
_open, _wopen
_tempnam, _wtempnam, tmpnam, _wtmpnam