_mktemp_s, _wmktemp_s

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

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


  • template
    File name pattern.

  • sizeInChars
    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



return value

new value in template





Incorrect format (see Remarks section for correct format)



empty string


<= number of X's


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 template argument, so that after the call, the template 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 does not handle multibyte-character strings.

Generic-Text Routine Mappings

Tchar.h routine

_UNICODE and _MBCS not defined

_MBCS defined

_UNICODE defined





The template 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 template must be an uppercase X. _mktemp_s preserves base and replaces the first trailing X with an alphabetic character. _mktemp_s replaces the following trailing X's with a five-digit value; this value is a unique number identifying the calling process, or in multithreaded programs, the calling thread.

Each successful call to _mktemp_s modifies template. In each subsequent call from the same process or thread with the same template 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 template is:


If FNA12345 does not 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 template values. Therefore, FNZ12345 is the last unique file name _mktemp_s can create for the base and template 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.



Required header




<io.h> or <wchar.h>

For more compatibility information, see Compatibility in the Introduction.


// 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

.NET Framework Equivalent

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.

See Also


File Handling

fopen, _wfopen



_open, _wopen


_tempnam, _wtempnam, tmpnam, _wtmpnam