Q114374: PRB: _mktemp() May Return the Same Name Between Threads

Article: Q114374
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbCRT kbVC kbprb
Last Modified: 22-JUL-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The C Run-Time (CRT), included with:
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, version 1.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

When calling _mktemp() with the same template name in different threads of the
same process, _mktemp() may return the same file name to each thread.

CAUSE
=====

The _mktemp() function is used to generate temporary file names. Starting with a
user supplied template consisting of a base followed by six X's, _mktemp()
generates temporary file names by concatenating a five digit value to the
specified base. To ensure that unique names are generated between processes, the
process id is encoded into the file name. In a multi- threaded application, the
scheme of using only the process id makes it possible for threads to receive the
same temporary file name. This could occur whenever one of the file names
returned by _mktemp had not actually been used to open a file yet.

RESOLUTION
==========

In Visual C++ 2.0, _mktemp() uses the thread id in generating the name, so this
problem will not occur.

In Visual C++ 1.0, the work-around is to pass in a different template name to
_mktemp() in each thread. A convenient way to create a unique template name for
a thread would be to use the thread id to generate the template name. In the
code fragment below, the thread id is obtained and used as an index into a set
of characters which are used as the first two characters in the template name.

  #define DIG_LEN 37

  static char digits [ DIG_LEN + 1 ] =
                             "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_";

  // Within the thread, generate a template name
  // using the thread id.

       char templte[9];
       unsigned tid = GetCurrentThreadId();

       strcpy ( templte , "@@XXXXXX" ); // initialize template name

       templte[0] = digits [ tid % DIG_LEN ];     // assign first character
                                             // in template name using
                                             // the tid to index into
                                             // the digits array
       templte[1] =
            digits [ ( tid / DIG_LEN ) % DIG_LEN ];  // second character

      path1 = _mktemp(templte);

MORE INFORMATION
================

Sample Code
-----------

  /* Compile options needed:    /MT /D_CONSOLE
  */ 
  #include <windows.h>
  #include <process.h>    /* _beginthread, _endthread */ 
  #include <io.h>         /* _mktemp */ 
  #include <stdio.h>

  void Check1(void *);
  void Check2(void *);
  char *path1, *path2, *path3;
  char templateName[4][9] = {"fnXXXXXX","fnXXXXXX","fnXXXXXX",
                   "tnXXXXXX"};
  void main()
  {
    _beginthread(Check1, 0, NULL );
    _beginthread(Check2,0,NULL);
    Sleep(200L);
    printf("Program finished\n");
  }

  /* Check */ 
  void Check1( void * dummy  )
  {
    FILE *fp;

    path1 = _mktemp(templateName[0]);

    Sleep(100L);  // Give thread 2 a chance to call mktemp before the
                    // the file is actually opened
    if (path1 != NULL)
       if ((fp = fopen(path1,"w")) == NULL) {
       printf("Thread 1 - Failed to open file %s\n",path2);
         _endthread();
       }
    fclose(fp);
    remove(path1);
    printf("Thread 1 - File %s successfully opened\n",path1);
    _endthread();
  }

  /* Check 2 */ 
  void Check2( void * dummy  )
  {
    FILE *fp;

    // Path2 is set to the same as path1 which is INCORRECT
    path2 = _mktemp(templateName[2]);

    // Workaround is to specify an unused template name

    if (path2 != NULL)
       if ((fp = fopen(path2,"w")) == NULL) {
       printf("Thread 2 - Failed to open file %s\n",path2);
         _endthread();
       }
    fclose(fp);
    remove(path2);
    printf("Thread 2 - File %s successfully opened\n",path2);
    _endthread();
  }

Additional query words: 1.00

======================================================================
Keywords          : kbCRT kbVC kbprb 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : winnt:
Issue type        : kbprb

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.