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

• The C Run-time (CRT), included with: Microsoft Visual C++ 32-bit Edition, 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

                           "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();