Microsoft KB Archive/114374

= PRB: _mktemp May Return the Same Name Between Threads =

Article ID: 114374

Article Last Modified on 10/2/2003

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APPLIES TO


 * The C Run-Time (CRT)

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This article was previously published under Q114374



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.
 * 1) 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);



Sample Code
/* Compile options needed:   /MT /D_CONSOLE
 * 1) include 
 * 1) include 


 * 1) include    /* _beginthread, _endthread */
 * 2) include         /* _mktemp */
 * 3) include 

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 kbprb KB114374

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