prefix can be NULL for GetTempFileNameW.

dlls/kernel/path.c

@@ -651,7 +651,7 @@ UINT WINAPI GetTempFileNameW( LPCWSTR path, LPCWSTR prefix, UINT unique, LPWSTR
     int i;
     LPWSTR p;
 
-    if ( !path || !prefix || !buffer )
+    if ( !path || !buffer )
     {
         SetLastError( ERROR_INVALID_PARAMETER );
         return 0;
@@ -663,7 +663,8 @@ UINT WINAPI GetTempFileNameW( LPCWSTR path, LPCWSTR prefix, UINT unique, LPWSTR
     /* add a \, if there isn't one  */
     if ((p == buffer) || (p[-1] != '\\')) *p++ = '\\';
 
-    for (i = 3; (i > 0) && (*prefix); i--) *p++ = *prefix++;
+    if (prefix)
+        for (i = 3; (i > 0) && (*prefix); i--) *p++ = *prefix++;
 
     unique &= 0xffff;
 

dlls/kernel/tests/path.c

@@ -335,9 +335,18 @@ static void test_InitPathA(CHAR *newdir, CHAR *curDrive, CHAR *otherDrive)
   sprintf(tmpstr1,"pat%x.tmp",id & 0xffff);
   ok(lstrcmpiA(newdir+lstrlenA(tmppath),tmpstr)==0 ||
      lstrcmpiA(newdir+lstrlenA(tmppath),tmpstr1)==0,
-     "GetTempPath returned '%s' which doesn't match '%s' or '%s'. id=%x\n",
+     "GetTempFileNameA returned '%s' which doesn't match '%s' or '%s'. id=%x\n",
      newdir,tmpstr,tmpstr1,id);
 
+  ok((id=GetTempFileNameA(tmppath,NULL,0,newdir)),"GetTempFileNameA failed\n");
+  sprintf(tmpstr,"%.4x.tmp",id & 0xffff);
+  sprintf(tmpstr1,"%x.tmp",id & 0xffff);
+  ok(lstrcmpiA(newdir+lstrlenA(tmppath),tmpstr)==0 ||
+     lstrcmpiA(newdir+lstrlenA(tmppath),tmpstr1)==0,
+     "GetTempFileNameA returned '%s' which doesn't match '%s' or '%s'. id=%x\n",
+     newdir,tmpstr,tmpstr1,id);
+
+
 /* Find first valid drive letter that is neither newdir[0] nor curDrive */
   drives = GetLogicalDrives() & ~(1<<(newdir[0]-'A'));
   if( *curDrive != NOT_A_VALID_DRIVE)