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Reply"So now, rather than issuing a CreateMutex call and then having to check the error status when it returns a valid handle, which has always been a little jarring to me, you first try to open the mutex. If you can open it, you know it already exists and can act accordingly. If you can't open it, you can create it."
There's no guarantee that the mutex will not exist when you try to create it. Consider threads A, B in a single processor (the least dangerous) scenario:
time A B
| open->FILE_NOT_FOUND
| // OK, so go and create
| (unscheduled)
| (scheduled)
| open->FILE_NOT_FOUND
| // OK, so go and create
|
| create->OK
|
| (unscheduled)
| (scheduled)
|
| create->ACCESS_DENIED
|
|
v
Obviously what's needed is a mutex to protect the {open ... create} logic! :-(
What options do you have?
a) loop around doing {open? ...create} until one succeeds - but how many times?
b) nominate a master process that will create the mutex (maybe your configuration involves a service and n user processes - then this would make sense).
c) the logic suggested by Dr GUI is {create? ... open} which may be less susceptible to the race condition (the creating process is presumably performing some operations while it has the mutex, so allowing time for another process's {create? ... open} to complete.
d) anything else?
Reply
Possibly because of Windows XP SP2 tightening up security, I found the rights assigned here weren't enough to access mutexes or events created in a service in a client running in a limited user logon account and using OpenMutex/OpenEvent. If you're experiencing this, try replacing 'GENERIC_ALL' in the ::AddAccessAllowedAce call with (GENERIC_ALL|STANDARD_RIGHTS_ALL|SPECIFIC_RIGHTS_ALL) & (~(WRITE_OWNER|WRITE_DAC)). This seemed to work with my code.
ReplyOriginally posted by: mrr
Thanks--I did a web search for null DACL and found this very useful article. I knew what I wanted to do, but I didn't know how to do it.
ReplyOriginally posted by: soda
This saved me a ton of work :)
ReplyOriginally posted by: Kathleen Langone
if ( ( g_hMutexAppRunning != NULL ) && ( GetLastError() == ERROR_ALREADY_EXISTS) ||
g_hMutexAppRunning = CreateMutex( &my_attr, FALSE, "Global\\myapp.exe");
I found some of the above information in a good Microsoft Knowledge Base articles... but don't have a link to it.
In essence the return error code of: ERROR_ALREADY_EXISTS didn't occur w/ my logic... so I ended up checking for 'access_denied' when a second app tried to access/create the same mutex:
( g_hMutexAppRunning == NULL ) && ( GetLastError() == ERROR_ACCESS_DENIED))
Also for the CreateMutex I used the "global" area... which helps facilities apps from two different XP logics to communicate:
Originally posted by: Bill Gates
Saved me a ton of time. Could not figure out why the service and the UI app could not share a mutex.
Originally posted by: Jason Welco
Many thanks. Microsoft's documentation has not kept up with the complexity of their APIs. This helps me quickly solve my problem with getting a client to connect to my pipe server.
I still need to dig into this further, however, because I don't want anybody to be able to toast my pipe by feeding it garbage.
ReplyOriginally posted by: Sidder Aal
Just the piece of information that was missing to solve a mutex sharing problem between our service and another application. Thanks !
Reply