Re: thread safe

"Heinz Ozwirk" <>
25 Jun 2006 09:45:20 -0400
"" <> schrieb im Newsbeitrag

I have a monitor like class and a thread-safe queue:

class Monitor {
    void lock();
    void unlock();

class MTQueue {
    void put();
    void get();
    std::queue<item> queue_
    Monitor getLock_;
    Monitor putLock_;
    Monitor emptyLock_;
    Monitor serializeLock_;

Here is the get() and put() implementation:

void put()

void get()
    while (queue_.empty()) emptyLock_.wait();
    item = queue_.front();

Are the put() and get() thread safe? I used two locks for
serialization and one lock for signaling. This allows two threads
access to the queue: one for put and the other for get.

Since there are no threads or locks in standard C++ (at least not yet) you
should better ask in a group for your system. So without knowing what
exactly your Monitor class really does, the following comments may be of
little value.

No. I don't think your code is thread safe. You should better use a single
lock for reading and writing. Inserting a new item to a container might use
iterators or pointers and while these are used, they must not become
invalid. OTH, removing an item from a container may invalidate all
iterators/pointers to elements in that container might comflict with
simultanous insertion. It is the container you must protect, not a single

Also, you should unlock the container as early as possible. Therefor it
would be better if put would unlock the container before signaling. But even
more important, get must not keep the container locked while it is waiting
for something to be written to it. How can another thread write something
into the queue while it is locked?


aka "working"

implementation would use only one lock and allows only one thread to
access queue:

void put()

void get()
    while (queue_.empty()) serializeLock_.wait();
    item = queue_.front();

Which is the right way to implemt the put() and get()? Thanks in

IMHO none of above. Protecting data against concurrent manipulation and
sending messages from one thread to another are different tasks, so there
should be different types objects or at least different instances of them.
Some systems use mutexes to protect critical sections of code and events to
signal an important change of an object's state. And there may be even more
types of syncronisation objects. If your Monitor can do both it might be OK,
but in general it seems better to use different classes for different tasks.


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