How to correctly use boost::timed_mutex and scoped_lock

Question

I am trying to use both timed_mutex with the scoped_lock. I have successfully used the scoped_lock before by following some examples but now I don't seem to find my way around neither I am able to properly understand the boost documentation.

The desired behavior is the following: try to acquire an scoped_lock for x time, if successful return true otherwise return false.

Currently I have:

boost::timed_mutex _mutex;
boost::timed_mutex::scoped_lock scoped_lock(_mutex, boost::get_system_time() + boost::posix_time::miliseconds(10));

However when I try to find (through boost documentation or examples) if this scoped_lock will return a boolean or not I find nothing or find really different ways to do it.

Therefore I ask which is the correct way to do it, how does it exactly works and maybe some indication as to how correctly "read" the documentation of boost.

UPDATE:

So

boost::timed_mutex _mutex;
boost::timed_mutex::scoped_lock scoped_lock(_mutex, boost::get_system_time() + boost::posix_time::miliseconds(10));

if(scoped_lock.owns_lock()) {
    // exclusive code 
}

Will create a mutex which when I try to lock with scoped_lock.owns_lock() will try to acquire the lock during 10 miliseconds (in this case) and return false if the time's up and the lock wasn't acquired?

This question might help you: http://stackoverflow.com/questions/14200305/usage-of-boostunique-locktimed-lock — Martijn Wijns, Jun 07 '14 at 13:25

T.C. · Accepted Answer · 2014-06-07T19:10:41.443

If you look at the documentation, boost::timed_mutex::scoped_lock is just an alias for boost::unique_lock<timed_mutex>:

class timed_mutex:
    boost::noncopyable
{
public:
    // ...

    typedef unique_lock<timed_mutex> scoped_timed_lock;
    typedef unspecified-type scoped_try_lock;
    typedef scoped_timed_lock scoped_lock;

    // ...
};

Now checking out the documentation for boost::unique_lock, it shows that there are two ways to determine if you own the lock:

template<typename Lockable>
class unique_lock
{
public:
    // ...

    explicit operator bool() const noexcept;
    bool owns_lock() const noexcept;

    // ...
};

Therefore, you can do either

if(scoped_lock) {
    // we have the lock, yay!
}

or

if(scoped_lock.owns_lock()) {
    // we have the lock, yay!
}

Incidentally, unique_lock has a constructor that takes relative time as a chrono::duration, which may or may not be cleaner than using absolute time.

Edit: Given this code:

boost::timed_mutex _mutex;
boost::timed_mutex::scoped_lock scoped_lock(_mutex,
           boost::get_system_time() + boost::posix_time::miliseconds(10)); // <-- attempt to acquire mutex happens here!

if(scoped_lock.owns_lock()) {
    // exclusive code 
}

The attempt to acquire the mutex happens at the time the lock is constructed, not at the time owns_lock() is called. Yes, the exclusive code will only execute if you successfully acquired the mutex. I'm not sure what you mean by "return false" - this code doesn't return anything. If owns_lock() returns false, then you have failed to acquire the mutex and can't run the exclusive code, and you can communicate this to your caller anyway you want.

I've updated my question to reflect your information and make sure I've understood it well, please confirm I am right! — Pol Alvarez Vecino, Jun 07 '14 at 17:40

How to correctly use boost::timed_mutex and scoped_lock

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