Subj : Re: pthread_mutex_trylock problem
To   : comp.programming.threads
From : David Schwartz
Date : Wed Aug 31 2005 11:17 am


<rajendra.stalekar@gmail.com> wrote in message 
news:1125489761.189542.167460@g49g2000cwa.googlegroups.com...

>I would like to know if one thread acquires a lock(by using
> pthread_mutex_lock) and another thread tries to acquire it it would
> sleep on it and once the thread which acquired releases it the first
> thread which is sleeping acquires.
> When would we use pthread_mutex_trylock() then? What is the use of such
> a function.

    It's sometimes useful in very rare cases where you cannot wait for a 
lock, but can do something very efficiently if you can get the lock 
immediately.

    For example, consider a server where each client is protected by a lock. 
Threads that are handling a client hold the lock on that client, and they 
might hold it for awhile if that client is doing a lot of work.

    One thread, handling an administrative client, receives a command to 
disconnect another client. If it can get that client's lock immediately, 
then it can shut the client down very quickly. But it can't afford to wait 
for that client's lock to be released if the client is in the middle of 
something complicated (because that would stall the administrative client 
for too long).

    It's also good in cases where there's potential deadlock. Imagine if you 
called 'pthread_mutex_lock' and two administrative clients each tried to 
disconnect the other. If you used 'pthread_mutex_lock', each client could 
hold a lock on itself and try to get a lock on the other. Very bad.

    With 'pthread_mutex_trylock', you can try to acquire the lock safely and 
do things the easy way. If you fail to get the lock, you can fall back to 
assigning another thread (which would hold no locks) to do the 
disconnection, and it can wait for the busy client to finish and the lock to 
be released.

    DS

.