Re: [Boost-users] [interprocess] Interprocess mutex not working / throwing exception when trying to write lock

Subject: Re: [Boost-users] [interprocess] Interprocess mutex not working / throwing exception when trying to write lock
From: Malko (malko.bravi_at_[hidden])
Date: 2015-10-16 04:26:58

Copying over here my initial reply:

Hi, 

1. Your lockWithWriteLock and lockWithReadLock create a lock, which locks on
construction and unlocks on destruction, hence when the function returns no
lock is held on the mutex. 

2. The fact that you put a scope here makes me think that your intention is
for the lock to be unlocked at the end of this scope (which it won't, as it
is unlocked way before, as per #1). 

    { 
        std::cout<<"Program 1: Before first locking
-------------------------- 1 v\n"; 
        temp.sharedMutex->lockWithWriteLock(); 
        const unsigned int SLEEP_TIME_IN_SECOND = 60; 
        std::cout<<"Program 1: sleep for "<< SLEEP_TIME_IN_SECOND << "\n"; 
        sleep(SLEEP_TIME_IN_SECOND); 
        std::cout<<"Program 1: Finished sleeping\n"; 
    } 

In this case though, you would perform the wait in bold while still holding
the lock, and hence preventing the 2nd program from locking it. 
Once out of the scope you destroy the object right away. 

NOTE: both the issue in #1 and #2 lead to your first program possibly
deleting the mutex before the 2nd program is done with it. Which leads to
the issue you are experiencing. 

3. If you need to have the 1st program wait for the 2nd program to be done,
then do not use sleeps, use a condition_variable. 

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