When multiple threads want to synchronize access to a shared resource they use mutexes and condition variables. A mutex enables a thread to block other threads from accessing a shared resource, while a condition variable is used by a thread to make itself wait for the state of a resource to be changed by other threads.
In my next two blogs I’ll illustrate how to simplify Pthread mutex and condition variable functions by wrapping them in C++ classes, starting with mutexes.
Pthread Mutex Functions
These are the standard Pthread functions that will be incorporated in the Mutex class.
Each function returns
0 if successful or an error number if it fails.
Mutex class provides a method that wraps each of the Pthread mutex functions and contains a private
pthread_mutex_t variable which is use by the function calls. This hides the parameter so applications that call the
Mutex class methods do not have to provide it. Note that I declare a friend class
CondVar which will need to have access to the private
m_mutex data member as you’ll see in my next blog.
Owing to its simplicity the class can be defined in a single include file Mutex.h. Each method simply calls the corresponding Pthread mutex function and returns whatever it returns. When
pthread_mutex_init() is called, the attribute argument is set to
NULL to accept the default behavior.
Mutex Test Application
You can get the source code for this project from GitHub – https://github.com/vichargrave/mutex.git.
Mutex class test program relies on my
Thread class which I wrote about awhile back. The application declares a MutexTest thread class which works with the
main() thread to lock a single
The trick here is to give the MutexTest thread time to lock the
main() so we see the main thread waiting to acquire lock. So after creating the MutexTest thread,
main() sleeps for 1 second during which time the
MutexTest thread acquires the lock.
MutexTest then sleeps 10 seconds before unlocking the
Mutex at which point
main() acquires the lock and proceeds to run.
When the test application is run, the output looks like this after 1 second:
This indicates the Mutex is working. The application pauses at this point while main() waits 10 seconds to acquire the lock. When this time elapses the output looks like this: