Do I need a mutex for reading?

c++ multithreading mutex
Tom picture Tom · Oct 6, 2009 · Viewed 32.1k times · Source

I have a class that has a state (a simple enum) and that is accessed from two threads. For changing state I use a mutex (boost::mutex). Is it safe to check the state (e.g. compare state_ == ESTABLISHED) or do I have to use the mutex in this case too? In other words do I need the mutex when I just want to read a variable which could be concurrently written by another thread?

Answer

jalf picture jalf · Oct 13, 2009

It depends.

The C++ language says nothing about threads or atomicity.

But on most modern CPU's, reading an integer is an atomic operation, which means that you will always read a consistent value, even without a mutex.

However, without a mutex, or some other form of synchronization, the compiler and CPU are free to reorder reads and writes, so anything more complex, anything involving accessing multiple variables, is still unsafe in the general case.

Assuming the writer thread updates some data, and then sets an integer flag to inform other threads that data is available, this could be reordered so the flag is set before updating the data. Unless you use a mutex or another form of memory barrier.

So if you want correct behavior, you don't need a mutex as such, and it's no problem if another thread writes to the variable while you're reading it. It'll be atomic unless you're working on a very unusual CPU. But you do need a memory barrier of some kind to prevent reordering in the compiler or CPU.

Related questions

Mutex example / tutorial?

I'm new to multithreading, and was trying to understand how mutexes work. Did a lot of Googling but it still left some doubts of how it works because I created my own program in which locking didn't work. One absolutely …

Read More
Example for boost shared_mutex (multiple reads/one write)?

I have a multithreaded app that has to read some data often, and occasionally that data is updated. Right now a mutex keeps access to that data safe, but it's expensive because I would like multiple threads to be able …

Read More
understanding of pthread_cond_wait() and pthread_cond_signal()

Generally speaking, pthread_cond_wait() and pthread_cond_signal() are called as below: //thread 1: pthread_mutex_lock(&mutex); pthread_cond_wait(&cond, &mutex); do_something() pthread_mutex_unlock(&mutex); //thread 2: pthread_mutex_lock(&mutex); pthread_…

Read More