Mutexes in multithread Linux application

Question

Could you help me to understand how to use mutexes in multithread Linux application, where:

• during data writing it is need to lock variable on write and read
• during data reading from the variable it is need to lock it on write.

So it is possible to read simultaneously, but writing opertion is a single opertaion in the same time. During writing, all other operation should wait before it finishes.

Answer 1

You're asking about something that is a bit higher level than mutexes. A mutex is a simple, low-level device. When you lock a thread with a mutex, the CPU is either executing code in the thread that obtained the lock or it is executing some other process entirely. In other words, the mutex has locked out all other threads that belong to the same (heavyweight) process.

You are asking about a read-write lock. Read-write locks use mutexes underneath the hood. The POSIX functions that deal with read-write locks start with pthread_rwlock_. Since you are on a Linux machine, just type man pthread and look for the section marked "READ/WRITE LOCK ROUTINES".

Answer 2

You need a reader/writer lock to allow multiple readers/single writer.

Boost.Thread has one of these (boost::shared_mutex), if you have no other preferred threading library. This uses PThreads primitives under the covers, and will probably save you time in wrapping the raw APIs yourself.

I would not recommend implementing this yourself - it's easy to get something that appears to work, but under load either crashes or kills performance or (worst of all) silently modifies your data in a way it should not be, so you get bad results.

A simple boost::mutex can also be used here as noted by @Als, but won't allow multiple concurrent reads. That is simpler to implement, and may be sufficient for your needs, depending on your read/write access profile.

Answer 3

You will need to use mutexes, if you have global or static objects which are being accessed(read and written to) from different threads.