do need to use mutex lock?

Simple edition: In a C++ program I'm using two different threads to work with some integer variable. but I'm sure one is always writing some value into it and the other one is only reading That. do I still need to use mutex lock when reading/writing data?

Now details : The main idea is that first thread generates some information and saves them into an array, and the second thread reads data from that array and process them. this array represents a queue. meaning I have two index values pointing to the first and last item in queue. Now I'm wondering if I have to lock these two index values whenever I'm reading or writing values or is it ok to chec开发者_如何学Ck them without locking? note that generator thread is the only thread changing index of queue_back, and processor thread has exclusive permission to change queue_front.

if makes any change I'm developing for a linux based system and the code is compiled using gcc.

PS: In some codes which use threading, I've seen keyword volatile around variables shared between different threads, do I need to use that too?

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No the read and write are not atomic, You will need to synchronize it using some synchronization mechanism.

Also, You MUST mark the shared integer as volatile, otherwise the optimizer might think the variable is never updated in one of your threads.

gcc allows you to do atomic operations on int‘s, long‘s and long long‘s (and their unsigned counterparts).

Look up for the functions:

type __sync_fetch_and_add (type *ptr, type value);
type __sync_fetch_and_sub (type *ptr, type value);
type __sync_fetch_and_or (type *ptr, type value);
type __sync_fetch_and_and (type *ptr, type value);
type __sync_fetch_and_xor (type *ptr, type value);
type __sync_fetch_and_nand (type *ptr, type value);

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Do I still need to use mutex lock when reading/writing data?

Yes, you will need a lock. You may be interested in a more specific implementation called a read/write lock.

You can also use atomics and/or memory barriers. Using these will require a better understanding of your target architectures. Reproducing multithreading bugs can be very difficult, and these alternatives should be considered an optimization which may not be portable.

I've seen keyword volatile around variables shared between different threads, do I need to use that too?

Yikes. No! That is not a safe or portable solution to multithreaded reads and writes in C++. Use atomics, locks, copying, immutable and pure implementations (etc...) instead.

Interpretation of volatile can vary by platform and/or compiler, and it's not specified to operate any particular way in C or C++ for the purpose of multithreaded reads and writes (there's an old false legend that it can be used reliably as an atomic read/write). I once tested the effectiveness of volatile in a multithreaded C++ program for fun (on an intel-mac with apple's gcc). I won't provide the results because it worked well enough that some people might consider using it, although they should not because 'almost' isn't good enough.

And to qualify the use of volatile: It exists in my (large, strictly written, multithreading aware) codebase for the sole purpose of interfacing with platform dependent atomics APIs. And being entirely honest: there are a few other uses from earlier days, but they can and should be removed.

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Yes, you need to synchronize access to the variable, either with a mutex, a critical section, interlocked access, etc to make sure that the reading thread does not read incomplete bytes while the writing thread is still saving them. This is especially important on multi-core/CPU systems, where the two threads can truely access the variable in parallel.

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Reads and writes of properly aligned data no larger than a machine word (usually whatever int resolves to) are atomic on most major architectures. That does not mean every architecture.

This means that no, you cannot just read head and tail and expect that data is consistent. However, if for example sizeof(int) happens to be 4 and sizeof(short) happens to be 2, and if you don't care about "not mainstream" platforms, you can do some union trickery and get away without atomic operations or a mutex.

If you want your code to be portable, there is no way around proper locking or atomic compare/exchange.

About volatile, this does insert a memory barrier for Microsoft Visual C++ (as a compiler-specific sophistry) but the standard does not guarantee anything special other than that the compiler won't optimize the variable. Insofar, making something volatile does not really help much, and it guarantees thread safety in no way.

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Yes, you need to protect the queue indexes in both the generator and the reader threads by using some kind of synchronization mechanism, like a mutex.

HTH

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If you're really just sharing one single integer, then std::atomic<int> sounds like the right type to use, from the <atomic> header. (There should also be Boost or TR1 versions if you have an old compiler.) This ensures atomic reads and writes. There's no need for a volatile qualifier as far as I understand.