C++ Multithreading Note

Following is the summary note categorizing into different category of STL functions related to multithreading programming in C++ upon my studying. There is also example code demonstrating some of its usage as well as noted.

Locking Stategy

STL Names Note
std::defer_lock Do not acquire ownership of the mutex. Defer locking. Use when users want to create a mutex-lock (for example std::lock, std::unique_lock) without owning the mutex before actually create ones that own it; note that we can specify locking strategy via mutex wrappers (i.e. std::lock_guard, std::unique_lock)’ constructor’s parameter.
std::try_to_lock Try to acquire ownership of the mutex without locking. It call std::mutex::try_lock at the constructor method to determine whether it should own such std::mutex or not.
std::adopt_lock Assume the calling thread already has ownership of the mutex (assume previously locked).

Locks (Mutex Wrapper)

STL Names Note
std::lock_guard Mutex lock which will automatically unlock itself at the end of the block scope.
std::unique_lock Similar to std::lock_guard but provides optional flexibility to manually lock and unlock manually.
std::shared_lock Used in situation of several readers, but one writer, so readers can share a single mutex.
std::scoped_lock Enhanced version of std::lock_guard but supports multiple mutuxes at once using deadlock avoidance algorithm.


STL Names Note
std::lock Acquire locks from multiple mutexes by using deadlock avoidance algorithm.


STL Names Note
std::atomic Hold the atomic object for performing atomic operations. Due to its implementation, it does not guarantee lock-free. Atomic relies on CPU features and ability.
std::atomic_flag Lower level of atomic operation with major difference that it guarantees lock-free but doesn’t provide any load and store operation.


STL Names Note
std::thread This is meat of everything as to run a task concurrently, one need to spawn a new thread although in certain case we run a task in the current thread.
std::mutex Also a meat of everything. It’s the basic for almost parts of multithreading programming environment dealing with C++. Deep down inside it’s implemented with atomic operation; thus mutex itself doesn’t need another mutex to achieve the goal.
std::recursive_mutex Recursive version of mutex. It allows unspecified number of lock() call on itself if locked already, until std::system_error will be thrown. Thus it allows to be locked even though it’s already locked but we cannot control how many times it can do just that.
std::condition_variable A logic control to allow other thread(s) to wait for a condition to be true and notified by a thread which is able to modity a value affecting the logic condition, and notify other thread(s). It can be worked with only std::unique_lock for performance reason.
std::condition_variable_any Same as std::condition_variable but relax such requirement to allow it to work with any locks.
std::shared_timed_mutex Optimized version upon std::unique_lock especially for situation of several readers, and one writer. It will usually work better than using std::unique_lock, but if it’s just a few of readers, no significant performance gained thus it’s still advised to use std::unique_lock.


STL Names Note
std::async Execute a function on newly spawned thread (via std::launch::async), or on the calling thread only the first time to compute its result (via std::launch::deferred). It return std::future which we use std::future::get() to retrieve the result. Note: Its destructor is blocked to properly destroy all its shared data and states. Only std::future created from this way will block the call (in its destructor).
std::promise Similar to std::async but allow users flexibility to fullfil resultant std::future as created by it later separately rather than at the end of function call. It is used by producer/writer of asynchronized operation. Internally it uses std::condition_variable to notify the associated std::future.
std::packaged_task Suitable for sending it into other classes, then execute later. Same as std::promise which allows such task to be executed later.
std::future Resultant mechasnim in getting result from other means. All those std::async, std::promise, and std::packaged_task return std::future either immediately in case of std::async or via get_future() in case of the other twos. It is used by consumer/reader operation. Underlying it is implemented using std::thread.

Code Example

I’ve done some of the code examples covering usage of above categories although not all, but it should be useful. Locks and Mutex should be used in almost all examples, so the Topic mentioned below is for specific category we’re interested in. See below.

Topic URLs
std::async Async.cpp
std::async effect of blocking on its destructor Async2.cpp
std::condition_variable ConditionVariable.cpp
std::packaged_task PackagedTask.cpp
std::atomic Atomic.cpp

First published on Oct, 8, 2019

Nov, 30, 2019

Dec, 26, 2020

Written by Wasin Thonkaew
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