With C++14 came reader-writer locks. The idea is straightforward and promising. Arbitrary reading threads can access the critical region simultaneously, but only one thread is allowed to write.
Reader-writer locks do not solve the fundamental problem – threads competing for access to a critical region. But reader-writer locks help a lot – to minimize the bottleneck. Let’s give an example.
The telebook in line 9 is the shared variable, which has to be protected. Eight threads want to read the telephone book; two threads want to modify it (lines 30 – 39). To access the telephone book simultaneously, the reading threads use the std::shared_lock<std::shared_timed_mutex>> in line 22. This opposes the writing threads, which need exclusive access to the critical section. The exclusivity is given by the std::lock_guard<std::shared_timed_mutex>> in line 14. In the end, the program displays (lines 54 – 57) the updated telephone book.
The screenshot shows that the output of the reading threads overlaps while the writing thread is executed one after the other. It means that the reading operations are performed at the same time.
That was easy. Too easy.
The program has undefined behavior. What? Before you continue, stop for a few seconds and think. By the way, the concurrent access of std::cout is not the issue.
The characteristic of a race condition is that at least two threads access the shared variable simultaneously, and at least one of them is a writer. Precisely what is happening in the program. A characteristic of the ordered associative container is that the reading of the container can modify it. It happens if the element is unavailable in the container – the case of “Bjarne”. If “Bjarne” is not found in the telephone book, a pair (“Bjarne”, 0) will be created from the read operation. For the details, have a look at cppreference.com.
In the next post, I will continue with thread-safe data initialization in multithreading programs.
Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschläger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mühlhaus, Matthieu Bolt, Stephen Kelley, Kyle Dean, Tusar Palauri, Dmitry Farberov, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Fütterer, Matthias Grün, Phillip Diekmann, Ben Atakora, Ann Shatoff, Rob North, Bhavith C Achar, Marco Parri Empoli, moon, and Philipp Lenk.
Thanks, in particular, to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, John Nebel, Mipko, Alicja Kaminska, Slavko Radman, and David Poole.
|My special thanks to Embarcadero|
|My special thanks to PVS-Studio|
|My special thanks to Tipi.build|
|My special thanks to Take Up Code|
|My special thanks to SHAVEDYAKS|
I’m happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.
- Embedded Programmierung mit modernem C++ 12.12.2023 – 14.12.2023 (Präsenzschulung, Termingarantie)
Standard Seminars (English/German)
Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.
- C++ – The Core Language
- C++ – The Standard Library
- C++ – Compact
- C++11 and C++14
- Concurrency with Modern C++
- Design Pattern and Architectural Pattern with C++
- Embedded Programming with Modern C++
- Generic Programming (Templates) with C++
- Clean Code with Modern C++
- Phone: +49 7472 917441
- Mobil:: +49 176 5506 5086
- Mail: schulung@ModernesCpp.de
- German Seminar Page: www.ModernesCpp.de
- Mentoring Page: www.ModernesCpp.org
Modernes C++ Mentoring,