<\/p>\n
In case the program has a race condition, the program behavior is undefined. To say it differently, anything can happen.<\/p>\n
A nice way to visualize a race condition is to let a few threads write to std::cout<\/span>. std::cout<\/span> is the shared object (output stream), that should be protected from simultaneous access by multiple threads.<\/p>\n <\/p>\n <\/p>\n The boss assigns three work packages (lines 11 – 17) to each of its six workers (lines 32 – 36). When a worker is done with their work package it screams out loudly to the boss (line 16). When the boss has gotten notifications from all workers, it sends them home (line 45).<\/span><\/p>\n What a mess!<\/strong><\/p>\n The same mess the next day. The workers scream out loudly. Totally unsynchronized.<\/p>\n The first solution is a mutex. A mutex ensures, that each thread exclusively accesses the shared variable std::cout<\/span>. <\/span><\/p>\n Mutex stands for mut<\/strong>ual ex<\/strong>clusion. It ensures, that only one thread can access a critical section.<\/p>\n <\/p>\n <\/p>\n <\/p>\n The key difference to the first example is lines 19 to 21. By invoking the methods coutMutex.lock()<\/span> and coutMutex.unlock()<\/span>; you define the exclusive section. This section can only be accessed by, at most, a single thread. The access to std::cout<\/span> is synchronized, and the mess becomes harmonious.<\/p>\n <\/p>\n\n\n
\n \n 1\r\n 2\r\n 3\r\n 4\r\n 5\r\n 6\r\n 7\r\n 8\r\n 9\r\n10\r\n11\r\n12\r\n13\r\n14\r\n15\r\n16\r\n17\r\n18\r\n19\r\n20\r\n21\r\n22\r\n23\r\n24\r\n25\r\n26\r\n27\r\n28\r\n29\r\n30\r\n31\r\n32\r\n33\r\n34\r\n35\r\n36\r\n37\r\n38\r\n39\r\n40\r\n41\r\n42\r\n43\r\n44\r\n45\r\n46\r\n47\r\n48\r\n49<\/pre>\n<\/td>\n
\n \/\/ coutUnsynchronized.cpp<\/span>\r\n\r\n#include <chrono><\/span>\r\n#include <iostream><\/span>\r\n#include <thread><\/span>\r\n\r\nclass<\/span> Worker<\/span>{\r\npublic:\r\n Worker(std::string n):name(n){};\r\n \r\n void<\/span> operator() (){\r\n for<\/span> (int<\/span> i= 1; i <= 3; ++i){\r\n\t\/\/ begin work<\/span>\r\n\tstd::this_thread::sleep_for(std::chrono::milliseconds(200));\r\n\t\/\/ end work<\/span>\r\n\tstd::cout << name << \": \"<\/span> << \"Work \"<\/span> << i << \" done !!!\"<\/span> << std::endl;\r\n }\r\n \r\n }\r\nprivate:\r\n std::string name;\r\n};\r\n\r\n\r\nint<\/span> main(){\r\n\r\n std::cout << std::endl;\r\n \r\n std::cout << \"Boss: Let's start working.\\n\\n\"<\/span>;\r\n \r\n std::thread<\/span> herb= std::thread<\/span>(Worker(\"Herb\"<\/span>));\r\n std::thread<\/span> andrei= std::thread<\/span>(Worker(\" Andrei\"<\/span>));\r\n std::thread<\/span> scott= std::thread<\/span>(Worker(\" Scott\"<\/span>));\r\n std::thread<\/span> bjarne= std::thread<\/span>(Worker(\" Bjarne\"<\/span>));\r\n std::thread<\/span> andrew= std::thread<\/span>(Worker(\" Andrew\"<\/span>));\r\n std::thread<\/span> david= std::thread<\/span>(Worker(\" David\"<\/span>));\r\n \r\n herb.join();\r\n andrei.join();\r\n scott.join();\r\n bjarne.join();\r\n andrew.join();\r\n david.join();\r\n \r\n std::cout << \"\\n\"<\/span> << \"Boss: Let's go home.\"<\/span> << std::endl;\r\n \r\n std::cout << std::endl;\r\n\r\n}\r\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n![\"bossWorker\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2016\/04\/bossWorker.png\")
<\/p>\n![\"bossWorker1\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2016\/04\/bossWorker1.png\")
<\/p>\n<\/dd>\nA side note: std::cout<\/span> is thread-safe
<\/span><\/h3>\n<\/dd>\nMutex<\/h2>\n
\n\n
\n \n 1\r\n 2\r\n 3\r\n 4\r\n 5\r\n 6\r\n 7\r\n 8\r\n 9\r\n10\r\n11\r\n12\r\n13\r\n14\r\n15\r\n16\r\n17\r\n18\r\n19\r\n20\r\n21\r\n22\r\n23\r\n24\r\n25\r\n26\r\n27\r\n28\r\n29\r\n30\r\n31\r\n32\r\n33\r\n34\r\n35\r\n36\r\n37\r\n38\r\n39\r\n40\r\n41\r\n42\r\n43\r\n44\r\n45\r\n46\r\n47\r\n48\r\n49\r\n50\r\n51<\/pre>\n<\/td>\n
\n \/\/ coutSynchronized.cpp<\/span>\r\n\r\n#include <chrono><\/span>\r\n#include <iostream><\/span>\r\n#include <mutex><\/span>\r\n#include <thread><\/span>\r\n\r\nstd::mutex coutMutex;\r\n\r\nclass<\/span> Worker<\/span>{\r\npublic:\r\n Worker(std::string n):name(n){};\r\n \r\n void<\/span> operator() (){\r\n for<\/span> (int<\/span> i= 1; i <= 3; ++i){\r\n\t\/\/ begin work<\/span>\r\n\tstd::this_thread::sleep_for(std::chrono::milliseconds(200));\r\n\t\/\/ end work<\/span>\r\n\tcoutMutex.lock();\r\n\tstd::cout << name << \": \"<\/span> << \"Work \"<\/span> << i << \" done !!!\"<\/span> << std::endl;\r\n\tcoutMutex.unlock();\r\n }\r\n }\r\nprivate:\r\n std::string name;\r\n};\r\n\r\n\r\nint<\/span> main(){\r\n\r\n std::cout << std::endl;\r\n \r\n std::cout << \"Boss: Let's start working.\"<\/span> << \"\\n\\n\"<\/span>;\r\n \r\n std::thread<\/span> herb= std::thread<\/span>(Worker(\"Herb\"<\/span>));\r\n std::thread<\/span> andrei= std::thread<\/span>(Worker(\" Andrei\"<\/span>));\r\n std::thread<\/span> scott= std::thread<\/span>(Worker(\" Scott\"<\/span>));\r\n std::thread<\/span> bjarne= std::thread<\/span>(Worker(\" Bjarne\"<\/span>));\r\n std::thread<\/span> andrew= std::thread<\/span>(Worker(\" Andrew\"<\/span>));\r\n std::thread<\/span> david= std::thread<\/span>(Worker(\" David\"<\/span>));\r\n \r\n herb.join();\r\n andrei.join();\r\n scott.join();\r\n bjarne.join();\r\n andrew.join();\r\n david.join();\r\n \r\n std::cout << \"\\n\"<\/span> << \"Boss: Let's go home.\"<\/span> << std::endl;\r\n \r\n std::cout << std::endl;\r\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n![\"bossWorkerSynchonized\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2016\/04\/bossWorkerSynchonized.png\")
<\/p>\nWhat’s next?<\/h2>\n