Note: This paper was adopted into draft C++14 on Saturday at the Bristol UK ISO C++ meeting.

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Document number: N3659

Date: 2013-04-19

Shared locking in C++ (a.k.a. reader/writer locks)

by Howard Hinnant, Detlef Vollmann, Hans Boehm

Excerpt:

N3568 was presented at the Spring 2013 meeting in Bristol to SG1 (Concurrency and Parallelism). The decision was to bring shared_mutex only forward for C++14. Also the specification should allow for spurious failures.

From N3568:

This proposal adds functionality to allow clients to easily code the well-known multiple-readers / single-writer locking pattern. ...

... a shared_mutex can be locked in one of two ownership modes:

1. Exclusive, using lock(), or
2. Shared, using lock_shared().

... The recommended pattern for locking a shared_mutex in shared ownership mode is not to call m.lock_shared() directly, but to use shared_lock<shared_mutex>:

shared_mutex mut;
...
void foo()
{
     shared_lock<shared_mutex> _(mut);
     // mut lock_shared'd here
     // ...
}    // mut.unlock_shared() called here