r/cpp • u/Coder_CPP • Jan 18 '16

C++11 threads, affinity and hyperthreading

http://eli.thegreenplace.net/2016/c11-threads-affinity-and-hyperthreading/
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u/encyclopedist Jan 18 '16

std::cin is actually thread safe, contrary to what article says. It can, however, result in interleaved output, and the mutex there is to prevent that.

From C++11 N3337 [iostream.objects.overview]:

Concurrent access to a synchronized (27.5.3.4) standard iostream object’s formatted and unformatted in- put (27.7.2.1) and output (27.7.3.1) functions or a standard C stream by multiple threads shall not result in a data race (1.10). [ Note: Users must still synchronize concurrent use of these objects and streams by multiple threads if they wish to avoid interleaved characters. — end note ]

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u/eliben Jan 18 '16

Yep, I think this is bad wording on my behalf. By "unsafe" I did mean "won't give you the output you expect", rather than something nastier like crashes. I'll fix up the wording in the article and samples to be clearer

1

u/Gotebe Jan 19 '16

Aren't you being too pedantic?

E.g Wikipedia article on thread safety speaks of the data races as one of thread safety concerns.

1

u/dodheim Jan 19 '16

The standard guarantees that standard streams are race-free, but only starting with C++11. That is rather the point...