Mutex locking when shared memory is only read

Question

Is my impression correct that if shared memory between threads is only read and never written, by any of the threads, mutex protection is unneccessary?

It's because wherever a thread is preempted, it can always take off again when rescheduled, and the memory contents would not have changed.

This [previous SO](http://stackoverflow.com/a/15744475/1079907) might answer it. — Sunil Bojanapally, Feb 05 '14 at 19:43

score 4 · Accepted Answer · answered Feb 05 '14 at 19:38

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Yes, you're right, multiple threads reading the same variable do not introduce a race.

answered Feb 05 '14 at 19:38

Alex Telishev

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@KerrekSB Lol, pun was not intended) – Alex Telishev Feb 05 '14 at 19:39
They can't race with each other, but if anything is writing to the memory, you need some kind of barrier between the write and reads to avoid data races. – R.. GitHub STOP HELPING ICE Feb 05 '14 at 20:08
In which case the typical mechanism used is the [readers-writer lock](http://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock). – To마SE Feb 06 '14 at 05:08

score 3 · Answer 2 · answered Feb 05 '14 at 19:38

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Both C++11 and C11 define a data race to be access to a memory location by more than one execution context without ordering where at least one of the accesses is a write. If you have no writes, you have no races.

answered Feb 05 '14 at 19:38

Kerrek SB

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Mutex locking when shared memory is only read

2 Answers2

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