Why doesn't POSIX mmap return a volatile void*?

Question!

Mmap returns a void*, but not a volatile void*. If I'm using mmap to map shared memory, then another process could be writing to that memory, which means two subsequent reads from the same memory location can yield different values -- the exact situation volatile is meant for. So why doesn't it return a volatile void*?

My best guess is that if you have a process that's exclusively writing to the shared memory segment, it doesn't need to look at the shared memory through volatile pointers because it will always have the right understanding of what's present; any optimizations the compiler does to prevent redundant reads won't matter since there is nothing else writing and changing the values under its feet. Or is there some other historical reason? I'm inclined to say returning volatile void* would be a safer default, and those wanting this optimization could then manually cast to void*.

POSIX mmap description: http://opengroup.org/onlinepubs/007908775/xsh/mmap.html



Answers

It's probably done that way for performance reasons, providing nothing extra by default. If you know that on your particular architecture that writes/reads won't be reordered by the processor you may not need volatile at all (possibly in conjuction with other synchronization). EDIT: this was just an example - there may be a variety of other cases where you know that you don't need to force a reread every time the memory is accessed.

If you need to ensure that all the addresses are read from memory each time they're accessed, const_cast (or C-style cast) volatile onto the return value yourself.

By : Mark B


The type volatile void * or void * volatile is nonsensical: you cannot dereference a void *, so it doesn't make sense to specify type qualifiers to it.

And, since you anyway need a cast to char * or whatever your data type, then perhaps that is the right place to specify volatility. Thus, the API as defined nicely side-steps the responsibility of marking the memory changable-under-your-feet/volatile.

That said, from a big picture POV, I agree with you: mmap should have a return type stating that the compiler should not cache this range.

By : terminus


I don't think volatile does what you think it does.

Basically, it just tells the compiler not to optimize the variable by storing its value in a register. This forces it to retrieve the value each time you reference it, which is a good idea if another thread (or whatever) could have updated it in the interim.

The function returns a void*, but it's not going to be updated, so calling it volatile is meaningless. Even if you assigned the value to a local volatile void*, nothing would be gained.



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