What does 'threadsafe' mean?

Alastair Reid alastair at reid-consulting-uk.ltd.uk
Fri Nov 29 03:41:45 EST 2002

[This question came from the ghc-users thread on native threads but it
seems to belong on the ffi list so I'm copying it and my response to

Wolfgang Thaller writes:
> I reread Section 3.3 of the FFI document (RC7), and now I think I
> cannot clarify my specification in this respect without first asking
> others to clarify the current specs - can someone explain the
> distinction between unsafe, safe and threadsafe in the current FFI
> to me? I think I know what it does in GHC, but what's the general
> definition?

My reading of the spec is that it means that execution of other
runnable Haskell threads and of the foreign function can be
interleaved.  That is, neither will be 'starved'.

Operationally, this requires that the Haskell runtime and the foreign
call operate in separate threads.

I think it is a mistake for the ffi spec to say that single-threaded
systems may safely ignore this spec.  Who would specify 'threadsafe'
if they were not worried about starvation?  How could an
implementation that allows starvation be considered correct?

I think Haskell compilers that cannot implement 'threadsafe' as it is
implemented in GHC should reject Haskell programs which use it.  I
think the ffi spec should be amended to say this (the spec currently
says that such compilers may treat 'threadsafe' as meaning 'safe').

I suspect that 'threadsafety' is actually orthogonal to the
'safe/unsafe' distinction.  I think it would be perfectly meaningful
to make calls which must execute in a separate thread (i.e., are
'threadsafe') but do not make callbacks into Haskell (i.e., could
otherwise be 'unsafe').  We currently ignore this case because the
overhead of a context switch is so much greater than the overhead of a
safe call.

Alastair Reid                 alastair at reid-consulting-uk.ltd.uk  
Reid Consulting (UK) Limited  http://www.reid-consulting-uk.ltd.uk/alastair/

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