expose strlen from Foreign.C.String

[Carter Schonwald]

andrew! this is a really good point. would the with# or touch# combinators
be needed to fix it (to force gc liveness?)? OR would we need to have the
foreign c call  defined to have an -> IO result, then use unsafePerformIO
to "purefy it correctly"?

i think the best way to explain *why* the proposed definition runs into
trouble is to look at how we annotate delicate/complicated prims in primops
are annotated
https://gitlab.haskell.org/ghc/ghc/-/blob/4bb9a349b5d002463b9fc4e9a3b6dbf77ea7c085/compiler/GHC/Builtin/primops.txt.pp#L139-178

otoh, the last time i was playing with an ostensibly pure primop that had
really delicate effect ordering, the prefetch stuff in the NCG,
my conclusion was that it *needed* explicit state tokens to make sure it
didn't get reordered,  and for this primop that pure version would need to
be via unsafeperformio i think

On Fri, Jan 22, 2021 at 8:46 AM Andrew Martin <andrew.thaddeus at gmail.com>
wrote:

> This is unsound:
>
> x <- malloc ...
> memcpy ... copy a nul-terminated string into x
> let len = cstringLength x
> free x
>
> Because GHC can float the let binding down to where it is used after free.
>
> Sent from my iPhone
>
> On Jan 21, 2021, at 7:45 PM, Zemyla <zemyla at gmail.com> wrote:
>
> 
> That doesn't sound right. I don't think it allocates any data on the heap
> which could cause reallocation and move an unpinned ByteArray#, which is
> the only way I can think it would be unsafe.
>
> On Thu, Jan 21, 2021, 17:50 Andrew Martin <andrew.thaddeus at gmail.com>
> wrote:
>
>> Both the unboxed variant and the wrapper are only sound on primitive
>> string literals. You cannot use them on anything that was allocated at
>> runtime, only on stuff baked into the rodata section. This is a pretty
>> onerous restriction. What use case did you have in mind?
>>
>> Sent from my iPhone
>>
>> > On Jan 20, 2021, at 11:02 PM, Viktor Dukhovni <ietf-dane at dukhovni.org>
>> wrote:
>> >
>> > 
>> >>
>> >>> On Jan 21, 2021, at 1:39 AM, chessai <chessai1996 at gmail.com> wrote:
>> >>> On Wed, Jan 20, 2021, 17:01 Viktor Dukhovni <ietf-dane at dukhovni.org>
>> wrote:
>> >>>> On Wed, Jan 20, 2021 at 09:54:30AM -0800, chessai wrote:
>> >>>>
>> >>>> I've wanted the following before:
>> >>>>
>> >>>> foreign import ccall unsafe "strlen"
>> >>>>   cstringLength# :: Addr# -> Int#
>> >>>>
>> >>>> cstringLength :: CString -> Int
>> >>>> cstringLength (Ptr s) = I# (cstringLength# s)
>> >>>>
>> >>>> A natural place for this seems to be Foreign.C.String.
>> >>>
>> >>> Why a new FFI call, rather than `cstringLength#` from ghc-prim:
>> GHC.CString
>> >>> (as of GHC 9.0.1):
>> >>
>> >> I forgot about that addition. In that case we would just need the
>> lifted wrapper
>> >
>> > No worries, sure the lifted wrapper makes sense, and Foreign.C.String
>> does
>> > look like a reasonable place in which to define, and from which to
>> export it.
>> >
>> > --
>> >    Viktor.
>> >
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