Haskell FFI double pointer argument

Question

I would like to wrap a set of C functions with the following sig.:

ErrorCode Initialize(int *argc,char ***args, ...)

How is the double pointer represented in the FFI call? It is a pointer to a list of strings; is the following plausible?

foreign import ccall unsafe "lib.h Initialize"
    c_initialize :: Ptr CInt 
                 -> Ptr [String]
                 -> IO (Ptr CInt)

Or is the second argument a Ptr (Ptr Char)? I can't find this case in the literature I've read so far (Real World Haskell and the Wikibook) and my C is a bit rusty. Thanks in advance

-- Correction : RWH actually shows the interface to pcre_compile():

-- PCRE-compile.hs
foreign import ccall unsafe "pcre.h pcre_compile"
  c_pcre_compile :: CString
                 -> PCREOption
                 -> Ptr CString
                 -> Ptr CInt
                 -> Ptr Word8
                 -> IO (Ptr PCRE)

Which corresponds to:

-- pcre.h
pcre *pcre_compile(const char *pattern,
               int options,
               const char **errptr,
               int *errofset,
               const unsigned char *tableptr);

Maybe it's a Ptr (Ptr (Ptr CChar)) since we have three * in front of args (and CChar is not Char). — chi
– chi, Commented Mar 23, 2015 at 0:17
I think the translation from C types to Haskell FFI types is pretty straightforward. char*** is Ptr (Ptr (Ptr CChar)). As far as I know, there is no way to produce or consume a Ptr [String] (other than castPtr) because [String] is not Storable. So having your function take a Ptr [String] would be pretty silly - you can never actually call it from Haskell, unless you have some other FFI function specifically for creating a Ptr [String] for use with c_initialize. — user2407038
– user2407038, Commented Mar 23, 2015 at 4:46

ocramz · Accepted Answer · 2015-03-23 14:47:32Z

2

So, taking all these suggestions into account, it seems like the most fitting signature is something like

foreign import ccall unsafe "lib.h Initialize"
c_initialize :: Ptr CInt 
             -> Ptr (Ptr CString)
             -> IO (Ptr CInt)

Thank you all! will keep you posted

answered Mar 23, 2015 at 14:47

ocramz

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