Haskell FFI how to accept local value address

Question

I am trying to work with this simple C code from Haskell:

time_t rawTime = time( NULL );
struct tm* timeInfo = localtime( &rawTime );  /* Using address of a local variable */
printf( "The current date is: %s", asctime( timeInfo ) );

So I write:

data CTmStruct = CTmStruct
type CTmStructPtr = Ptr CTmStruct

foreign import ccall "time"      c_time      :: Ptr CTime    -> IO CTime
foreign import ccall "localtime" c_localtime :: Ptr CTime    -> IO CTmStructPtr
foreign import ccall "asctime"   c_asctime   :: CTmStructPtr -> CString

main :: IO ()
main = do
    rawTime <- c_time nullPtr
    tmStructPtr <- c_localtime ???  -- How to get an address of rawTime?
    print $ c_asctime tmStructPtr

So, how can I get the address of a local value rawTimeto place it as a function argument c_localtime? I read the documentation for the module Foreign, but I did not find the answer to this question. I would really appreciate any help.

Answer 1

I could:

import Foreign.C.String
import Foreign.Marshal.Alloc

-- Your declarations and foreign bindings here

getTime :: IO String
getTime = do
    p <- malloc
    c_time p
    tmPtr <- c_localtime p
    str <- peekCString $ c_asctime tmPtr
    free p
    return str

main :: IO ()
main = do
    currentTime <- getTime
    putStrLn currentTime

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---

, , alloca:

getTime :: IO String
getTime = alloca $ \p -> do
    c_time p
    tm <- c_localtime p
    peekCString $ c_astime tm

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