FFI Query: How to free a CString allocated in Haskell
Hi, I am calling a Haskell function from my C code, wherein I export a Haskell function xyz that takes as an argument string and returns a string: Foreign export ccall xyz::CString -> IO CString xyz = do -- extract the input string and act on it-- -- at the end I return a string like this newCString str Now once I call this function from C code, I am freeing the allocated memory using free function. I want to confirm that this is the right thing to do. - Anurag
Hello Verma, Monday, April 28, 2008, 4:11:51 PM, you wrote:
newCString str
Now once I call this function from C code, I am freeing the allocated memory using free function. I want to confirm that this is the right thing to do.
yes, i've traced this function down to mallocArray -> mallocBytes -> malloc() calls -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com
On 28/04/2008, at 7:23 PM, Bulat Ziganshin wrote:
Hello Verma,
Monday, April 28, 2008, 4:11:51 PM, you wrote:
newCString str
Now once I call this function from C code, I am freeing the allocated memory using free function. I want to confirm that this is the right thing to do.
yes, i've traced this function down to mallocArray -> mallocBytes -> malloc() calls
Depending on how you feel about it, Bulat's response either entirely answers your question or is neither here nor there. If you want to write portable code, you should check with the FFI spec. If you just want to write code that works with GHC as it presently is, you can strace it or whatever. As Bulat has already given you the pragmatic answer, allow me to grind my axe a little. In the FFI spec on p33, we find:
newCString :: String -> IO CString newCStringLen :: String -> IO CStringLen
Allocate a memory area for a Haskell string and marshal the string into its C representation. There are two variants of the routine, one for each supported string representation. The memory area allocated by these routines may be deallocated using MarshalAlloc.free.
OK, so back on p19 we find:
free :: Ptr a -> IO ()
Free a block of memory that was allocated with malloc, mallocBytes, realloc, reallocBytes, or any of the allocation functions from MarshalArray (see Section 5.9).
and we can draw the inference (reading between the lines) that this function need not have a relation to C's malloc. (Roughly, the intention is to allow a Haskell implementation to use whatever memory management it likes.) So no, using C's free is not the right thing to do. It will probably work in the real world, but you should try to use Haskell's free if possible. cheers peter
Peter, A naïve question I have now after reading your mail. How do I call MarshallAlloc.free from my C code because that's where I need to free it? Anurag
So no, using C's free is not the right thing to do. It will probably work in the real world, but you should try to use Haskell's free if possible.
cheers peter
On Apr 29, 2008, at 1:45 , Verma Anurag-VNF673 wrote:
A naïve question I have now after reading your mail. How do I call MarshallAlloc.free from my C code because that's where I need to free it?
Provide a Haskell wrapper function to deallocate / free it, as a parallel to the function that allocates and returns it to the C code. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
On 29/04/2008, at 12:58 PM, Brandon S. Allbery KF8NH wrote:
On Apr 29, 2008, at 1:45 , Verma Anurag-VNF673 wrote:
A naïve question I have now after reading your mail. How do I call MarshallAlloc.free from my C code because that's where I need to free it?
Provide a Haskell wrapper function to deallocate / free it, as a parallel to the function that allocates and returns it to the C code.
I would also point out the existence of withCString and friends. Tread carefully if you care about Unicode! cheers peter
participants (4)
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Brandon S. Allbery KF8NH -
Bulat Ziganshin -
Peter Gammie -
Verma Anurag-VNF673