11. Foreign function interface (FFI)

ForeignFunctionInterface

Since:6.8.1

Allow use of the Haskell foreign function interface.

GHC (mostly) conforms to the Haskell Foreign Function Interface as specified in the Haskell Report. Refer to the relevant chapter of the Haskell Report for more details.

FFI support is enabled by default, but can be enabled or disabled explicitly with the ForeignFunctionInterface flag.

GHC implements a number of GHC-specific extensions to the FFI Chapter of the Haskell 2010 Report. These extensions are described in GHC extensions to the FFI Chapter, but please note that programs using these features are not portable. Hence, these features should be avoided where possible.

The FFI libraries are documented in the accompanying library documentation; see for example the Foreign module.

11.1. GHC differences to the FFI Chapter

11.1.1. Guaranteed call safety

The Haskell 2010 Report specifies that safe FFI calls must allow foreign calls to safely call into Haskell code. In practice, this means that the garbage collector must be able to run while these calls are in progress, moving heap-allocated Haskell values around arbitrarily.

This greatly constrains library authors since it implies that it is not safe to pass any heap object reference to a safe foreign function call. For instance, it is often desirable to pass an unpinned ByteArray#s directly to native code to avoid making an otherwise-unnecessary copy. However, this can only be done safely if the array is guaranteed not to be moved by the garbage collector in the middle of the call.

The Chapter does not require implementations to refrain from doing the same for unsafe calls, so strictly Haskell 2010-conforming programs cannot pass heap-allocated references to unsafe FFI calls either.

In previous releases, GHC would take advantage of the freedom afforded by the Chapter by performing safe foreign calls in place of unsafe calls in the bytecode interpreter. This meant that some packages which worked when compiled would fail under GHCi (e.g. #13730).

However, since version 8.4 this is no longer the case: GHC guarantees that garbage collection will never occur during an unsafe call, even in the bytecode interpreter, and further guarantees that unsafe calls will be performed in the calling thread.

11.2. GHC extensions to the FFI Chapter

The FFI features that are described in this section are specific to GHC. Your code will not be portable to other compilers if you use them.

11.2.1. Unlifted FFI Types

UnliftedFFITypes

Since:6.8.1

The following unlifted unboxed types may be used as basic foreign types (see FFI Chapter, Section 8.6) for both safe and unsafe foreign calls: Int#, Word#, Char#, Float#, Double#, Addr#, and StablePtr# a. Several unlifted boxed types may be used as arguments to FFI calls, subject to these restrictions:

• Valid arguments for foreign import unsafe FFI calls: Array#, SmallArray#, ArrayArray#, ByteArray#, and the mutable counterparts of these types.
• Valid arguments for foreign import safe FFI calls: ByteArray# and MutableByteArray#. The byte array must be pinned.
• Mutation: In both foreign import unsafe and foreign import safe FFI calls, it is safe to mutate a MutableByteArray. Mutating any other type of array leads to undefined behavior. Reason: Mutable arrays of heap objects record writes for the purpose of garbage collection. An array of heap objects is passed to a foreign C function, the runtime does not record any writes. Consequently, it is not safe to write to an array of heap objects in a foreign function. Since the runtime has no facilities for tracking mutation of a MutableByteArray#, these can be safely mutated in any foreign function.

None of these restrictions are enforced at compile time. Failure to heed these restrictions will lead to runtime errors that can be very difficult to track down. (The errors likely will not manifest until garbage collection happens.) In tabular form, these restrictions are:

Restrictions on unlifted boxed arguments passed to foreign C calls. Cells marked as “Unsound” represent combinations that lead to undefined runtime behavior. GHC does not reject such unsound programs at compile time.

	When value is used as argument to FFI call that is
	foreign import safe		foreign import unsafe
Argument Type	reads are	writes are	reads are	writes are
Array# MutableArray# SmallArray# MutableSmallArray# ArrayArray# MutableArrayArray# unpinned ByteArray# unpinned MutableByteArray# pinned ByteArray# pinned MutableByteArray#	Unsound Unsound Unsound Unsound Unsound Unsound Unsound Unsound Sound Sound	Unsound Unsound Unsound Unsound Unsound Unsound Unsound Unsound Unsound Sound	Sound Sound Sound Sound Sound Sound Sound Sound Sound Sound	Unsound Unsound Unsound Unsound Unsound Unsound Unsound Sound Unsound Sound

When passing any of the unlifted array types as an argument to a foreign C call, a foreign function sees a pointer that refers to the payload of the array, not to the StgArrBytes/StgMutArrPtrs/StgSmallMutArrPtrs heap object containing it [1]. By contrast, a foreign Cmm call, introduced by foreign import prim, sees the heap object, not just the payload. This means that, in some situations, the foreign C function might not need any knowledge of the RTS closure types. The following example sums the first three bytes in a MutableByteArray# [2] without using anything from Rts.h:

// C source
uint8_t add_triplet(uint8_t* arr) {
  return (arr[0] + arr[1] + arr[2]);
}

-- Haskell source
foreign import ccall unsafe "add_triplet"
  addTriplet :: MutableByteArray# RealWorld -> IO Word8

In other situations, the C function may need knowledge of the RTS closure types. The following example sums the first element of each ByteArray# (interpreting the bytes as an array of CInt) element of an ArrayArray## [3]:

// C source, must include the RTS to make the struct StgArrBytes
// available along with its fields: ptrs and payload.
#include "Rts.h"
int sum_first (StgArrBytes **bufs) {
  StgArrBytes **bufs = (StgArrBytes**)bufsTmp;
  int res = 0;
  for(StgWord ix = 0;ix < arr->ptrs;ix++) {
    res = res + ((int*)(bufs[ix]->payload))[0];
  }
  return res;
}

-- Haskell source, all elements in the argument array must be
-- either ByteArray# or MutableByteArray#. This is not enforced
-- by the type system in this example since ArrayArray is untyped.
foreign import ccall unsafe "sum_first"
  sumFirst :: ArrayArray# -> IO CInt

Although GHC allows the user to pass all unlifted boxed types to foreign functions, some of them are not amenable to useful work. Although Array# is unlifted, the elements in its payload are lifted, and a foreign C function cannot safely force thunks. Consequently, a foreign C function may not dereference any of the addresses that comprise the payload of the Array#.

11.2.2. Newtype wrapping of the IO monad

The FFI spec requires the IO monad to appear in various places, but it can sometimes be convenient to wrap the IO monad in a newtype, thus:

newtype MyIO a = MIO (IO a)

(A reason for doing so might be to prevent the programmer from calling arbitrary IO procedures in some part of the program.)

The Haskell FFI already specifies that arguments and results of foreign imports and exports will be automatically unwrapped if they are newtypes (Section 3.2 of the FFI addendum). GHC extends the FFI by automatically unwrapping any newtypes that wrap the IO monad itself. More precisely, wherever the FFI specification requires an IO type, GHC will accept any newtype-wrapping of an IO type. For example, these declarations are OK:

foreign import foo :: Int -> MyIO Int
foreign import "dynamic" baz :: (Int -> MyIO Int) -> CInt -> MyIO Int

11.2.3. Explicit “forall”s in foreign types

The type variables in the type of a foreign declaration may be quantified with an explicit forall by using the ExplicitForAll language extension, as in the following example:

{-# LANGUAGE ExplicitForAll #-}
foreign import ccall "mmap" c_mmap :: forall a. CSize -> IO (Ptr a)

Note that an explicit forall must appear at the front of the type signature and is not permitted to appear nested within the type, as in the following (erroneous) examples:

foreign import ccall "mmap" c_mmap' :: CSize -> forall a. IO (Ptr a)
foreign import ccall quux :: (forall a. Ptr a) -> IO ()

11.2.4. Primitive imports

GHC extends the FFI with an additional calling convention prim, e.g.:

foreign import prim "foo" foo :: ByteArray# -> (# Int#, Int# #)

This is used to import functions written in Cmm code that follow an internal GHC calling convention. The arguments and results must be unboxed types, except that an argument may be of type Any (by way of unsafeCoerce#) and the result type is allowed to be an unboxed tuple or the type Any.

This feature is not intended for use outside of the core libraries that come with GHC. For more details see the GHC developer wiki.

11.2.5. Interruptible foreign calls

InterruptibleFFI

Since:7.2.1

This concerns the interaction of foreign calls with Control.Concurrent.throwTo. Normally when the target of a throwTo is involved in a foreign call, the exception is not raised until the call returns, and in the meantime the caller is blocked. This can result in unresponsiveness, which is particularly undesirable in the case of user interrupt (e.g. Control-C). The default behaviour when a Control-C signal is received (SIGINT on Unix) is to raise the UserInterrupt exception in the main thread; if the main thread is blocked in a foreign call at the time, then the program will not respond to the user interrupt.

The problem is that it is not possible in general to interrupt a foreign call safely. However, GHC does provide a way to interrupt blocking system calls which works for most system calls on both Unix and Windows. When the InterruptibleFFI extension is enabled, a foreign call can be annotated with interruptible instead of safe or unsafe:

foreign import ccall interruptible
   "sleep" sleepBlock :: CUint -> IO CUint

interruptible behaves exactly as safe, except that when a throwTo is directed at a thread in an interruptible foreign call, an OS-specific mechanism will be used to attempt to cause the foreign call to return:

Unix systems

The thread making the foreign call is sent a SIGPIPE signal using pthread_kill(). This is usually enough to cause a blocking system call to return with EINTR (GHC by default installs an empty signal handler for SIGPIPE, to override the default behaviour which is to terminate the process immediately).

Windows systems

[Vista and later only] The RTS calls the Win32 function CancelSynchronousIo, which will cause a blocking I/O operation to return with the error ERROR_OPERATION_ABORTED.

If the system call is successfully interrupted, it will return to Haskell whereupon the exception can be raised. Be especially careful when using interruptible that the caller of the foreign function is prepared to deal with the consequences of the call being interrupted; on Unix it is good practice to check for EINTR always, but on Windows it is not typically necessary to handle ERROR_OPERATION_ABORTED.

11.2.6. The CAPI calling convention

CApiFFI

Since:7.10.1

The CApiFFI extension allows a calling convention of capi to be used in foreign declarations, e.g.

foreign import capi "header.h f" f :: CInt -> IO CInt

Rather than generating code to call f according to the platform’s ABI, we instead call f using the C API defined in the header header.h. Thus f can be called even if it may be defined as a CPP #define rather than a proper function.

When using capi, it is also possible to import values, rather than functions. For example,

foreign import capi "pi.h value pi" c_pi :: CDouble

will work regardless of whether pi is defined as

const double pi = 3.14;

or with

#define pi 3.14

In order to tell GHC the C type that a Haskell type corresponds to when it is used with the CAPI, a CTYPE pragma can be used on the type definition. The header which defines the type can optionally also be specified. The syntax looks like:

data    {-# CTYPE "unistd.h" "useconds_t" #-} T = ...
newtype {-# CTYPE            "useconds_t" #-} T = ...

11.2.7. hs_thread_done()

void hs_thread_done(void);

GHC allocates a small amount of thread-local memory when a thread calls a Haskell function via a foreign export. This memory is not normally freed until hs_exit(); the memory is cached so that subsequent calls into Haskell are fast. However, if your application is long-running and repeatedly creates new threads that call into Haskell, you probably want to arrange that this memory is freed in those threads that have finished calling Haskell functions. To do this, call hs_thread_done() from the thread whose memory you want to free.

Calling hs_thread_done() is entirely optional. You can call it as often or as little as you like. It is safe to call it from a thread that has never called any Haskell functions, or one that never will. If you forget to call it, the worst that can happen is that some memory remains allocated until hs_exit() is called. If you call it too often, the worst that can happen is that the next call to a Haskell function incurs some extra overhead.

11.2.8. Freeing many stable pointers efficiently

The standard function hs_free_stable_ptr locks the stable pointer table, frees the given stable pointer, and then unlocks the stable pointer table again. When freeing many stable pointers at once, it is usually more efficient to lock and unlock the table only once.

extern void hs_lock_stable_ptr_table (void);

extern void hs_unlock_stable_ptr_table (void);

extern void hs_free_stable_ptr_unsafe (HsStablePtr sp);

hs_free_stable_ptr_unsafe must be used only when the table has been locked using hs_lock_stable_ptr_table. It must be unlocked afterwards using hs_unlock_stable_ptr_table. The Haskell garbage collector cannot run while the table is locked, so it should be unlocked promptly. The following operations are forbidden while the stable pointer table is locked:

• Calling any Haskell function, whether or not that function manipulates stable pointers.
• Calling any FFI function that deals with the stable pointer table except for arbitrarily many calls to hs_free_stable_ptr_unsafe and the final call to hs_unlock_stable_ptr_table.
• Calling hs_free_fun_ptr.

Note

GHC versions before 8.8 defined undocumented functions hs_lock_stable_tables and hs_unlock_stable_tables instead of hs_lock_stable_ptr_table and hs_unlock_stable_ptr_table. Those names are now deprecated.

11.3. Using the FFI with GHC

The following sections also give some hints and tips on the use of the foreign function interface in GHC.

11.3.1. Using foreign export and foreign import ccall "wrapper" with GHC

When GHC compiles a module (say M.hs) which uses foreign export or foreign import "wrapper", it generates a M_stub.h for use by C programs.

For a plain foreign export, the file M_stub.h contains a C prototype for the foreign exported function. For example, if we compile the following module:

module Foo where

foreign export ccall foo :: Int -> IO Int

foo :: Int -> IO Int
foo n = return (length (f n))

f :: Int -> [Int]
f 0 = []
f n = n:(f (n-1))

Then Foo_stub.h will contain something like this:

#include "HsFFI.h"
extern HsInt foo(HsInt a0);

To invoke foo() from C, just #include "Foo_stub.h" and call foo().

The Foo_stub.h file can be redirected using the -stubdir option; see Redirecting the compilation output(s).

11.3.1.1. Using your own main()

Normally, GHC’s runtime system provides a main(), which arranges to invoke Main.main in the Haskell program. However, you might want to link some Haskell code into a program which has a main function written in another language, say C. In order to do this, you have to initialize the Haskell runtime system explicitly.

Let’s take the example from above, and invoke it from a standalone C program. Here’s the C code:

#include <stdio.h>
#include "HsFFI.h"

#if defined(__GLASGOW_HASKELL__)
#include "Foo_stub.h"
#endif

int main(int argc, char *argv[])
{
  int i;

  hs_init(&argc, &argv);

  for (i = 0; i < 5; i++) {
    printf("%d\n", foo(2500));
  }

  hs_exit();
  return 0;
}

We’ve surrounded the GHC-specific bits with #if defined(__GLASGOW_HASKELL__); the rest of the code should be portable across Haskell implementations that support the FFI standard.

The call to hs_init() initializes GHC’s runtime system. Do NOT try to invoke any Haskell functions before calling hs_init(): bad things will undoubtedly happen.

We pass references to argc and argv to hs_init() so that it can separate out any arguments for the RTS (i.e. those arguments between +RTS...-RTS).

After we’ve finished invoking our Haskell functions, we can call hs_exit(), which terminates the RTS.

There can be multiple calls to hs_init(), but each one should be matched by one (and only one) call to hs_exit(). The outermost hs_exit() will actually de-initialise the system. Note that currently GHC’s runtime cannot reliably re-initialise after this has happened; see The Foreign Function Interface.

Note

When linking the final program, it is normally easiest to do the link using GHC, although this isn’t essential. If you do use GHC, then don’t forget the flag -no-hs-main, otherwise GHC will try to link to the Main Haskell module.

Note

On Windows hs_init treats argv as UTF8-encoded. Passing other encodings might lead to unexpected results. Passing NULL as argv is valid but can lead to <unknown> showing up in error messages instead of the name of the executable.

To use +RTS flags with hs_init(), we have to modify the example slightly. By default, GHC’s RTS will only accept “safe” +RTS flags (see Options affecting linking), and the -rtsopts[=⟨none|some|all|ignore|ignoreAll⟩] link-time flag overrides this. However, -rtsopts[=⟨none|some|all|ignore|ignoreAll⟩] has no effect when -no-hs-main is in use (and the same goes for -with-rtsopts=⟨opts⟩). To set these options we have to call a GHC-specific API instead of hs_init():

#include <stdio.h>
#include "HsFFI.h"

#if defined(__GLASGOW_HASKELL__)
#include "Foo_stub.h"
#include "Rts.h"
#endif

int main(int argc, char *argv[])
{
  int i;

#if __GLASGOW_HASKELL__ >= 703
  {
      RtsConfig conf = defaultRtsConfig;
      conf.rts_opts_enabled = RtsOptsAll;
      hs_init_ghc(&argc, &argv, conf);
  }
#else
  hs_init(&argc, &argv);
#endif

  for (i = 0; i < 5; i++) {
    printf("%d\n", foo(2500));
  }

  hs_exit();
  return 0;
}

Note two changes: we included Rts.h, which defines the GHC-specific external RTS interface, and we called hs_init_ghc() instead of hs_init(), passing an argument of type RtsConfig. RtsConfig is a struct with various fields that affect the behaviour of the runtime system. Its definition is:

typedef struct {
    RtsOptsEnabledEnum rts_opts_enabled;
    const char *rts_opts;
} RtsConfig;

extern const RtsConfig defaultRtsConfig;

typedef enum {
    RtsOptsNone,         // +RTS causes an error
    RtsOptsSafeOnly,     // safe RTS options allowed; others cause an error
    RtsOptsAll           // all RTS options allowed
  } RtsOptsEnabledEnum;

There is a default value defaultRtsConfig that should be used to initialise variables of type RtsConfig. More fields will undoubtedly be added to RtsConfig in the future, so in order to keep your code forwards-compatible it is best to initialise with defaultRtsConfig and then modify the required fields, as in the code sample above.

11.3.1.2. Making a Haskell library that can be called from foreign code

The scenario here is much like in Using your own main(), except that the aim is not to link a complete program, but to make a library from Haskell code that can be deployed in the same way that you would deploy a library of C code.

The main requirement here is that the runtime needs to be initialized before any Haskell code can be called, so your library should provide initialisation and deinitialisation entry points, implemented in C or C++. For example:

#include <stdlib.h>
#include "HsFFI.h"

HsBool mylib_init(void){
  int argc = 2;
  char *argv[] = { "+RTS", "-A32m", NULL };
  char **pargv = argv;

  // Initialize Haskell runtime
  hs_init(&argc, &pargv);

  // do any other initialization here and
  // return false if there was a problem
  return HS_BOOL_TRUE;
}

void mylib_end(void){
  hs_exit();
}

The initialisation routine, mylib_init, calls hs_init() as normal to initialise the Haskell runtime, and the corresponding deinitialisation function mylib_end() calls hs_exit() to shut down the runtime.

11.3.2. Using header files

C functions are normally declared using prototypes in a C header file. Earlier versions of GHC (6.8.3 and earlier) #included the header file in the C source file generated from the Haskell code, and the C compiler could therefore check that the C function being called via the FFI was being called at the right type.

GHC no longer includes external header files when compiling via C, so this checking is not performed. The change was made for compatibility with the native code generator (-fasm) and to comply strictly with the FFI specification, which requires that FFI calls are not subject to macro expansion and other CPP conversions that may be applied when using C header files. This approach also simplifies the inlining of foreign calls across module and package boundaries: there’s no need for the header file to be available when compiling an inlined version of a foreign call, so the compiler is free to inline foreign calls in any context.

The -#include option is now deprecated, and the include-files field in a Cabal package specification is ignored.

11.3.3. Memory Allocation

The FFI libraries provide several ways to allocate memory for use with the FFI, and it isn’t always clear which way is the best. This decision may be affected by how efficient a particular kind of allocation is on a given compiler/platform, so this section aims to shed some light on how the different kinds of allocation perform with GHC.

alloca

Useful for short-term allocation when the allocation is intended to scope over a given IO computation. This kind of allocation is commonly used when marshalling data to and from FFI functions.

In GHC, alloca is implemented using MutableByteArray#, so allocation and deallocation are fast: much faster than C’s malloc/free, but not quite as fast as stack allocation in C. Use alloca whenever you can.

mallocForeignPtr

Useful for longer-term allocation which requires garbage collection. If you intend to store the pointer to the memory in a foreign data structure, then mallocForeignPtr is not a good choice, however.

In GHC, mallocForeignPtr is also implemented using MutableByteArray#. Although the memory is pointed to by a ForeignPtr, there are no actual finalizers involved (unless you add one with addForeignPtrFinalizer), and the deallocation is done using GC, so mallocForeignPtr is normally very cheap.

malloc/free

If all else fails, then you need to resort to Foreign.malloc and Foreign.free. These are just wrappers around the C functions of the same name, and their efficiency will depend ultimately on the implementations of these functions in your platform’s C library. We usually find malloc and free to be significantly slower than the other forms of allocation above.

Foreign.Marshal.Pool

Pools are currently implemented using malloc/free, so while they might be a more convenient way to structure your memory allocation than using one of the other forms of allocation, they won’t be any more efficient. We do plan to provide an improved-performance implementation of Pools in the future, however.

11.3.4. Multi-threading and the FFI

In order to use the FFI in a multi-threaded setting, you must use the -threaded option (see Options affecting linking).

11.3.4.1. Foreign imports and multi-threading

When you call a foreign imported function that is annotated as safe (the default), and the program was linked using -threaded, then the call will run concurrently with other running Haskell threads. If the program was linked without -threaded, then the other Haskell threads will be blocked until the call returns.

This means that if you need to make a foreign call to a function that takes a long time or blocks indefinitely, then you should mark it safe and use -threaded. Some library functions make such calls internally; their documentation should indicate when this is the case.

If you are making foreign calls from multiple Haskell threads and using -threaded, make sure that the foreign code you are calling is thread-safe. In particularly, some GUI libraries are not thread-safe and require that the caller only invokes GUI methods from a single thread. If this is the case, you may need to restrict your GUI operations to a single Haskell thread, and possibly also use a bound thread (see The relationship between Haskell threads and OS threads).

Note that foreign calls made by different Haskell threads may execute in parallel, even when the +RTS -N flag is not being used (RTS options for SMP parallelism). The -N ⟨x⟩ flag controls parallel execution of Haskell threads, but there may be an arbitrary number of foreign calls in progress at any one time, regardless of the +RTS -N value.

If a call is annotated as interruptible and the program was multithreaded, the call may be interrupted in the event that the Haskell thread receives an exception. The mechanism by which the interrupt occurs is platform dependent, but is intended to cause blocking system calls to return immediately with an interrupted error code. The underlying operating system thread is not to be destroyed. See Interruptible foreign calls for more details.

11.3.4.2. The relationship between Haskell threads and OS threads

Normally there is no fixed relationship between Haskell threads and OS threads. This means that when you make a foreign call, that call may take place in an unspecified OS thread. Furthermore, there is no guarantee that multiple calls made by one Haskell thread will be made by the same OS thread.

This usually isn’t a problem, and it allows the GHC runtime system to make efficient use of OS thread resources. However, there are cases where it is useful to have more control over which OS thread is used, for example when calling foreign code that makes use of thread-local state. For cases like this, we provide bound threads, which are Haskell threads tied to a particular OS thread. For information on bound threads, see the documentation for the Control.Concurrent module.

11.3.4.3. Foreign exports and multi-threading

When the program is linked with -threaded, then you may invoke foreign exported functions from multiple OS threads concurrently. The runtime system must be initialised as usual by calling hs_init(), and this call must complete before invoking any foreign exported functions.

11.3.4.4. On the use of hs_exit()

hs_exit() normally causes the termination of any running Haskell threads in the system, and when hs_exit() returns, there will be no more Haskell threads running. The runtime will then shut down the system in an orderly way, generating profiling output and statistics if necessary, and freeing all the memory it owns.

It isn’t always possible to terminate a Haskell thread forcibly: for example, the thread might be currently executing a foreign call, and we have no way to force the foreign call to complete. What’s more, the runtime must assume that in the worst case the Haskell code and runtime are about to be removed from memory (e.g. if this is a Windows DLL, hs_exit() is normally called before unloading the DLL). So hs_exit() must wait until all outstanding foreign calls return before it can return itself.

The upshot of this is that if you have Haskell threads that are blocked in foreign calls, then hs_exit() may hang (or possibly busy-wait) until the calls return. Therefore it’s a good idea to make sure you don’t have any such threads in the system when calling hs_exit(). This includes any threads doing I/O, because I/O may (or may not, depending on the type of I/O and the platform) be implemented using blocking foreign calls.

The GHC runtime treats program exit as a special case, to avoid the need to wait for blocked threads when a standalone executable exits. Since the program and all its threads are about to terminate at the same time that the code is removed from memory, it isn’t necessary to ensure that the threads have exited first. If you want this fast and loose version of hs_exit(), you can call:

void hs_exit_nowait(void);

instead. This is particularly useful if you have foreign libraries that need to call hs_exit() at program exit (perhaps via a C++ destructor): in this case you should use hs_exit_nowait(), because the thread that called exit() and is running C++ destructors is in a foreign call from Haskell that will never return, so hs_exit() would deadlock.

11.3.4.5. Waking up Haskell threads from C

Sometimes we want to be able to wake up a Haskell thread from some C code. For example, when using a callback-based C API, we register a C callback and then we need to wait for the callback to run.

One way to do this is to create a foreign export that will do whatever needs to be done to wake up the Haskell thread - perhaps putMVar - and then call this from our C callback. There are a couple of problems with this:

1. Calling a foreign export has a lot of overhead: it creates a complete new Haskell thread, for example.
2. The call may block for a long time if a GC is in progress. We can’t use this method if the C API we’re calling doesn’t allow blocking in the callback.

For these reasons GHC provides an external API to tryPutMVar, hs_try_putmvar, which you can use to cheaply and asynchronously wake up a Haskell thread from C/C++.

void hs_try_putmvar (int capability, HsStablePtr sp);

The C call hs_try_putmvar(cap, mvar) is equivalent to the Haskell call tryPutMVar mvar (), except that it is

• non-blocking: takes a bounded, short, amount of time
• asynchronous: the actual putMVar may be performed after the call returns (for example, if the RTS is currently garbage collecting). That’s why hs_try_putmvar() doesn’t return a result to say whether the put succeeded. It is your responsibility to ensure that the MVar is empty; if it is full, hs_try_putmvar() will have no effect.

Example. Suppose we have a C/C++ function to call that will return and then invoke a callback at some point in the future, passing us some data. We want to wait in Haskell for the callback to be called, and retrieve the data. We can do it like this:

import GHC.Conc (newStablePtrPrimMVar, PrimMVar)

makeExternalCall = mask_ $ do
  mvar <- newEmptyMVar
  sp <- newStablePtrPrimMVar mvar
  fp <- mallocForeignPtr
  withForeignPtr fp $ \presult -> do
    cap <- threadCapability =<< myThreadId
    scheduleCallback sp cap presult
    takeMVar mvar `onException`
      forkIO (do takeMVar mvar; touchForeignPtr fp)
    peek presult

foreign import ccall "scheduleCallback"
    scheduleCallback :: StablePtr PrimMVar
                     -> Int
                     -> Ptr Result
                     -> IO ()

And inside scheduleCallback, we create a callback that will in due course store the result data in the Ptr Result, and then call hs_try_putmvar().

There are a few things to note here.

• There’s a special function to create the StablePtr: newStablePtrPrimMVar, because the RTS needs a StablePtr to the primitive MVar# object, and we can’t create that directly. Do not just use newStablePtr on the MVar: your program will crash.

• The StablePtr is freed by hs_try_putmvar(). This is because it would otherwise be difficult to arrange to free the StablePtr reliably: we can’t free it in Haskell, because if the takeMVar is interrupted by an asynchronous exception, then the callback will fire at a later time. We can’t free it in C, because we don’t know when to free it (not when hs_try_putmvar() returns, because that is an async call that uses the StablePtr at some time in the future).

• The mask_ is to avoid asynchronous exceptions before the scheduleCallback call, which would leak the StablePtr.

• We find out the current capability number and pass it to C. This is passed back to hs_try_putmvar, and helps the RTS to know which capability it should try to perform the tryPutMVar on. If you don’t care, you can pass -1 for the capability to hs_try_putmvar, and it will pick an arbitrary one.

  Picking the right capability will help avoid unnecessary context switches. Ideally you should pass the capability that the thread that will be woken up last ran on, which you can find by calling threadCapability in Haskell.

• If you want to also pass some data back from the C callback to Haskell, this is best done by first allocating some memory in Haskell to receive the data, and passing the address to C, as we did in the above example.

• takeMVar can be interrupted by an asynchronous exception. If this happens, the callback in C will still run at some point in the future, will still write the result, and will still call hs_try_putmvar(). Therefore we have to arrange that the memory for the result stays alive until the callback has run, so if an exception is thrown during takeMVar we fork another thread to wait for the callback and hold the memory alive using touchForeignPtr.

For a fully working example, see testsuite/tests/concurrent/should_run/hs_try_putmvar001.hs in the GHC source tree.

11.3.5. Floating point and the FFI

The standard C99 fenv.h header provides operations for inspecting and modifying the state of the floating point unit. In particular, the rounding mode used by floating point operations can be changed, and the exception flags can be tested.

In Haskell, floating-point operations have pure types, and the evaluation order is unspecified. So strictly speaking, since the fenv.h functions let you change the results of, or observe the effects of floating point operations, use of fenv.h renders the behaviour of floating-point operations anywhere in the program undefined.

Having said that, we can document exactly what GHC does with respect to the floating point state, so that if you really need to use fenv.h then you can do so with full knowledge of the pitfalls:

• GHC completely ignores the floating-point environment, the runtime neither modifies nor reads it.
• The floating-point environment is not saved over a normal thread context-switch. So if you modify the floating-point state in one thread, those changes may be visible in other threads. Furthermore, testing the exception state is not reliable, because a context switch may change it. If you need to modify or test the floating point state and use threads, then you must use bound threads (Control.Concurrent.forkOS), because a bound thread has its own OS thread, and OS threads do save and restore the floating-point state.
• It is safe to modify the floating-point unit state temporarily during a foreign call, because foreign calls are never pre-empted by GHC.

11.3.6. Pinned Byte Arrays

A pinned byte array is one that the garbage collector is not allowed to move. Consequently, it has a stable address that can be safely requested with byteArrayContents#. There are a handful of primitive functions in GHC.Prim <GHC-Prim.html> used to enforce or check for pinnedness: isByteArrayPinned#, isMutableByteArrayPinned#, and newPinnedByteArray#. A byte array can be pinned as a result of three possible causes:

1. It was allocated by newPinnedByteArray#.
2. It is large. Currently, GHC defines large object to be one that is at least as large as 80% of a 4KB block (i.e. at least 3277 bytes).
3. It has been copied into a compact region. The documentation for ghc-compact and compact describes this process.

[1]	Prior to GHC 8.10, when passing an ArrayArray# argument to a foreign function, the foreign function would see a pointer to the StgMutArrPtrs rather than just the payload.

[2]	(1, 2) In practice, the FFI should not be used for a task as simple as reading bytes from a MutableByteArray#. Users should prefer GHC.Exts.readWord8Array# for this.

[3]	As in [2], the FFI is not actually needed for this. GHC.Exts includes primitives for reading from on ArrayArray#.