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"

#ifdef __GLASGOW_HASKELL__
#include "foo_stub.h"
#endif

#ifdef __GLASGOW_HASKELL__
extern void __stginit_Foo ( void );
#endif

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

  hs_init(&argc, &argv);
#ifdef __GLASGOW_HASKELL__
  hs_add_root(__stginit_Foo);
#endif

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

  hs_exit();
  return 0;
}

We've surrounded the GHC-specific bits with #ifdef __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).

Next, we call hs_add_root, a GHC-specific interface which is required to initialise the Haskell modules in the program. The argument to hs_add_root should be the name of the initialization function for the "root" module in your program - in other words, the module which directly or indirectly imports all the other Haskell modules in the program. In a standalone Haskell program the root module is normally Main, but when you are using Haskell code from a library it may not be. If your program has multiple root modules, then you can call hs_add_root multiple times, one for each root. The name of the initialization function for module M is __stginit_M, and it may be declared as an external function symbol as in the code above. Note that the symbol name should be transformed according to the Z-encoding:

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()^[11].

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.

The scenario here is much like in Section 8.2.1.1, “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:

 HsBool mylib_init(void){
   int argc = ...
   char *argv[] = ...

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

   // Tell Haskell about all root modules
   hs_add_root(__stginit_Foo);

   // 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() and hs_add_root() as normal to initialise the Haskell runtime, and the corresponding deinitialisation function mylib_end() calls hs_exit() to shut down the runtime.

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 Section 8.2.4.2, “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 (Section 4.14.2, “RTS options for SMP parallelism”). The +RTS -N 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.

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.

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 hs_add_root, and these calls must complete before invoking any foreign exported functions.

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. (Unofficially, if you want to use this fast and loose version of hs_exit(), then call shutdownHaskellAndExit() instead).