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(): strange things will undoubtedly happen.

We pass 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.

After we've finished invoking our Haskell functions, we can call hs_exit(), which terminates the RTS. It runs any outstanding finalizers and generates any profiling or stats output that might have been requested.

There can be multiple calls to hs_init(), but each one should be matched by one (and only one) call to hs_exit()^[9].

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.

When foreign import ccall "wrapper" is used in a Haskell module, The C stub file M_stub.c generated by GHC contains small helper functions used by the code generated for the imported wrapper, so it must be linked in to the final program. When linking the program, remember to include M_stub.o in the final link command line, or you'll get link errors for the missing function(s) (this isn't necessary when building your program with ghc ––make, as GHC will automatically link in the correct bits).

Header files named by the -#include option or in a foreign import declaration are searched for using the C compiler's usual search path. You can add directories to this search path using the -I option (see Section 4.10.3, “Options affecting the C pre-processor”).

Note: header files are ignored unless compiling via C. If you had been compiling your code using the native code generator (the default) and suddenly switch to compiling via C, then you can get unexpected errors about missing include files. Compiling via C is enabled automatically when certain options are given (eg. -O and -prof both enable -fvia-C).