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Calling Haskell from C

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Current revision (19:43, 11 June 2012) (edit) (undo)
 
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Line 5: Line 5:
<haskell>
<haskell>
-
{-# LANGUAGE ForeignFunctionInterface #-}
+
{-# LANGUAGE ForeignFunctionInterface #-}
-
module Safe where
+
module Safe where
-
import Foreign.C.Types
+
import Foreign.C.Types
-
fibonacci :: Int -> Int
+
fibonacci :: Int -> Int
-
fibonacci n = fibs !! n
+
fibonacci n = fibs !! n
-
where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
+
where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
-
fibonacci_hs :: CInt -> CInt
+
fibonacci_hs :: CInt -> CInt
-
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
+
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
-
foreign export ccall fibonacci_hs :: CInt -> CInt
+
foreign export ccall fibonacci_hs :: CInt -> CInt
</haskell>
</haskell>
Line 26: Line 26:
And call it from C:
And call it from C:
-
<haskell>
+
#include <HsFFI.h>
-
#include "A_stub.h"
+
#ifdef __GLASGOW_HASKELL__
-
#include <stdio.h>
+
#include "Safe_stub.h"
-
 
+
extern void __stginit_Safe(void);
-
int main(int argc, char *argv[]) {
+
#endif
-
int i;
+
#include <stdio.h>
-
hs_init(&argc, &argv);
+
-
 
+
int main(int argc, char *argv[])
-
i = fibonacci_hs(42);
+
{
-
printf("Fibonacci: %d\n", i);
+
int i;
-
 
+
hs_init(&argc, &argv);
-
hs_exit();
+
#ifdef __GLASGOW_HASKELL__
-
return 0;
+
hs_add_root(__stginit_Safe);
-
}
+
#endif
-
</haskell>
+
 +
i = fibonacci_hs(42);
 +
printf("Fibonacci: %d\n", i);
 +
 +
hs_exit();
 +
return 0;
 +
}
Now, first compile the Haskell file:
Now, first compile the Haskell file:
-
$ ghc -c -O A.hs
+
$ ghc -c -O Safe.hs
-
Which creates some *.c and *.h headers, which you import into
+
Which creates Safe_stub.c, Safe_stub.o, Safe_stub.h, which you import into
your C program. Now compile your C code with ghc (!), passing
your C program. Now compile your C code with ghc (!), passing
the Haskell objects on the command line:
the Haskell objects on the command line:
-
$ ghc -optc-O test.c A.o A_stub.o -o test
+
$ ghc -optc-O test.c Safe.o Safe_stub.o -o test
 +
 
 +
Note, this will not work for ghc >= 7.2 due to ghc not generating Safe_stub.o file (thanks to Daniel Fischer for pointing this out), use the following command instead:
 +
 
 +
$ ghc --make -no-hs-main -optc-O test.c Safe -o test
-
How run your C code:
+
Then run your C code:
-
$ ./test
+
$ ./test
-
Fibonacci: 267914296
+
Fibonacci: 267914296
And that's it.
And that's it.
[[Category:Tutorials]]
[[Category:Tutorials]]

Current revision

It is not uncommon to want to call a Haskell function from C code. Here's how to do that.

We define the fibonacci function in Haskell: 

{-# LANGUAGE ForeignFunctionInterface #-}
 
module Safe where
 
import Foreign.C.Types
 
fibonacci :: Int -> Int
fibonacci n = fibs !! n
    where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
 
fibonacci_hs :: CInt -> CInt
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
 
foreign export ccall fibonacci_hs :: CInt -> CInt

Note the foreign export. When GHC sees this, it will generate stubs for C, to help it work out the Haskell types.

And call it from C: 

#include <HsFFI.h>
#ifdef __GLASGOW_HASKELL__
#include "Safe_stub.h"
extern void __stginit_Safe(void);
#endif
#include <stdio.h>

int main(int argc, char *argv[])
{
    int i;
    hs_init(&argc, &argv);
#ifdef __GLASGOW_HASKELL__
    hs_add_root(__stginit_Safe);
#endif

    i = fibonacci_hs(42);
    printf("Fibonacci: %d\n", i);

    hs_exit();
    return 0;
}

Now, first compile the Haskell file: 

$ ghc -c -O Safe.hs

Which creates Safe_stub.c, Safe_stub.o, Safe_stub.h, which you import into your C program. Now compile your C code with ghc (!), passing the Haskell objects on the command line: 

$ ghc -optc-O test.c Safe.o Safe_stub.o -o test

Note, this will not work for ghc >= 7.2 due to ghc not generating Safe_stub.o file (thanks to Daniel Fischer for pointing this out), use the following command instead: 

$ ghc --make -no-hs-main -optc-O test.c Safe -o test

Then run your C code: 

$ ./test 
Fibonacci: 267914296

And that's it.

Retrieved from "http://www.haskell.org/haskellwiki/Calling_Haskell_from_C"

Category: Tutorials