[Haskell-cafe] FFI ccall struct return by value on the stack?

☂Josh Chia (謝任中) joshchia at gmail.com
Tue Jun 16 14:44:18 UTC 2020

Suppose I have the following C code:

typedef struct Foo {
    char* p1;  /* Some data on the heap. */
    size_t s1; /* Size of data. */
    char* p2;  /* More data on the heap. */
    size_t s2;
} Foo;

/* Allocates and writes two pieces of data on the heap and returns them in
a Foo. */
Foo makeFoo(size_t x);

Based on my limited understanding of Haskell FFI according to the Haskell
2010 Language Report (
https://www.haskell.org/onlinereport/haskell2010/haskellch8.html), it is
not possible to have a ccall for makeFoo() because Foo is not a
marshallable foreign result type. Is my understanding correct?

However, I believe I can have a ccall if I change makeFoo() to either of
the following:
Foo* makeFoo(size_t x);
void makeFoo(Foo* out, size_t x);

The first involves the C code allocating a Foo and returning a pointer to
it (so now there's one more pointer for the C code to deallocate later in
another function). The second involves the C code writing a Foo value to a
piece of memory allocated in Haskell (possibly using
Foreign.Marshall.Alloc.alloca). Both signatures work because Foo* is
marshallable but are clumsier to use than the original signature. Is there
just no way to return a struct by value on the stack? Is there a cleaner
way than the above two?

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