Re: [Haskell-cafe] FFI nested structs - malloc - free

Alexey, That is an interesting insight. I suppose there are C API like that. In my case, the FFI calls ioctl, which calls i2cdev_ioctl, which calls i2cdev_ioctl_rdwr. The only function that can assume anything about the structure is i2cdev_ioctl_rdwr, which as you can see below copies the data, but does not free it. So in this particular case, I need the FFI to free it. On the other hand, I could write a wrapper in C that frees the structure if that is the way FFI is supposed to be used. Does anyone know if there is a FFI solution that would not require a wrapper? Mike static noinline int i2cdev_ioctl_rdrw(struct i2c_client *client, unsigned long arg) { struct i2c_rdwr_ioctl_data rdwr_arg; struct i2c_msg *rdwr_pa; u8 __user **data_ptrs; int i, res; if (copy_from_user(&rdwr_arg, (struct i2c_rdwr_ioctl_data __user *)arg, sizeof(rdwr_arg))) return -EFAULT; On May 4, 2015, at 10:40 PM, Alexey Shmalko wrote:

Hi!

Disclaimer: I haven't worked much with FFI, so I'd like someone confirmed my words.

Seems that allocated memory in your example is supposed to be freed from inside C. It's not a memory leak.

If I understand correctly documentation [1], malloc/free are just a simple wrappers around C's malloc/free. It's mallocForeignPtr that sets finalizer.

Best regards, Alexey Shmalko

[1]: https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/ffi-ghc.html

On Tue, May 5, 2015 at 6:45 AM, Proclivis wrote:

...

I should have mentioned GHC 7.8.3 Ubuntu 64bit

Sent from my iPad

...

On May 4, 2015, at 9:42 PM, Proclivis wrote:

FFI Gurus,

I created a c2hs FFI of a nested C structure, where struct A has a pointer to a struct B. To do so, I used a malloc, but I am unsure if the memory will be freed when the resulting Ptr is freed.

The example at this link uses the same technique, so it will serve as an example.

https://github.com/ifesdjeen/haskell-ffi-tutorial/blob/master/src/Example.hs...

Line 48 and 51 do the malloc and assign the pointer in the struct, from inside a Storable poke implementation.

But, there is no explicit free, nor a finalizer.

Will the memory be freed when the Ptr of the Storable is freed?

If it is, it implies that some magic keeps track of mallocs inside a poke, and creates finalizers. Or, this example leaks. If it leaks, how do I create a finalizer from inside a poke implementation?

Mike

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