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node.jspointersbuffernode-ffi

Why is a `node-ffi` Callback returning an empty Buffer with `void *` but not with `int *`?

I'm working on implementing a callback using node-ffi with the following signature:

const callback = ffi.Callback('void', ['int', 'void *'], (eMsg, pData) => {
    console.log(pData);
    console.log(`Buffer deref: ${ref.get(pData)}`);
});

The underlying C library calls this with a void * value for pData. About half of the callbacks actually return an int * value and, in the other half, pData is more complex data (i.e. Struct). The ref.get(pData) ends up being null. If I change the Callback definition to use int * instead of void *, ref.get(pData) returns the correct value. What I can't figure out is why the void * ends up being a Buffer with a size of 0 that does not dereference properly (even when pData is an int *).

I feel like I'm missing something but can't, for the life of me, see what that would be. Can anyone help?

Solution

  • I had a similar problem, and what I did was set the length property of the returned Buffer to the size of the thing that will be returned. Because it is void * type, node-ffi basically does not know how long of Buffer will be returned to it.

    so in this case, pData.length = <size of structure> should fix it.