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From: Joel Reymont <joelr1@gmail.com>
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Date: Sun, 15 May 2011 15:50:34 +0200
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To: caml-list <caml-list@inria.fr>
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Subject: [Caml-list] problem with array access in a c function

The types are at the bottom here, in the Poll module:

https://github.com/wagerlabs/ocaml-zmq/blob/master/src/ZMQ.ml

The C function in question is caml_zmq_poll_of_pollitem_array here:

https://github.com/wagerlabs/ocaml-zmq/blob/master/src/poll.c

I can grab the number of elements in the array just fine, e.g.
	
	int n = Wosize_val(pollitem_array); 

This will give me n of 1 or 2, depending on the number of tuples in the array. 

This sets current_elem to 0x400 (1024)

	current_elem = Field(pollitem_array, n); 

This returns 1

	int block = Is_block(current_elem); 

And this crashes

	int tag = Tag_val(current_elem); 

Subsequent attempts to pull out the custom data crash as well. 

Interestingly enough, Field(current_value, 0) and Field(current_value, 1) are 8 bytes apart. 

I'm on a 64-bit machine and I expect tuple elements to be pointers since one is a custom block and the other a variant.

What am I doing wrong?

	Thanks, Joel

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