self referencing struct issue

[rhasson]

when creating a struct where one or more of its members are pointers to the same struct and used with node-ffi these nested structs are filled in with invalid memory addresses so dereferencing them causes a seg fault.

Example structs:

var pst_index_ll = Struct ({
    'i_id': 'uint64',
    'offset': 'uint64',
    'size': 'uint64',
    'u1': 'int64'
});
pst_index_ll.defineProperty('next', ref.refType(pst_index_ll));

var pst_desc_tree = Struct ({
    'd_id': 'uint64',
    'parent_d_id': 'uint64',
    'no_child': 'int32'
});
pst_desc_tree.defineProperty('desc', ref.refType(pst_index_ll));
pst_desc_tree.defineProperty('assoc_tree', ref.refType(pst_index_ll));
pst_desc_tree.defineProperty('prev', ref.refType(pst_desc_tree));
pst_desc_tree.defineProperty('next', ref.refType(pst_desc_tree));
pst_desc_tree.defineProperty('parent', ref.refType(pst_desc_tree));
pst_desc_tree.defineProperty('child', ref.refType(pst_desc_tree));
pst_desc_tree.defineProperty('child_tail', ref.refType(pst_desc_tree));

In this example accessing "next" or "child", etc. would result in a seg fault because the memory address stored at these offsets in the buffer are invalid.

Thanks, Roy

[TooTallNate]

Struct instances start of with undefined data (this is true in C as well). You must initialize the struct, either manually or through other means like node-ffi, before accessing the data from it.

The problem you're describing is true for any "pointer" type, but the solution is simple: don't access uninitialized fields. If this were C, the compiler would be giving you a warning ;)

[rhasson]

Before passing these structs to a function (via node-ffi) I initialize them by doing tree = new pst_desc_tree(); The challenge is that within the C library the nested struct pointers are allocated and filled in correctly, but when that pointer is passed back to node-ffi I get an invalide memory address.

Am I not initializing the structs correctly? Do I need to initialize each of the nested structs as well?

Can you give an example of manually initializing the structs?

[TooTallNate]

Can you show me some of your example ffi code?

[rhasson]

https://github.com/rhasson/node-libpst/blob/master/ffi_nodepst.js

[TooTallNate]

Cool, and do you have a test script that uses that and crashes?

[rhasson]

any time I try to access any of the nested structs inside the instantiated pst_file struct called "f". That means when I try to do f.d_head.d_id for example would cause a seg fault.

In the file on github I try to copy the memory address from the instantiated buffer into another buffer and try to read it but that also fails. I would ignore that bit of code since I don't think it's even correct.