node-ffi ref-struct question

[rhasson]

Nathan, 

I started using node-ffi which works great.  I need to create some structs so I'm using ref-struct.  Now some of my structs have pointers to themselves like this:

typedef struct pst_block_recorder {

    struct pst_block_recorder  *next;

    int64_t                     offset;

    size_t                      size;

    int                         readcount;

} pst_block_recorder;

I tried to recreate this with ref-struct like this:

var pst_block_recorder = Struct ({

    //struct pst_block_recorder  *next;

    'next': ref.refType(pst_block_recorder),

    //int64_t                     offset;

    'offset': 'int64',

    //size_t                      size;

    'size': 'size_t',

    //int                         readcount;

    'readcount': 'int'

});

But I get the following error:

assert.js:102

  throw new assert.AssertionError({

        ^

AssertionError: could not determine a proper "type" from: undefined

    at Object.coerceType (/home/roy/node_modules/ref/lib/ref.js:389:3)

    at Object.refType (/home/roy/node_modules/ref/lib/ref.js:304:23)

I also tried making those struct fields "pointer" types but the C library doesn't seem to populate them correctly.

Any thoughts?

Thanks,

Roy

[Nathan Rajlich]

So basically when you're calling "ref.refType(pst_block_recorder)",
pst_block_recorder is still undefined. I ran into this problem with a
struct that libffi uses, and the solution is to progressively create
the struct instance using the "defineProperty()" function. Try
something like this:

var pst_block_recorder = Struct();
pst_block_recorder.defineProperty('next', ref.refType(pst_block_recorder));
pst_block_recorder.defineProperty('offset', 'int64');
pst_block_recorder.defineProperty('size', 'size_t');
pst_block_recorder.defineProperty('readcount', 'int');

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[rhasson]

Good point.  I'll do that.


Another question, 

In C I do this:

typedef struct pst_file {

    pst_desc_tree  *d_head

} pst_file;

pst_desc_tree *d_ptr;  //this is another struct

d_ptr = pstfile.d_head;

How do I make this assignment with ref-struct?

[Nathan Rajlich]

> In C I do this:
> typedef struct pst_file {
> pst_desc_tree *d_head
> } pst_file;
>
> pst_desc_tree *d_ptr; //this is another struct
> d_ptr = pstfile.d_head;

It's a pointer to a struct actually. I think you need to explain more
what you intend to do with the variable after. Are you trying to make
a duplicate of the struct? It seems to me that what you have above
would be achieved with "pstfile.d_head.slice(0)" but I can't really
imagine what you would need that for.

[rhasson]

Sorry for not being very clear.  You're correct, pst_file is a struct with d_head being a pointer to another struct.  Within libpst (the C library I'm working with), when you call pst_open() you pass pst_file by reference which opens the PST file (MS outlook file) and populates the pst_file struct.  d_head is a pointer to another struct that holds individual records pulled out of the PST file.  When the pst_open() call returns I need to take the d_head pointer, assign it to another pointer variable of type pst_desc_tree and use that to pass into another function.

    pst_item *item = NULL;

    pst_desc_tree *d_ptr;

    pst_open(&pstfile, file_name, NULL);

    d_ptr = pstfile.d_head; // first record is main record

    item  = pst_parse_item(&pstfile, d_ptr, NULL);

The way I did it which seems to work is:

    var f = new pst_file();  //the main pst_file struct

    var pstdesctree_Ptr = ref.refType(pst_desc_tree);

    var d_ptr = ref.alloc(pstdesctree_Ptr);

    ref.writePointer(d_ptr, 0, f.d_head);

This seems to copy the correct pointer, however when I do:

ref.deref(d_ptr) 

I get a segmentation fault.

How would I get access to the variables stored in that struct inside node?

Here is my code:  https://github.com/rhasson/node-libpst

I didn't finish converting all the structs yet but give you an idea of what the code looks like.  In the SRC directory I included the .c and .h files so you can see the structures.

Thanks,

Roy

[Nathan Rajlich]

I think that whole step is unnecessary (saving the reference into a
"d_ptr" pointer). Just pass `f.d_head` directly to the
pst_parse_item() function.