Re: Segmentation fault when passing Poppler objects

[Emmanuele Bassi <ebassi gmail com>]

On Tue, 6 Nov 2018 at 09:55, Радомир Хаџић via gtk-app-devel-list <
gtk-app-devel-list gnome org> wrote:

> Hi. I get segmentation fault if I try to access a Poppler object whose
> pointer is passed through g_signal_connect. There is no such problem
> with normal pointers, though.


This:


> void draw_cb(GtkWidget *drawing_area, struct Colors *colors)

Is *not* the signature for a GtkWidget::draw signal callback.

The signature is:

  gboolean (* draw) (GtkWidget *widget, cairo_t *context, gpointer
user_data);


0x5571faa9c6e0

> 0x5571fac68200
> current colors are:
> red 0.000000
> green 0.000000
> blue 0.000000
> As we can see, colors in main and colors in draw_cb have different
> values, but this doesn't stop us from accessing the object (I wonder
> how this works, though it's not important in this case).

It "works" because you're getting passed a pointer to a memory area, and
you're trying to access it by 3 `sizeof(double)` offsets; the cairo_t
structure is large enough to accommodate those accesses without generating
a segmentation fault, but of course cairo_t does not contain 3 doubles at
the very beginning of its structure, so you're getting garbage that C
helpfully translates to a double representation. I also assume that you're
running on a 64 bit architecture, because if you tried the same of 32 bits
archs, you'd very much get a segmentation fault.

Of course, this will never work for a PopplerDocument instance, because
you're not trying to access the first `3 * sizeof(double)` bytes of it:
you're calling a Poppler function, which expects a PopplerDocument
instance, instead of a cairo_t one:

void doc_cb(GtkWidget *drawing_area, PopplerDocument *doc)
> {
>         g_print("%p\n", doc);
>         g_print("%d\n", poppler_document_get_n_pages(doc));
> }
Which is why you're getting a segmentation fault.

I strongly encourage you to read the GTK API reference:

https://developer.gnome.org/gtk3/stable/GtkWidget.html#GtkWidget-draw

In general, you should *always* read the documentation for each signal
you're using, to know the signature of the callback associated to the
signal. The signal machinery disables a lot of the type safety at compile
time in order to allow generic functions to be invoked without ad hoc
emitters.

Ciao,
 Emmanuele.

-- 
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[@] ebassi [@gmail.com]