Re: Another GNode question

[vladimir <vladeck sezampro yu>]

THANKS, once again Olexiy. As I see I haven't finished with pointers
yet. Still a lot to learn, but I __WILL__ try to be more careful. It is
pure __LOGIC__ now that You have explained it to me:) THANKS.

On Mon, 2002-10-21 at 10:04, Olexiy Avramchenko wrote:
> {reposted in text format, sorry}
>
> vladimir wrote:
>
> > #include <glib.h>
> >
> > int
> > main (int argc, char *argv[])
> > {
> >     GNode *node;
> >     GArray *array1;
> >     GArray *array2;
> >     guint number;
> >     
> >     node = g_node_new (NULL);
> >     array1 = g_array_new (FALSE, FALSE, sizeof (guint));
> >     
> >     number = 2;
> >     array1 = g_array_append_val (array1, number);
> >     
> >     number = 5;
> >     array1 = g_array_append_val (array1, number);
> >     
> >     number = 4;
> >     array1 = g_array_append_val (array1, number);
> >     
> >     node->data = g_memdup (array1, sizeof (array1) * array1->len);
> >     
> This line is completely broken:
> 0) GArray uses the *data* pointer to point the data. Assume you've 
> alloced the data
> at address 0x00400000 and initialized the pointer to it. Then you're 
> copying data with
> g_memdup to address 0x00400100, your pointer value stays at 0x00400000. 
> All looks
> right till you free the data at 0x00400000.
> 1) Variable array1 is a pointer, as I see you've expected that you would 
> get a size of GArray
> structure, but you will get an arch-depended size of pointer (4 for the 
> modern x86 running pm).
> 2) GArray is a public top of a larger structure GRealArray. You cant 
> just copy *sizeof(GArray)+sizeof(YourData)*array->len*
> bytes of memory. You have to create a new array and fill it with values 
> of an old one.
>
> >     g_array_free (array1, TRUE);
> >             
> >     array2 = (GArray *)node->data;
> >     g_print ("testing...\n");
> >     g_print ("array2, (1): %d\n", g_array_index (array2, guint, 0));
> >     g_print ("array2, (2): %d\n", g_array_index (array2, guint, 1));
> >     g_print ("array2, (3): %d\n", g_array_index (array2, guint, 2));
> >     
> >     return 0;
> > }
> >
> > So if I free array1 like this: g_array_free (array1, TRUE); the result
> > is:
> > testing...
> > array2, (1): 1078834872
> > array2, (2): 1078834872
> > array2, (3): 667182
> > (junk memory)
> That's ok - array1 is freed, so data can be overwritten.
>
> > But, if I __DON'T__ free array, the result is:
> > testing...
> > array2, (1): 2
> > array2, (2): 5
> > array2, (3): 4
> That's ok too - cause you've copied the pointer and data lying at the 
> same addresses.
>
> > Doesn't g_memdup copies mem? Or I'm missing something else? I'm using
> > GLib 2.0.6 & GTK+ 2.0.6.
> Cut from GLib reference (you can reach it @ gtk.org):
>
>
>       g_memdup ()
>
> gpointer <mailbox:///home/ath/.mozilla/ath/sy6a9i4u.slt/Mail/cs-1.stu/Sent?number=2201165&part=1.2>    
> g_memdup                        (gconstpointer 
> <mailbox:///home/ath/.mozilla/ath/sy6a9i4u.slt/Mail/cs-1.stu/Sent?number=2201165&part=1.3> mem,
>                                              guint 
> <mailbox:///home/ath/.mozilla/ath/sy6a9i4u.slt/Mail/cs-1.stu/Sent?number=2201165&part=1.4> byte_size);
>
> Allocates /byte_size/ bytes of memory, and copies /byte_size/ bytes into 
> it from /mem/. If /mem/ is NULL it returns NULL .
>
>
> Olexiy