Re: Another GNode question

[Olexiy Avramchenko <ath beast stu cn ua>]

{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