Re: GtkImage/GdkPixbuf memory management

[Tristan Van Berkom <tvb gnome org>]

On Sun, 2007-06-03 at 16:56 +0100, Michael Tweedale wrote:
> Could someone clarify something that doesn't seem to be explicit in the
> API documentation?
>
> Consider the following code:
>
> static GtkWidget *image; /* initialized to a fixed GtkImage */
>  
> void update(const char *s)
> {
>   GdkPixbuf *pixbuf;
>   
>   pixbuf=gdk_pixbuf_new_from_file(s, NULL);
>   if(pixbuf) {
>     gtk_image_set_from_pixbuf(GTK_IMAGE(image), pixbuf);  /* <---| */
>     g_object_unref(G_OBJECT(pixbuf));  
>   } else
>     gtk_image_clear(GTK_IMAGE(image));
> }
>
> The question is: have I done enough to prevent a memory leak?

Yes.

> Suppose I call this function twice in succession:
> update("file1"); update("file2");
> When the indicated line executes the second time, is GtkImage smart
> enough to realize that it's replacing an existing image got from a
> GdkPixbuf, that when it _set_from_pixbuf that image it added a reference
> to the pixbuf, and that it should therefore now unref the pixbuf before
> replacing the image?

The cool part of a refcounted api here is that it doesnt matter
how GtkImage handles this, GtkImage could copy the pixbuf pixels
into a private pixmap or similar, or it could use the pixbuf
directly and hold a reference to it; in which case the previous
pixbuf would be released when updating the image for the second
time (or destroying the image), i.e. either:
 - the pixbuf is just copied into an internal buffer and the pixbuf
   freed when you call g_object_unref() (the api name kindof implies
   this).
 - ownership of the pixbuf is passed along to the image when you
   call g_object_unref().

Cheers,
                -Tristan