Re: (gtk_pixmap_new): assertion `val != NULL' failed

[lah micropp se (Lars Hallberg)]

On Thu, Jun 07, 2001 at 01:42:54PM +0200, Ronald Bultje wrote:
> 
> ---
> #include "image.xpm"
> void create_pixmap(GtkWidget *w) {
>   GdkPixmap *pixmap;
>   GdkBitmap *mask;
>   pixmap = gdk_pixmap_colormap_create_from_xpm_d (NULL,
>      gtk_widget_get_default_colormap(),
>      &mask, NULL, (gchar **)image_xpm);
>   w = gtk_pixmap_new (pixmap, mask);
> }
> void test() {
>   GtkWidget *w = NULL;
>   create_pixmap(w);
>   if (w == NULL) g_print("I am stupid\n");
>   else g_print("I am not that stupid\n");
> }
> ---
> 
> Should return "I am not that stupid", right? (I don't have a xwindows
> computer on my hands here so I can't test it - grom).

unless create_pixmap(w) is C++ with an referens argument this program 
*will* output "I am stupid\n". In C, ther is *no* way the create_pixmap
funktion can change its argument. It can change the memory the pointer
points to (but then it have to be pointing to valid memory, not NULL),
but it can *not* change the pointer itself. So, i C, w is garanteed 
to still point to NULL! You actuly have to send a pointer to a pointer 
for the funktion to be able to change the pointer.

That is a poor interface tho, in C a initsilasion funktion (like fopen) 
that returns a alocated chunk of memory whit a fiting pointer type is 
commonly used.

/Lars