RE: radio button label

["Dugas, Alan" <alan dugas analog com>]

Thanks to all who reponded to my questions.  Based on what has been said, I need
to bear in mind what a function does when checking its prototype.  Thanks again
to all!



				-- Stupid Genius

> ----------
> From: 	Eric Monsler[SMTP:eric_monsler radixwireless com]
> Sent: 	Thursday, October 19, 2000 6:34 PM
> To: 	Dugas, Alan
> Cc: 	Darin Fisher; gtk-list gtk org
> Subject: 	Re: radio button label
> 
> "Dugas, Alan" wrote:
> > 
> > BINGO!!!  Nice work Darin!  ...and thanks also to Jeff!  I don't understand
> why
> > the changes in Darin's code work however.  The online documentation declares
> > gtk_label_get() as follows;
> > 
> > void gtk_label_get(GtkLabel *label, gchar **str);
> > 
> > not
> > 
> > void gtk_label_get(GtkLabel *label, gchar *str);
> > 
> > Could either of you enlighten me?  Thanks in advance.
> > 
> 
> 
> This is a common C error with pointers.  Basically, the gtk_label_get
> wants to tell the calling code where the string is.  The calling code
> needs a char * type, to contain this information.  The intent of the
> call is that the address, i.e. storage location, into which the location
> of the desired string whould be written, it specified in the call. 
> Clear as mud?  
> 
> So, if you use a variable on the stack or in memory, and specify the
> address of that variable, the correct char * pointer value gets written
> into that variable.
> 
> If you create a char ** variable, pointer to a pointer, and pass that
> in, the gtk_label_get function will try to write the correct char *
> pointer value into the location specified by the value of the char **
> variable.  But, you've never set the value of the char ** to the address
> of a valid pointer.  Hence, the write by gtk_label_get is attempted into
> whatever random space corresponds to the data in that char ** memory.
> 
> Another valid construct (IIUC, barring tyops) could be:
> 
> gchar **radio_button_handle = (gchar **) NULL;
> gchar *radio_button_label = NULL;
> 
> /* No casting should be required here, this is important,
>    it shows that we are handling things properly */
> radio_button_handle = &radio_button_label;
> 
> gtk_label_get(GTK_LABEL(label), radio_button_handle);
>  
> 
> 
> Hope that helps!
> 
> Using addresses of variables for function returns, and for input/output
> parameters, is powerful but can get confusing, and the prototyping (even
> if syntactically valid) does not capture the subtleties of some usages.
> 
> Eric Monsler
> 
> _______________________________________________
> gtk-list mailing list
> gtk-list gnome org
> http://mail.gnome.org/mailman/listinfo/gtk-list
>