Thanks, Jonathan, for your patience, and detailed answer. I'll need to take some time to digest he wikipedia article. I was thinking of structure instances, not definitions, when questioning their positioning and padding. I can see now that an instance of a structure of 9 ints followed by 8 doubles would need to be positioned carefully to avoid that dreaded hole. Thanks, but I may need to come back to you after studying that article. JJ On 8 September 2013 15:33, Jonathan Wakely-4 [via gcc] < ml-node+s1065356n966628h59@xxxxxxxxxxxxx> wrote: > On 8 September 2013 14:42, JimJoyce wrote: > > > > My question 'Are Structures doublewoord aligned?' was not about the > second > > half of the structure, but the beginning: an array of 9 ints. > > It appears, having decided to place them on a doubleword boundary, it > then > > had to pad after the 9 ints to get back to an 8-byte boundary. > > Yes, there's a "hole" in the middle of the struct. > > > Was it pure mischance that the structure happened to start on a > doubleword > > that the extra int was needed. Had it started 4 bytes later, there would > be > > no padding? > > If you think about it that question doesn't make sense. The layout of > a struct is always the same, irrespective of where an particular > instance of that struct happens to be positioned in memory. A > struct's definition does not "start" anywhere in memory, only an > instance of the struct has an address. > > > Or do structures always start on a doubleword? > > It depends on the types in the struct and the ABI of the target platform. > > There's plenty of information about this on the web, e.g. > > http://en.wikipedia.org/wiki/Data_structure_alignment#Typical_alignment_of_C_structs_on_x86 > > > ------------------------------ > If you reply to this email, your message will be added to the discussion > below: > http://gcc.1065356.n5.nabble.com/gcc-structures-tp966595p966628.html > To unsubscribe from gcc structures, click here<http://gcc.1065356.n5.nabble.com/template/NamlServlet.jtp?macro=unsubscribe_by_code&node=966595&code=amltQGppbWpveWNlLmNvLnVrfDk2NjU5NXwtOTM4MDcwNDA4> > . > NAML<http://gcc.1065356.n5.nabble.com/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml> > -- JimJoyce T: 0(044) 1280 813 899 E: jim@xxxxxxxxxxxxxx -- View this message in context: http://gcc.1065356.n5.nabble.com/gcc-structures-tp966595p966657.html Sent from the gcc - Help mailing list archive at Nabble.com.
