Yes, it is great to know the reason behind a decision in the C Standard (fix size pointer for unknown structure) and it is cool to know a people who is involving in the process answering my post :) The DG machine example describes the scenario to me vividly. Thank you! And now, I understand why Brian and you previously said this feature is ISO C complaint but some other info in the internet said not. struct foo {} is not, but struct bar is. On Fri, Oct 17, 2008 at 12:50 AM, MICHAEL R. MEISSNER <meissner@xxxxxxxxxxxxxxxxxx> wrote: > > On Fri, 2008-10-17 at 00:28 +0800, tiande wrote: > > Thank you for your further comment, especially on the standard size of pointer. > > > > So the pointer of the empty struct definition always has a standard > > size just like any other pointer type, we can use this pointer to > > refer to a "complete struct definition" known only by private module. > > Yes. When the original C90 standard came out, there were several > implementations of C on machines that had different sizes or internal > formats of pointers. > > For the early part of the standard process, I was representing the Data > General corporation whose initial machine was a 16-bit machine that only > had word pointers, and later added byte pointers by shifting the word > pointer left by one bit (the word pointer format used the top bit for an > indirection bit). On a DG machine, structure pointers were always word > pointers, even if the only items in a structure were bytes due to the > requirement of supporting pointers to unknown structures. > > > We still need the empty struct definition to tell the compiler about > > the "hidden/private" content of such struct use in current module. > > My reading of the grammar in section 6.7.2.1 of the ISO C99 standard is > that it is not standards compliant for GCC to accept struct foo {}, > since struct-declaration-list must contain at least one > struct-declaration, and struct-declaration must contain at least one > declarator. GCC does not seem to warn about this. > -- Regards, Chong, Tian Teck tiande@xxxxxxxxx -- Regards, Chong, Tian Teck tiande@xxxxxxxxx