From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Berlin To: Cc: Eli Zaretskii , Subject: Re: Cast to a struct in expressions Date: Sat, 28 Apr 2001 11:30:00 -0000 Message-id: References: <200104280912.CAA09949@tully.CS.Berkeley.EDU> X-SW-Source: 2001-04/msg00212.html On Sat, 28 Apr 2001, Paul Hilfinger wrote: > > > > > I'm still missing something, because I don't see how "*(struct foo *)x" > > > > differs from "*(struct {long foo; unsigned bar;} *)x". Both are valid > > > > C expression syntax, so the parser should be able to parse them both. > > > > > > No, they aren't both *expressions*, only one is. > > > One is a statement/declaration, and the other is an expression. > > > > I'm not a C language expert, but my references seem to disagree with > > you. > > Correct; both are expressions, or you couldn't write > > long z = (*(struct {long foo; unsigned bar;} *)x).foo; Except, I tried to compile this with a few c compilers, and it didn't work. Only gcc seems to accept it. > > whereas, in fact, you can. > > Of course, the semantics of > > *(struct {long foo; unsigned bar;} *)x > > is officially undefined, since, contrary to the fond beliefs of many C > programmers, the Standard only occasionally gives meaning to > dereferences of a cast of a pointer value to a different pointer type > (the anonymous type in the expression above necessarily differs from > that of x). > Therefore, GDB is not completely out of line in refusing > to recognize this, even if the reason it gives is maybe a little off. not "completely" out of line? It's not a c compiler. It doesn't pretend to be. I can't even get a lot of c compilers to recognize the expression. how could gdb be in any way out of line in refusing it, considering it's just a debugger? C also says that given int a[5]; 4[a] == a[4] We don't allow you to type "print 4[a] = 3", but we do allow "print a[4] = 3". Is this out of line? > Paul Hilfinger >