warning: taking the address of a label is non-standard

Question

Can anyone explain how the following code creates a label?

char memory[] = "hello";
&&memory[0];

error: label 'memory' used but not defined

You definitely got one `&` too much there, where the label stuff is coming from I don't know though. — AliciaBytes, Sep 11 '14 at 23:37
@user4032914: The address of `memory` is just `memory` - it will decay to a pointer to the first character. — Mike Seymour, Sep 11 '14 at 23:38
It's a GNU extension -- or, at least, addresses of labels are a GNU extension, and they use the `&&` notation. See [Labels as values](https://gcc.gnu.org/onlinedocs/gcc-4.9.1/gcc/Labels-as-Values.html#Labels-as-Values) in the GCC documentation. — Jonathan Leffler, Sep 11 '14 at 23:38
Same as this question http://stackoverflow.com/questions/13915121/error-label-used-but-not-defined-when-using-operator — radar, Sep 11 '14 at 23:39
@MikeSeymour: Although to be clear, `&memory` is not the same thing as `memory` (or `&memory[0]`). — Oliver Charlesworth, Sep 11 '14 at 23:39
@user4032914 `&memory[0]` gives you a temporary char pointer pointing to the first element. But you can't take the addresses of temporaries so the second `&` is illegal. — AliciaBytes, Sep 11 '14 at 23:41

score 6 · Answer 1 · answered Sep 11 '14 at 23:36

6

&&memory[0];

That's not valid C++, thus a conforming extension can assign any semantics one could want to it.

It so happens, that &&label is the GNU folks' way of taking the address of a label for computed goto's, a GNU extension.

That's it.

answered Sep 11 '14 at 23:36

Deduplicator

The whole meaning of the expression to the GNU compiler is that first the (undefined) label `memory` is resolved to an address [(of type `void *`)](http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html) and then this pointer is subscripted via the `[0]` part, which happens to be an invalid operation too. (You might want to include the link in your answer.) – 5gon12eder Sep 11 '14 at 23:50
Well, certainly they also have that irritating extension of allowing pointer-arithmetic on `void*` (I think that's not a conforming extension for c++). – Deduplicator Sep 11 '14 at 23:53
Oops, `[]` actually seems to bind more tightly than `&&` and `(&&label)[0]` crashes GCC 4.9.0. – 5gon12eder Sep 12 '14 at 00:02

1 Answers1