Re: zero length array example does not compile

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Tue, 25 Apr 2023 at 13:17, Jonathan Wakely wrote:
>
> On Tue, 25 Apr 2023 at 13:13, Jonny Grant wrote:
> >
> > Hello
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> >
> > I wondered 'this_length' refers to in that example, it doesn't compile.
>
> It's not supposed to be a complete program.
>
> >
> > <source>: In function 'main':
> > <source>:13:34: error: 'this_length' undeclared (first use in this function)
> >    13 |   malloc (sizeof (struct line) + this_length);
> >       |                                  ^~~~~~~~~~~
> >
> >
> > https://godbolt.org/z/PWEcWsrKv
> >
> > I probably the size of the struct? So that would be 4 bytes for me, as it is just the int. That doesn't seem very useful. Maybe I am missing something.
>
> Yes, you are. Look at how it's used: malloc is called to allocate
> sizeof(struct line) + this_length bytes. Why would it be the size of
> the struct?
>
> It's the number of bytes that the zero-length contents array can hold.

Maybe this change would help:

--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1705,6 +1705,9 @@ struct line *thisline = (struct line *)
thisline->length = this_length;
@end smallexample

+In this example, @code{thisline->contents} is an array of @code{char} that
+can hold up to @code{thisline->length} bytes.
+
Although the size of a zero-length array is zero, an array member of
this kind may increase the size of the enclosing type as a result of tail
padding.  The offset of a zero-length array member from the beginning