zero length array example does not compile

[Jonny Grant]

Hello

https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

I wondered 'this_length' refers to in that example, it doesn't compile.

<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.

Kind regards
Jonny