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