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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Learning functions; roots of the quadratic; two return values
Date: Sun, 04 Oct 2015 12:56:49 -0700
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
[...]
> I tried this with lcc but the code did not compile.  Cutting it down to
> a minimal example, the problem seems to be with compound literals that
> include an array:
>
>   typedef struct s { double r[2]; };
>    
>   struct s f1(double a)
>   {
>        return (struct s){ { a, 0 } };
>   }

Interesting example.  Is the compound literal necessary to trigger the
bug?  (BTW the "typedef" keyword can be omitted, since you didn't give
it a name.)

Returning a structure that contains an array creates an object with
*temporary lifetime*, a concept that wasn't introduced until C11.
In C99 and earlier, something like

    f1(1.5).r[0]

has undefined, or at least vaguely defined, behavior.  Array indexing
requires an array object, but prior to c11 there is no array object to
be indexed, just an array value that's part of the returned struct
value.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"