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From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: C89 "bug"
Date: Fri, 13 Dec 2024 15:24:34 -0300
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Em 12/13/2024 3:15 PM, Keith Thompson escreveu:
> Thiago Adams <thiago.adams@gmail.com> writes:
>> Does anyone knows how can I convert this code (external declaration) to C89?
>>
>> union U {
>>      int i;
>>      double d;
>> };
>>
>> union U  u = {.d=1.2};
>>
>> The problem is that in C89 only the first member of the union is
>> initialized.
> 
> The obvious solution is:
>      union U u;
>      u.d = 1.2;
> But that works only if u has automatic storage duration.
> 
> You could also define a function that takes a double argument and
> returns a union U result.

Like this?
union U {
     int i;
     double d;
};
union U f(){ union U u; u.d = 1.2; return u;}
union U u = f();

The problem is that f() is not a constant expression for external 
declarations.