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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: C89 "bug"
Date: Fri, 13 Dec 2024 22:02:22 -0800
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antispam@fricas.org (Waldek Hebisch) writes:
> Thiago Adams <thiago.adams@gmail.com> wrote:
>> 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 following:
>
> union U {
>     int i;
>     double d;
> };
>
> union U u = (union U)(1.2);
>
> is accepted by 'gcc -S -std=c90'.  I leave to others to check
> if this is valid C89.

It's not.  With "-std=c90 -pedantic", gcc (correctly) warns:

    ISO C forbids casts to union type [-Wpedantic]

The specific constraint in the C90 standard is:

    Unless the type name specifies void type, the type name shall
    specify qualified or unqualified scalar type and the operand
    shall have scalar type.

(Later editions have similar wording.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */