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From: bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: C89 "bug"
Date: Fri, 13 Dec 2024 18:39:26 +0000
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On 13/12/2024 15:41, Thiago Adams wrote:
> Em 12/13/2024 12:01 PM, Kaz Kylheku escreveu:
>> On 2024-12-13, Thiago Adams <thiago.adams@gmail.com> wrote:
>>> unfortunately, this solution does not work if we have two objects.
>>>
>>> union U {
>>>          double d;
>>>          int i;
>>>    };
>>>
>>> union U u1 = { .d=2.2 };
>>> union U u2 = { .i=1   };
>>
>> Idea: have several declarations of the union in different translation
>> units.
>>
>> /* translation unit for u1 */
>> union U {
>>    double d;
>>    int i;
>> };
>> extern union U u1 = { 2.2 };
>>
>> /* translation unit for u2 */
>> union U {
>>    int i;
>>    double d;
>> };
>> extern union U u2 = { 1 };
>>
> 
> another solution could be call a function that initializes before main.
> 
> extern union U u1;
> 
> void before_main()
> {
>    u1.d = 1.2;
> }
> 
> 

I assume the initialisation is at file-scope.

Then your suggestion could work, if you can arrange for it to be called.

But I think that will be a problem: this can occur in some arbitrary 
module compiled at a different time from the one containing the 
program's main(): either earlier, or later.

How will the compiler know which function(s) needs to be called from 
main(), when it translated the module containing main?

I can think of an approach, but it's not simple, and may be over-kill if 
this is something that is infrequently encountered.