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From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: C89 "bug"
Date: Fri, 13 Dec 2024 16:07:28 -0300
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Em 12/13/2024 3:59 PM, Thiago Adams escreveu:
> Em 12/13/2024 3:51 PM, David Brown escreveu:
>> On 13/12/2024 19:24, Thiago Adams wrote:
>>> 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.
>>>
>>
>> Can you use gcc extensions here, or are you looking for strict C89 
>> compliance?
>>
>> (To me, the "bug" is using C89 in the first place, but you have your 
>> reasons for that.)
>>
>> Another option if you are generating code is to make your union :
>>
>> union U {
>>      struct { unsigned int lo; unsigned int hi; } raw;
>>      int i;
>>      double d;
>> };
>>
>> and always initialise it with the underlying representation for the 
>> values and types that you want.  (I believe you are generating the 
>> code, so that should be practical even for floating point data.)
>>
>> However, that puts a dependency on the endianness and size of types.
>>
> 
> Interesting!
> Dependency on sizes is not a problem because the generated code is 
> intended to be used in a pipeline and discarded after feeding a C compiler.
> Your suggestion is basically to have a struct of "words" that have the 
> same size of the structs. Then convert 1.2 (double) to two words.
> It makes sense!
> 
> 
> 

https://godbolt.org/z/cKb4j35W8

#include <stdio.h>

union U {
     unsigned int _[2];
     int i;
     double d;
};

union U u1 = {1, 0}; /*1.2*/
union U u2 = {858993459, 1072902963}; /*1.2*/
int main(){
   printf("u1.i = %d\n", u1.i);
   printf("u2.d = %f\n", u2.d);
}