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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: question about linker
Date: Wed, 27 Nov 2024 10:53:43 +0100
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On 26/11/2024 20:42, Thiago Adams wrote:
> On 26/11/2024 16:25, Bart wrote:
>> On 26/11/2024 19:11, Thiago Adams wrote:
>>> On 26/11/2024 15:35, Thiago Adams wrote:


>>> Then adding:
>>>
>>> int strcmp();
>>>
>>> int main() {
>>>      strcmp("a", "b");
>>> }
>>>
>>> it works in C99 / C11
>>>
>>> I think in C23 empty parameter list means no args, while in the 
>>> previous versions (void) means no args.
>>>
>>> Considering that in previous versions of C we could call a function 
>>> without its signature I think the compiler only needs the caller 
>>> side. (of course I am not considering programmer mistakes)
>>>
>>> So, I think one extra simplification for small compilers is to ignore 
>>> function parameters.
>>
>> I don't think so. But you are welcome to look at godbolt.org and see 
>> for yourself. Try this for example:
> 
> 
> 
> Yes..I realized now I am wrong. Considering function calls uses 
> registers I think the old C model works only when passing everything on 
> stack.
> 
> 

No, it should work for other calling conventions too.  Passing 
everything on the stack has not been common practice for many decades, 
for most processor architectures.

What you have to consider here is the "default argument promotions".  If 
a function is defined to take a "double", and you call it using an int 
expression without using a function prototype, the result is UB (in a 
real and practical sense, not just hypothetically).  It doesn't matter 
if arguments are passed on the stack or registers, or if you have 32-bit 
or 64-bit or any other size of cpu (I don't know why Bart thinks that 
matters).  It is still a disaster.

If you want to write or generate code that calls a function, you need to 
know /exactly/ what type the parameters are.  And you need to call it 
with parameters of those types.  You can do that by having a function 
prototype and letting the compiler make the appropriate implicit 
conversions (assuming they are allowed by the language), or you can 
manually add any required conversions (such as casts) before the call, 
or you can rely on the default argument promotions if you know the 
result will be the correct type.

There is - to my knowledge - never a good reason for omitting a function 
prototype.  Implicit function declaration was IMHO one of the biggest 
design flaws in pre-standard C, and allowing it to continue in C90 after 
prototypes were added to the language, was a serious mistake.  Compilers 
should complain loudly if you try to call a function without a prototype 
declaration.  (I believe Bart's compiler treats it as a fatal error - it 
is a non-conformity of which I approve.)  And finally in C23 - some 
thirty years late - the standard finally requires proper prototypes.

I am not all sure why you are generating code C90 code here.  I don't 
think anyone much cares about using strict C90 other than a couple of 
people in this newsgroup.  (People sometimes do have to limit their C to 
a subset accepted by an old or limited compiler, but usually they then 
also want to use extensions or post-C90 features that their compiler 
supports.  And are they going to use your tool?)  But presumably you 
know more about your potential users than I do.  However, it seems to me 
that you should not be considering generating code that would be 
rejected by at least some C compilers - including all C23 compilers.