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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: =?UTF-8?Q?Re=3a_technology_discussion_=e2=86=92_does_the_world_need?=
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Date: Wed, 10 Jul 2024 09:41:04 +0200
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On 09/07/2024 23:43, BGB wrote:
> On 7/9/2024 3:22 PM, James Kuyper wrote:
>> On 7/9/24 14:55, BGB wrote:
>> ...
>>> The pass by reference, in this context, was referring to the ABI, not
>>> to C itself.
>>>
>>> It looks from C's POV as-if it were by-value.
>>>
>>>
>>> Which it is, depends on if one is looking at things at the language
>>> level, ABI level, or IR level, ...
>>
>> The C standard doesn't explicitly specify pass by value, or pass by
>> reference, or anything other passing mechanism. What it does say is what
>> a programmer needs to know to use the passing mechanism. It says that
>> the value of a function parameter that is seen by the code inside that
>> function is a copy of the value passed as an argument to the function.
>> The copy can be modified without changing the original. When a C
>> function's declaration looks as though it takes an array as an argument,
>> what that declaration actually means is that it takes a pointer value as
>> an argument, and it is a copy of that pointer's value which is seen
>> inside the function, and can be modified. The memory it points at is the
>> same as the memory pointed at by the corresponding argument.
>>
>>
> 
> 
> We can probably agree that, in C:
>    typedef struct Foo_s Foo;
>    struct Foo_s {
>      int x, y, z, a, b, c;
>    };
> 
>    int FooFunc(Foo obj)
>    {
>      obj.z = obj.x + obj.y;
>      return(obj.z);
>    }
> 
>    int main()
>    {
>      Foo obj;
>      int z1;
>      obj.x=3;
>      obj.y=4;
>      obj.z=0;
>      z1=FooFunc(obj);
>      printf("%d %d\n", obj.z, z1);
>    }
> 
> Should print "0 7" regardless of how the structure is passed in the ABI.
> 

ABI's are irrelevant to how the language is defined and how these 
expressions are evaluated.  ABI's go along with details of the target - 
they can affect implementation-dependent behaviour but no more than 
that.  (A clear example would be that alignment of fundamental types 
would normally be specified by an ABI.)

So code that does not depend on implementation-dependent behaviour, such 
as your code here, will necessarily give the same results on all 
conforming C implementations.

> 
> Though, one possibility being to relax the language such that both "0 7" 
> and "7 7" are valid possibilities (the latter potentially allowing more 
> performance by not needing to make a temporary local copy). Though, 
> AFAIK, C doesn't really allow for this.

It continues to astound me that people who claim to have written C 
compilers themselves - such as you and Bart - regularly show 
misunderstandings or ignorance about things that are clear in the C 
standards and which I would expect any experienced C programmer to know.

All arguments to function calls in C are passed by value.  This is in 
6.5.2.2p4 - it is a two sentence paragraph that you really ought to have 
read before even considering writing a C compiler.

Some languages do have pass by reference (or other types of parameter 
passing systems), and would give "7 7".  C is not such a language.  (And 
I have never heard of a language that would either result.)

> 
> An implementation could be clever though and only make local copies in 
> cases where the structure is modified by the callee, as in the example 
> above.
> 

A clever implementation would turn the whole of main() into a single 
puts("0 7") call.

Compliers can generate whatever code they like, as long as the results 
are correct in the end.