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Date: Wed, 10 Jul 2024 16:40:29 +0100
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On 10/07/2024 15:54, Janis Papanagnou wrote:

> Values passed (including values of pointers [used for arrays]) are
> handled (in the functions) as copies and cannot change the original
> entities (values or dereferenced objects) in the calling environment.
> 
> To make it possible to change entities in the calling environment
> in "C" you have to implement the necessary indirection by pointers.


You don't have to do anything at all:

   #include <stdio.h>
   typedef unsigned char byte;
   typedef byte vector[4];

   void F(vector a) {
       a[0]+=3;
       a[1]+=3;
   }

   int main(void) {
       vector v = {10,20,30,40};

       printf("%d %d %d %d\n", v[0], v[1], v[2], v[3]);  // 10 20 30 40
       F(v);
       printf("%d %d %d %d\n", v[0], v[1], v[2], v[3]);  // 13 23 30 49
   }

Here it looks superficially as though 'v' is passed by value (and it is 
of a size that the ABI /would/ pass by value), yet F changes its 
caller's data, perhaps unintentionally.

> Your insistence is amazing. 
/I/ am amazed at everyone's insistence that there is nothing remarkable 
about this, and that it is nothing at all like pass-by-reference.

So, how do I write F in C so that the caller's data is unchanged?

Sure, true pass-by-reference has some extra properties, but if I wanted 
to duplicate the behaviour of the above in my language, I have to use 
pass-by-reference.

In C you get that behaviour anyway (possibly to the surprise of many), 
in a language which only has pass-by-value, and without needing explicit 
pointers.

That really is remarkable. And not unsafe at all!