Re: Reordering of function calls - compiler bug or allowed by standard?

[Kai Henningsen <kai dot extern at googlemail dot com>]

Am Mittwoch, den 24.09.2008, 08:25 +0200 schrieb Jens Kilian:
> The following somewhat contrived example models what happens in our software
> when accessing some memory-mapped IO registers.  There is an address register
> into which the SW writes the address to be accessed, plus a data register
> from which it can then read the contents of that address.  Both accesses
> have been replaced by printf/scanf here.
> 
> ---8<--------------------------------------------------------------------8<---
> #include <stdio.h>
> 
> /* model internal state of address register */
> int N;
> 
> /* write address register */
> void
> writeit(int i)
> {
>   printf("Writing %d\n", i);
>   N = i;
> }
> 
> /* read data register */
> int
> readit(void)
> {
>   int x;
> 
>   printf("Reading %d\n", N);
>   scanf("%d", &x);
> 
>   return x;
> }
> 
> /* convenience macro to read from address i */
> #define RD(i) (writeit(i), readit())

A comma is a sequence point, so this part works.

> 
> int
> main(void)
> {
>   if (RD(42) == RD(23))
>     return 1;
> 
>   return 0;
> }

On the other hand, the == operator isn't, so this part doesn't work.

> Is there any way to force a consistent order of function calls - i.e.,

Yes, of course, in the simplest way possible. Put the write and the
read inside a function. Function calls are sequence points. You still
can't predict if it's going for 42 or 43 first, but they won't be
interleaved that way.