[TUHS] Unix stories

[random832@fastmail.com (Random832)]

On Thu, Feb 9, 2017, at 08:46, Steffen Nurpmeso wrote:
> so now i really got this a few minute ago after adding negative
> history number support (to count from history top):
> 
>   tty.c: In function 'c_history':
>   tty.c:4157:13: warning: operation on 'entry' may be undefined
>   [-Wsequence-point]
>          entry = isneg ? --entry : (siz_t)a_tty.tg_hist_size - entry;
>          ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> And in my opinion this is just plain terrible?  I think it is
> absolutely clear what i intend, there is not even a dereferenced
> pointer involved, the lhv is either a register or if all fails
> a stack location.  I don't understand why i have to write
> 
>       if(isneg)
>          --entry;
>       else
>          entry = (siz_t)a_tty.tg_hist_size - entry;
> 
> to get over this, it is exactly the same?

What's wrong with entry = isneg ? entry-1 : (siz_t)a_tty.tg_hist_size -
entry;

Same number of characters (two more if you put spaces around the minus
sign, but hardly a huge burden in any case).

You're basically asking for the standard to carve out an exception for
the cases, and precisely only those cases, where the meaning can be seen
to be 100% unambiguous (i.e. that the two values being assigned to a
variable are provably the same value, and there are no other reads) -
which would limit it exclusively to the prefix operator (and assignment
operators, I suppose, "x = x += 1" is as unambiguous as it is
pointless), and only when there is no other expression involved except
for the conditionals (you couldn't have "--entry + x", for example).