Re: Memo on coding practices: strcmp() does not yield bool

I checked through the code and found no other problem cases.

> I've occasionally griped that I do not like the coding practice of
> writing
>     if (strcmp(foo, bar))
> to mean
>     if (strcmp(foo, bar) != 0)
> or its inverse
>     if (!strcmp(foo, bar))
> to mean
>     if (strcmp(foo, bar) == 0)
> 
> My past objection to this has been purely stylistic: it's too easy
> to read these constructs backwards, eg to think "!strcmp()" means
> "not equal".  However, I've now had my nose rubbed in the fact that
> this habit is actually dangerous.
> 
> Up till just now, ruleutils.c contained code like this:
> 
>         bool        tell_as = FALSE;
> 
>         /* Check if we must say AS ... */
>         if (!IsA(tle->expr, Var))
>             tell_as = strcmp(tle->resdom->resname, "?column?");
> 
>     /* more code... */
> 
>         if (tell_as)
>             /* do something */
> 
> This is subtly wrong, because it will work as intended on many
> platforms.  But on some platforms, strcmp is capable of yielding
> values that are not 0 but whose low 8 bits are all 0.  Stuff that
> into a char-sized "bool" variable, and all of a sudden it's zero,
> reversing the intended behavior of the test.
> 
> Correct, portable coding of course is
> 
>             tell_as = strcmp(tle->resdom->resname, "?column?") != 0;
> 
> This error would not have happened if the author of this code had
> been in the habit of regarding strcmp's result as something to compare
> against 0, rather than as equivalent to a boolean value.  So, I assert
> that the above-mentioned coding practice is dangerous, because it can
> lead you to do things that aren't portable.
> 
> I'm not planning to engage in a wholesale search-and-destroy mission
> for misuses of strcmp and friends just at the moment, but maybe someone
> should --- we may have comparable portability bugs elsewhere.  In any
> case I suggest we avoid this coding practice in future.
> 
>             regards, tom lane
> 


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