man 3 switch

[Rick Stevens]

On 11/16/2009 01:06 PM, Steven W. Orr wrote:
> On 11/16/09 13:54, quoth Rick Stevens:
>> On 11/14/2009 01:55 PM, Frank Cox wrote:
>>> On Sat, 14 Nov 2009 14:50:57 -0500
>>> Steven W. Orr wrote:
>>>
>>>> There's nothing wrong with perl having all kinds of perldoc pages.
>>>> But perl
>>>> comes from one place. C, OTOH could come from lots of places besides
>>>> FSF and
>>>> the switch statement in gcc may not be exactly the same as the switch
>>>> statement in some other dialect.
>>>
>>> As C is an ISO standard, I sincerely doubt there would be any
>>> difference in the
>>> syntax and behaviour of the keywords between C compilers on any Unix-like
>>> operating system.
>>
>> Incorrect.  C, for example, does not guarantee the order of evaluation
>> of arithmetic operators of equal precedence in the same statement (in
>> other words, is something like "a + b + c" evaluated left to right, or
>> right to left?).  This can have significant effects if some of the
>> operands have "side effects"
>>
>> Another example is that a null pointer (or the value "NUL") is not
>> necessarily zero, only that it is guaranteed to not point at any valid
>> datum.
>>
>> C allows quite a bit of leeway to the compiler implementation.
>
>
> I think I disagree on this one. We jumped from standardization of keywords to
> how operators perform. I quote from page 53 of K&R: Table of Precedence and
> Associativity of Operators: a + b + c *always* goes from left to right. K&R is
> not the standard, but does the standard say otherwise? Lots of things are up
> to the compiler writer, but I'd be surprised if this was one of them. Sometime
> people worry about things like
>
> a++ + b++ + c++
>
> but even there, the precedence and the associativity are defined. In this case
>
> a++ + b++ + c++
>
> becomes (in psuedo stack code):
>
> a++
> b++
> c++
> a b +
> c +
>
> because binary + is lower precedence than ++.
>
> No?

I probably should have added "...subject to standard rules of 
arithmetic".  AFAIK, there's no guarantee to the order that any given
operand to an arithmetic operator will be evaluated if they're at the
same level of precedence.  In the examples you give, the binding of "++"
is higher than addition so that's unambiguous. But if a, b or c are, 
say, pointers that depend on the values of one of the others, then side
effects can be dangerous (the classic example of that is "getc(3)").

What I was trying to get across is, while the library is pretty 
standardized (with the exceptions usually called out in the man pages),
there are some things that different compilers will do differently and
one should take reasonable care to make sure to use order enforcing
syntax to make things work properly.

However even that is off the point of this thread.  The C _library_
is documented, but that's because it is the underpinnings of many of
the high-level languages used (C, C++, Fortran and most of the others).
The languages themselves normally aren't.  The perl, python and several
other groups do create man pages for their languages (which can be
massive) and that's a nice thing, but IMHO it's somewhat inappropriate.
If you need to know C, get a book.  Ditto C++ or python or other high-
level language.  Don't rely on man pages.

I'm not even sure I like all the stuff you get with "man bash", but you 
all know that I'm a curmudgeon.  :-)
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- Rick Stevens, Systems Engineer                      ricks at nerd.com -
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