From: Josh Stone <jistone@redhat.com>
To: Neo Liu <diabloneo@gmail.com>
Cc: SystemTap <systemtap@sourceware.org>
Subject: Re: operator precedence differs from C language
Date: Thu, 01 Apr 2010 17:15:00 -0000 [thread overview]
Message-ID: <4BB4D49D.4010102@redhat.com> (raw)
In-Reply-To: <4BB4C9DC.4040305@redhat.com>
On 04/01/2010 09:29 AM, Josh Stone wrote:
> On 03/31/2010 07:36 PM, Neo Liu wrote:
>> I found that the operator precedence of "&" and "==" differed from C
>> language. Take a look at the following statements.
>
> That's not true -- in C and in stap, relational operators bind more
> tightly than bitwise operators. http://tinyurl.com/9q6szp
You might also find this interesting:
http://cm.bell-labs.com/cm/cs/who/dmr/chist.html
"""
Neonatal C
Rapid changes continued after the language had been named, for example
the introduction of the && and || operators. In BCPL and B, the
evaluation of expressions depends on context: within if and other
conditional statements that compare an expression's value with zero,
these languages place a special interpretation on the and (&) and or (|)
operators. In ordinary contexts, they operate bitwise, but in the B
statement
if (e1 & e2) ...
the compiler must evaluate e1 and if it is non-zero, evaluate e2, and if
it too is non-zero, elaborate the statement dependent on the if. The
requirement descends recursively on & and | operators within e1 and e2.
The short-circuit semantics of the Boolean operators in such
`truth-value' context seemed desirable, but the overloading of the
operators was difficult to explain and use. At the suggestion of Alan
Snyder, I introduced the && and || operators to make the mechanism more
explicit.
Their tardy introduction explains an infelicity of C's precedence rules.
In B one writes
if (a==b & c) ...
to check whether a equals b and c is non-zero; in such a conditional
expression it is better that & have lower precedence than ==. In
converting from B to C, one wants to replace & by && in such a
statement; to make the conversion less painful, we decided to keep the
precedence of the & operator the same relative to ==, and merely split
the precedence of && slightly from &. Today, it seems that it would have
been preferable to move the relative precedences of & and ==, and
thereby simplify a common C idiom: to test a masked value against
another value, one must write
if ((a&mask) == b) ...
where the inner parentheses are required but easily forgotten.
"""
next prev parent reply other threads:[~2010-04-01 17:15 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-04-01 2:37 Neo Liu
2010-04-01 16:29 ` Josh Stone
2010-04-01 17:15 ` Josh Stone [this message]
[not found] ` <u2n861674c01004011849o94f1a327za4bc14aff28e3d9@mail.gmail.com>
2010-04-02 2:00 ` Fwd: " Neo Liu
2010-04-01 16:30 ` Frank Ch. Eigler
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