From 8255d2dcb4cefee36c9fd63b57bd62b961e4cb67 Mon Sep 17 00:00:00 2001
From: "benjamin.franksen" <benjamin.franksen@helmholtz-berlin.de>
Date: Sat, 28 Sep 2013 18:40:26 +0000
Subject: [PATCH] docs: added a note regarding lazy evaluation of logical
operators
---
documentation/Reference.txt | 28 +++++++++++++++++-----------
1 file changed, 17 insertions(+), 11 deletions(-)
diff --git a/documentation/Reference.txt b/documentation/Reference.txt
index c7bd0993..3cc33c35 100644
--- a/documentation/Reference.txt
+++ b/documentation/Reference.txt
@@ -1042,6 +1042,13 @@ Left-associative Binary Operators
expr: `expr` "&" `expr`
expr: `expr` "%" `expr`
+.. note:: Like in most programming languages, evaluation of conditional
+ expressions using ``&&`` and ``||`` is done *lazily*: the second operand
+ is not evaluated if evaluation of the first already determines the
+ result. This particularly applies to the boolean expressions in
+ :token:`transition` clauses. See the built-in function :c:func:`pvGetQ`
+ for an extended discussion.
+
Ternary Operator
~~~~~~~~~~~~~~~~
@@ -1461,22 +1468,21 @@ combining a call to pvGetQ with other conditions in the same
} state ...
would remove the head from the queue every time the condition gets
-evaluated, regardless of whether ``other_condition`` is ``TRUE``. This is most
-probably not the desired effect, as you would lose an
-unknown number of messages. (Of course it *could* be exactly what you want,
-especially if ``other_condition`` were in fact ``!suppress_output``.)
-Whereas ::
+evaluated, regardless of whether ``other_condition`` is ``TRUE``. This is
+most probably not the desired effect, as you would lose an unknown number of
+messages. (Of course it *could* be exactly what you want, for instance if
+``other_condition`` were something like ``!suppress_output``.) Whereas ::
when (other_condition && pvGetQ(msg)) {
printf(msg);
} state ...
-is "safe", in the sense that no messages will be lost. BTW, If you
-combine with a disjunction ("||") it is the other way around, i.e. pvGetQ
-should appear as the first operand. This is all merely a result of the
-evaluation order imposed by the C language; similar remarks apply
-whenever you want to use an expression inside a :token:`transition` clause that
-potentially has a side-effect.
+is "safe", in the sense that no messages will be lost. BTW, If you combine
+with a disjunction ("||") it is the other way around, i.e. pvGetQ should
+appear as the first operand. This is all merely a result of the evaluation
+order imposed by the C language (and thus by SNL); similar remarks apply
+whenever you want to use an expression inside a :token:`transition` clause
+that potentially has a side-effect.
pvFreeQ
--
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