Editing Sequence point (section)

{{Short description|Concept in computer programming}}
In [[C (programming language)|C]] and [[C++]], a '''sequence point''' defines any point in a [[computer program]]'s [[execution (computing)|execution]] at which it is guaranteed that all [[side effect (computer science)|side effect]]s of previous evaluations will have been performed, and no side effects from subsequent evaluations have yet been performed. They are a core concept for determining the validity of and, if valid, the possible results of expressions. Adding more sequence points is sometimes necessary to make an expression defined and to ensure a single valid order of evaluation.

With [[C11 (C standard revision)|C11]] and [[C++11]], usage of the term sequence point has been replaced by sequencing. There are three possibilities:<ref>{{cite web |title=ISO/IEC 14882:2011 |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=50372 |accessdate=2012-07-04}}</ref><ref>{{cite web |title=A finer-grained alternative to sequence points (revised) (WG21/N2239 J16/07-0099) |url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2239.html |accessdate=2012-07-05}}</ref><ref>{{cite web |title=Order of evaluation |url=http://en.cppreference.com/w/c/language/eval_order |accessdate=2015-10-14}}</ref>

#An expression's evaluation can be ''sequenced before'' that of another expression, or equivalently the other expression's evaluation is ''sequenced after'' that of the first.
#The expressions' evaluation is ''indeterminately sequenced,'' meaning one is sequenced before the other, but which is unspecified.
#The expressions' evaluation is ''unsequenced.''

The execution of unsequenced evaluations can overlap, leading to potentially catastrophic [[undefined behavior]] if they share [[State (computer science)|state]]. This situation can arise in [[parallel computation]]s, causing [[race condition]]s, but undefined behavior can also result in single-threaded situations. For example, <code>a[i] = i++;</code> (where {{code|a}} is an array and {{code|i}} is an integer) has undefined behavior.