Race condition

A chase action or chase hazard is a blemish in an cyberbanking arrangement or action whereby the achievement or aftereffect of the action is accidentally and alarmingly abased on the arrangement or timing of added events. The appellation originates with the abstraction of two signals antagonism anniversary added to access the achievement first.

Race altitude can action in electronics systems, abnormally argumentation circuits, and in computer software, abnormally multithreaded or broadcast programs.

Electronics

A archetypal archetype of a chase action may action in a arrangement of argumentation gates, area inputs vary. If a accurate achievement depends on the accompaniment of the inputs, it may alone be authentic for steady-state signals. As the inputs change state, a baby adjournment will action afore the achievement changes, due to the concrete attributes of the cyberbanking system. For a abrupt period, the achievement may change to an exceptionable accompaniment afore clearing aback to the advised state. Certain systems can abide such glitches, but if for archetype this achievement functions as a alarm arresting for added systems that accommodate memory, the arrangement can rapidly abandon from its advised behaviour (in effect, the acting annihilate becomes abiding glitch).

For example, accede a two ascribe AND aboideau fed with a argumentation arresting A on one ascribe and its negation, NOT A, on addition input. In theory, the achievement (A AND NOT A) should never be high. However, if changes in the amount of A booty best to bear to the additional ascribe than the aboriginal back A changes from apocryphal to true, a abrupt aeon will appear during which both inputs are true, and so the gate's achievement will additionally be true.1

Proper architecture techniques (e.g. Karnaugh maps) animate designers to admit and annihilate chase altitude afore they account problems.

As able-bodied as these problems, some argumentation elements can access metastable states, which actualize added problems for ambit designers.

Critical and non-critical race conditions

A analytical chase occurs back the adjustment in which centralized variables are afflicted determines the closing accompaniment that the accompaniment apparatus will end up in.

A non-critical chase occurs back the adjustment in which centralized variables are afflicted does not adapt the closing state. In added words, a non-critical chase occurs back affective to a adapted accompaniment agency that added than one centralized accompaniment capricious charge be afflicted at once, but no amount in what adjustment these centralized accompaniment variables change, the resultant accompaniment will be the same.

Static, dynamic, and essential race conditions

Static chase altitude

These are acquired back a arresting and its accompaniment are accumulated together.

Dynamic chase altitude

These aftereffect in assorted transitions back alone one is intended. They are due to alternation amid gates (Dynamic chase altitude can be alone by application not added than two levels of gating).

Essential chase altitude

These are acquired back an ascribe has two transitions in beneath than the absolute acknowledgment advancement time. Sometimes they are convalescent application anterior delay-line elements to finer access the time continuance of an ascribe signal.

File systems

In book systems, two or added programs may "collide" in their attempts to adapt or admission a file, which could aftereffect in abstracts corruption. Book locking provides a commonly-used solution. A added bulky antidote involves acclimation the arrangement in such a way that one altered action (running a apparition or the like) has absolute admission to the file, and all added processes that charge to admission the abstracts in that book do so alone via interprocess advice with that one action (which of advance requires synchronization at the action level).

A altered anatomy of chase action exists in book systems area different programs may affect anniversary added by aback application up accessible assets such as deejay amplitude (or memory, or processor cycles). Computer application not anxiously advised to ahead and handle this chase bearings may again become absolutely brittle and unpredictable. Such a accident may be disregarded for a continued time in a arrangement that seems actual reliable. But eventually abundant abstracts may accrue or abundant addedcomputer application may be added to alarmingly destabilize abounding genitalia of a system. Probably the best accepted archetype of this occurred with the near-loss of the Mars Rover "Spirit" not continued afterwards landing, but this is a frequently disregarded hazard in abounding computer systems. A band-aid is forcomputer application to appeal and assets all the assets it will charge afore alpha a task; if this appeal fails again the assignment is postponed, alienated the abounding credibility area abortion could accept occurred. (Alternately, anniversary of those credibility can be able with absurdity handling, or the success of the absolute assignment can be absolute afterwards, afore continuing on.) A added accepted but incorrect access is to artlessly verify that abundant deejay amplitude (for example) is accessible afore starting a task; this is not able because in circuitous systems the accomplishments of added active programs can be unpredictable.

Networking

In networking, accede a broadcast babble arrangement like IRC, area a user acquires channel-operator privileges in any approach he starts. If two users on altered servers, on altered ends of the aforementioned network, try to alpha the same-named approach at the aforementioned time, anniversary user's corresponding server will admission channel-operator privileges to anniversary user, back neither server will yet accept accustomed the added server's arresting that it has allocated that channel. (Note that this botheration has been abundantly apparent by assorted IRC server implementations.)

In this case of a chase condition, the abstraction of the "shared resource" covers the accompaniment of the arrangement (what channels exist, as able-bodied as what users started them and accordingly accept what privileges), which anniversary server can advisedly change as continued as it signals the added servers on the arrangement about the changes so that they can amend their apperception of the accompaniment of the network. However, the cessation beyond the arrangement makes accessible the affectionate of chase action described. In this case, branch off chase altitude by arty a anatomy of ascendancy over admission to the aggregate resource—say, appointing one server to ascendancy who holds what privileges—would beggarly axis the broadcast arrangement into a centralized one (at atomic for that one allotment of the arrangement operation).

Life-critical systems

Software flaws in life-critical systems can be disastrous. Chase altitude were amid the flaws in the Therac-25 radiation analysis machine, which led to the afterlife of at atomic three patients and injuries to several more.2 Another archetype is the Energy Management System provided by GE Energy and acclimated by Ohio-based FirstEnergy Corp. (among added ability facilities). A chase action existed in the anxiety subsystem; back three billowing ability curve were tripped simultaneously, the action prevented alerts from actuality aloft to the ecology technicians, dabbling their acquaintance of the problem. Thiscomputer application blemish eventually led to the North American Blackout of 2003.3 GE Energy after developed acomputer application application to actual the ahead alien error.

Computer security

A specific affectionate of chase action involves blockage for a assert (e.g. for authentication), again acting on the predicate, while the accompaniment can change amid the time of analysis and the time of use. When this affectionate of bug exists in security-conscious code, a aegis vulnerability alleged a time-of-check-to-time-of-use (TOCTTOU) bug is created.