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Boeing C-17 Globemaster III

The McDonnell Douglas/Boeing C-17 Globemaster III is a large military transport aircraft that was developed for the United States Air Force (USAF) from the 1980s to the early 1990s by McDonnell Douglas. The C-17 carries forward the name of two previous piston-engined military cargo aircraft, the Douglas C-74 Globemaster and the Douglas C-124 Globemaster II.

The C-17 is based upon the YC-15, a smaller prototype airlifter designed during the 1970s. It was designed to replace the Lockheed C-141 Starlifter, and also fulfill some of the duties of the Lockheed C-5 Galaxy. Compared to the YC-15, the redesigned airlifter differed in having swept wings, increased size, and more powerful engines. Development was protracted by a series of design issues, causing the company to incur a loss of nearly US$1.5 billion on the program's development phase. On 15 September 1991, roughly one year behind schedule, the first C-17 performed its maiden flight. The C-17 formally entered USAF service on 17 January 1995. Boeing, which merged with McDonnell Douglas in 1997, continued to manufacture the C-17 for almost two decades. The final C-17 was completed at the Long Beach, California, plant and flown on 29 November 2015.

The C-17 commonly performs tactical and strategic airlift missions, transporting troops and cargo throughout the world; additional roles include medical evacuation and airdrop duties. The transport is in service with the USAF along with air arms of India, the United Kingdom, Australia, Canada, Qatar, the United Arab Emirates, Kuwait, and the Europe-based multilateral organization Heavy Airlift Wing. The type played a key logistical role during both Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom in Iraq, as well as in providing humanitarian aid in the aftermath of various natural disasters, including the 2010 Haiti earthquake, the 2011 Sindh floods and the 2023 Turkey-Syria earthquake.

Background and design phase
In the 1970s, the U.S. Air Force began looking for a replacement for its Lockheed C-130 Hercules tactical cargo aircraft. The Advanced Medium STOL Transport (AMST) competition was held, with Boeing proposing the YC-14, and McDonnell Douglas proposing the YC-15. Though both entrants exceeded specified requirements, the AMST competition was canceled before a winner was selected. The USAF started the C-X program in November 1979 to develop a larger AMST with longer range to augment its strategic airlift.

By 1980, the USAF had a large fleet of aging C-141 Starlifter cargo aircraft. Compounding matters, increased strategic airlift capabilities was needed to fulfill its rapid-deployment airlift requirements. The USAF set mission requirements and released a request for proposals (RFP) for C-X in October 1980. McDonnell Douglas chose to develop a new aircraft based on the YC-15. Boeing bid an enlarged three-engine version of its AMST YC-14. Lockheed submitted both a C-5-based design and an enlarged C-141 design. On 28 August 1981, McDonnell Douglas was chosen to build its proposal, then designated C-17. Compared to the YC-15, the new aircraft differed in having swept wings, increased size, and more powerful engines. This would allow it to perform the work done by the C-141, and to fulfill some of the duties of the Lockheed C-5 Galaxy, freeing the C-5 fleet for outsize cargo.

Alternative proposals were pursued to fill airlift needs after the C-X contest. These were lengthening of C-141As into C-141Bs, ordering more C-5s, continued purchases of KC-10s, and expansion of the Civil Reserve Air Fleet. Limited budgets reduced program funding, requiring a delay of four years. During this time contracts were awarded for preliminary design work and for the completion of engine certification. In December 1985, a full-scale development contract was awarded, under Program Manager Bob Clepper. At this time, first flight was planned for 1990. The USAF had formed a requirement for 210 aircraft.

Development problems and limited funding caused delays in the late 1980s. Criticisms were made of the developing aircraft and questions were raised about more cost-effective alternatives during this time. In April 1990, Secretary of Defense Dick Cheney reduced the order from 210 to 120 aircraft. The maiden flight of the C-17 took place on 15 September 1991 from the McDonnell Douglas's plant in Long Beach, California, about a year behind schedule. The first aircraft (T-1) and five more production models (P1-P5) participated in extensive flight testing and evaluation at Edwards Air Force Base. Two complete airframes were built for static and repeated load testing.

Development difficulties
A static test of the C-17 wing in October 1992 resulted in its failure at 128% of design limit load, below the 150% requirement. Both wings buckled rear to the front and failures occurred in stringers, spars, and ribs. Some $100 million was spent to redesign the wing structure; the wing failed at 145% during a second test in September 1993. A review of the test data, however, showed that the wing was not loaded correctly and did indeed meet the requirement. The C-17 received the "Globemaster III" name in early 1993. In late 1993, the Department of Defense (DoD) gave the contractor two years to solve production issues and cost overruns or face the contract's termination after the delivery of the 40th aircraft. By accepting the 1993 terms, McDonnell Douglas incurred a loss of nearly US$1.5 billion on the program's development phase.

In March 1994, the Non-Developmental Airlift Aircraft program was established to procure a transport aircraft using commercial practices as a possible alternative or supplement to the C-17. Initial material solutions considered included: buy a modified Boeing 747-400 NDAA, restart the C-5 production line, extend the C-141 service life, and continue C-17 production. The field eventually narrowed to: the Boeing 747-400, the Lockheed Martin C-5D, and the McDonnell Douglas C-17. The NDAA program was initiated after the C-17 program was temporarily capped at a 40-aircraft buy pending further evaluation of C-17 cost and performance and an assessment of commercial airlift alternatives.

In April 1994, the program remained over budget and did not meet weight, fuel burn, payload, and range specifications. It failed several key criteria during airworthiness evaluation tests. Problems were found with the mission software, landing gear, and other areas. In May 1994, it was proposed to cut production to as few as 32 aircraft; these cuts were later rescinded. A July 1994 Government Accountability Office (GAO) report revealed that USAF and DoD studies from 1986 and 1991 stated the C-17 could use 6,400 more runways outside the U.S. than the C-5, but these studies had only considered runway dimensions, but not runway strength or load classification numbers (LCN). The C-5 has a lower LCN, but the USAF classifies both in the same broad load classification group. When considering runway dimensions and load ratings, the C-17's worldwide runway advantage over the C-5 shrank from 6,400 to 911 airfields. The report also stated "current military doctrine that does not reflect the use of small, austere airfields", thus the C-17's short field capability was not considered.

A January 1995 GAO report stated that the USAF originally planned to order 210 C-17s at a cost of $41.8 billion, and that the 120 aircraft on order were to cost $39.5 billion based on a 1992 estimate. In March 1994, the U.S. Army decided it did not need the 60000 lb low-altitude parachute-extraction system delivery with the C-17 and that the C-130's 42000 lb capability was sufficient. C-17 testing was limited to this lower weight. Airflow issues prevented the C-17 from meeting airdrop requirements. A February 1997 GAO report revealed that a C-17 with a full payload could not land on 3000 ft wet runways; simulations suggested a distance of 5000 ft was required. The YC-15 was transferred to AMARC to be made flightworthy again for further flight tests for the C-17 program in March 1997.

By September 1995, most of the prior issues were reportedly resolved and the C-17 was meeting all performance and reliability targets. The first USAF squadron was declared operational in January 1995.

Production and deliveries

In 1996, the DoD ordered another 80 aircraft for a total of 120. In 1997, McDonnell Douglas merged with domestic competitor Boeing. In April 1999, Boeing offered to cut the C-17's unit price if the USAF bought 60 more; in August 2002, the order was increased to 180 aircraft. In 2007, 190 C-17s were on order for the USAF. On 6 February 2009, Boeing was awarded a $2.95 billion contract for 15 additional C-17s, increasing the total USAF fleet to 205 and extending production from August 2009 to August 2010. On 6 April 2009, U.S. Secretary of Defense Robert Gates stated that there would be no more C-17s ordered beyond the 205 planned. However, on 12 June 2009, the House Armed Services Air and Land Forces Subcommittee added a further 17 C-17s.

In 2010, Boeing reduced the production rate to 10 aircraft per year from a high of 16 per year, due to dwindling orders and to extend the production line's life while additional orders were sought. The workforce was reduced by about 1,100 through 2012, a second shift at the Long Beach plant was also eliminated. By April 2011, 230 production C-17s had been delivered, including 210 to the USAF. The C-17 prototype "T-1" was retired in 2012 after use as a testbed by the USAF. In January 2010, the USAF announced the end of Boeing's performance-based logistics contracts to maintain the type. On 19 June 2012, the USAF ordered its 224th and final C-17 to replace one that crashed in Alaska in July 2010.

In September 2013, Boeing announced that C-17 production was starting to close down. In October 2014, the main wing spar of the 279th and last aircraft was completed; this C-17 was delivered in 2015, after which Boeing closed the Long Beach plant. Production of spare components was to continue until at least 2017. The C-17 is projected to be in service for several decades. In February 2014, Boeing was engaged in sales talks with "five or six" countries for the remaining 15 C-17s; thus Boeing decided to build ten aircraft without confirmed buyers in anticipation of future purchases.

In May 2015, The Wall Street Journal reported that Boeing expected to book a charge of under $100 million and cut 3,000 positions associated with the C-17 program, and also suggested that Airbus' lower cost A400M Atlas took international sales away from the C-17.

Design

The C-17 Globemaster III is a strategic transport aircraft, able to airlift cargo close to a battle area. The size and weight of U.S. mechanized firepower and equipment have grown in recent decades from increased air mobility requirements, particularly for large or heavy non-palletized outsize cargo. It has a length of 174 ft and a wingspan of 169 ft, and uses about 8% composite materials, mostly in secondary structure and control surfaces. The aircraft features an anhedral wing configuration, providing pitch and roll stability to the aircraft. The aircraft's stability is furthered by its T-tail design, raising the center of pressure even higher above the center of mass. Drag is also lowered, as the wings are far removed from the vortices generated by the two wings of the aircraft.

The C-17 is powered by four Pratt & Whitney F117-PW-100 turbofan engines, which are based on the commercial Pratt & Whitney PW2040 used on the Boeing 757. Each engine is rated at 40400 lbf of thrust. The engine's thrust reversers direct engine exhaust air upwards and forward, reducing the chances of foreign object damage by ingestion of runway debris, and providing enough reverse thrust to back up the aircraft while taxiing. The thrust reversers can also be used in flight at idle-reverse for added drag in maximum-rate descents. In vortex surfing tests performed by two C-17s, up to 10% fuel savings were reported.

For cargo operations the C-17 requires a crew of three: pilot, copilot, and loadmaster. The cargo compartment is 88 ft long by 18 ft wide by 12 ft high. The cargo floor has rollers for palletized cargo but it can be flipped to provide a flat floor suitable for vehicles and other rolling stock. Cargo is loaded through a large aft ramp that accommodates rolling stock, such as a 69-ton (63-metric ton) M1 Abrams main battle tank, other armored vehicles, trucks, and trailers, along with palletized cargo.

Maximum payload of the C-17 is 170900 lb, and its maximum takeoff weight is 585000 lb. With a payload of 160000 lb and an initial cruise altitude of 28000 ft, the C-17 has an unrefueled range of about 2400 nmi on the first 71 aircraft, and 2800 nmi on all subsequent extended-range models that include a sealed center wing bay as a fuel tank. Boeing informally calls these aircraft the C-17 ER. The C-17's cruise speed is about 450 kn (Mach 0.74). It is designed to airdrop 102 paratroopers and their equipment. According to Boeing the maximum unloaded range is 6,230 nmi.

The C-17 is designed to operate from runways as short as 3500 ft and as narrow as 90 ft. The C-17 can also operate from unpaved, unimproved runways (although with a higher probability to damage the aircraft). The thrust reversers can be used to move the aircraft backwards and reverse direction on narrow taxiways using a three- (or more) point turn. The plane is designed for 20 man-hours of maintenance per flight hour, and a 74% mission availability rate.

United States Air Force

The first production C-17 was delivered to Charleston Air Force Base, South Carolina, on 14 July 1993. The first C-17 unit, the 17th Airlift Squadron, became operationally ready on 17 January 1995. It has broken 22 records for oversized payloads. The C-17 was awarded U.S. aviation's most prestigious award, the Collier Trophy, in 1994. A Congressional report on operations in Kosovo and Operation Allied Force noted "One of the great success stories...was the performance of the Air Force's C-17A" It flew half of the strategic airlift missions in the operation, the type could use small airfields, easing operations; rapid turnaround times also led to efficient utilization.

In 2006, eight C-17s were delivered to March Joint Air Reserve Base, California; controlled by the Air Force Reserve Command (AFRC), assigned to the 452nd Air Mobility Wing and subsequently assigned to AMC's 436th Airlift Wing and its AFRC "associate" unit, the 512th Airlift Wing, at Dover Air Force Base, Delaware, supplementing the Lockheed C-5 Galaxy. The Mississippi Air National Guard's 172 Airlift Group received their first of eight C-17s in 2006. In 2011, the New York Air National Guard's 105th Airlift Wing at Stewart Air National Guard Base transitioned from the C-5 to the C-17.

C-17s delivered military supplies during Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom in Iraq as well as humanitarian aid in the aftermath of the 2010 Haiti earthquake, and the 2011 Sindh floods, delivering thousands of food rations, tons of medical and emergency supplies. On 26 March 2003, 15 USAF C-17s participated in the biggest combat airdrop since the United States invasion of Panama in December 1989: the night-time airdrop of 1,000 paratroopers from the 173rd Airborne Brigade occurred over Bashur, Iraq. These airdrops were followed by C-17s ferrying M1 Abrams, M2 Bradleys, M113s and artillery. USAF C-17s have also assisted allies in their airlift needs, such as Canadian vehicles to Afghanistan in 2003 and Australian forces for the Australian-led military deployment to East Timor in 2006. In 2006, USAF C-17s flew 15 Canadian Leopard C2 tanks from Kyrgyzstan into Kandahar in support of NATO's Afghanistan mission. In 2013, five USAF C-17s supported French operations in Mali, operating with other nations' C-17s (RAF, NATO and RCAF deployed a single C-17 each).

Since 1999, C-17s have flown annually to Antarctica on Operation Deep Freeze in support of the US Antarctic Research Program, replacing the C-141s used in prior years. The initial flight was flown by the USAF 62nd Airlift Wing. The C-17s fly round trip between Christchurch Airport and McMurdo Station around October each year and take 5 hours to fly each way. In 2006, the C-17 flew its first Antarctic airdrop mission, delivering 70,000 pounds of supplies. Further air drops occurred during subsequent years.

A C-17 accompanies the President of the United States on his visits to both domestic and foreign arrangements, consultations, and meetings. It is used to transport the Presidential Limousine, Marine One, and security detachments. On several occasions, a C-17 has been used to transport the President himself, temporarily gaining the Air Force One call sign while doing so.

Debate arose over follow-on C-17 orders, the USAF requested line shutdown while Congress called for further production. In FY2007, the USAF requested $1.6 billion (~$ in ) in response to "excessive combat use" on the C-17 fleet. In 2008, USAF General Arthur Lichte, Commander of Air Mobility Command, indicated before a House of Representatives subcommittee on air and land forces a need to extend production to another 15 aircraft to increase the total to 205, and that C-17 production may continue to satisfy airlift requirements. The USAF finally decided to cap its C-17 fleet at 223 aircraft; the final delivery was on 12 September 2013.

In 2015, as part of a missile-defense test at Wake Island, simulated medium-range ballistic missiles were launched from C-17s against THAAD missile defense systems and the USS John Paul Jones (DDG-53). In early 2020, palletized munitions–"Combat Expendable Platforms"– were tested from C-17s and C-130Js with results the USAF considered positive. In 2021, the Air Force Research Laboratory further developed the concept into the Rapid Dragon system which transforms the C-17 into a lethal cruise missile arsenal ship capable of mass launching 45 JASSM-ER with 500 kg warheads from a standoff distance of 925 km. Future anticipated improvements includes support for JDAM-ER, mine laying, drone dispersal as well as improved standoff range when full production of the 1900 km JASSM-XR delivers large inventories in 2024.

On 15 August 2021, USAF C-17 02-1109 from the 62nd Airlift Wing and 446th Airlift Wing at Joint Base Lewis-McChord departed Hamid Karzai International Airport in Kabul, Afghanistan, while crowds of people trying to escape the 2021 Taliban offensive ran alongside the aircraft. The C-17 lifted off with people holding on to the outside, and at least two died after falling from the aircraft. There were an unknown number possibly crushed and killed by the landing gear retracting, with human remains found in the landing-gear stowage. Also that day, C-17 01-0186 from the 816th Expeditionary Airlift Squadron at Al Udeid Air Base transported 823 Afghan citizens from Hamid Karzai International Airport on a single flight, setting a new record for the type which was previously over 670 people during a 2013 typhoon evacuation from Tacloban, Philippines.

Royal Air Force
Boeing marketed the C-17 to many European nations including Belgium, Germany, France, Italy, Spain and the United Kingdom. The Royal Air Force (RAF) has established an aim of having interoperability and some weapons and capabilities commonality with the USAF. The 1998 Strategic Defence Review identified a requirement for a strategic airlifter. The Short-Term Strategic Airlift competition commenced in September of that year, but the tender was canceled in August 1999 with some bids identified by ministers as too expensive, including the Boeing/BAe C-17 bid, and others unsuitable. The project continued, with the C-17 seen as the favorite. In the light of Airbus A400M delays, the UK Secretary of State for Defence, Geoff Hoon, announced in May 2000 that the RAF would lease four C-17s at an annual cost of £100 million from Boeing for an initial seven years with an optional two-year extension. The RAF had the option to buy or return the aircraft to Boeing. The UK committed to upgrading its C-17s in line with the USAF so that if they were returned, the USAF could adopt them. The lease agreement restricted the C-17's operational use, meaning that the RAF could not use them for para-drop, airdrop, rough field, low-level operations and air to air refueling.

The first C-17 was delivered to the RAF at Boeing's Long Beach facility on 17 May 2001 and flown to RAF Brize Norton by a crew from No. 99 Squadron. The RAF's fourth C-17 was delivered on 24 August 2001. The RAF aircraft were some of the first to take advantage of the new center wing fuel tank found in Block 13 aircraft. In RAF service, the C-17 has not been given an official service name and designation (for example, C-130J referred to as Hercules C4 or C5), but is referred to simply as the C-17 or "C-17A Globemaster". Although it was to be a fallback for the A400M, the Ministry of Defence (MoD) announced on 21 July 2004 that they had elected to buy their four C-17s at the end of the lease, though the A400M appeared to be closer to production. The C-17 gives the RAF strategic capabilities that it would not wish to lose, for example a maximum payload of 169500 lb compared to the A400M's 82000 lb. The C-17's capabilities allow the RAF to use it as an airborne hospital for medical evacuation missions.

Another C-17 was ordered in August 2006, and delivered on 22 February 2008. The four leased C-17s were to be purchased later in 2008. Due to fears that the A400M may suffer further delays, the MoD announced in 2006 that it planned to acquire three more C-17s, for a total of eight, with delivery in 2009–2010. On 3 December 2007, the MoD announced a contract for a sixth C-17, which was received on 11 June 2008. On 18 December 2009, Boeing confirmed that the RAF had ordered a seventh C-17, which was delivered on 16 November 2010. The UK announced the purchase of its eighth C-17 in February 2012. The RAF showed interest in buying a ninth C-17 in November 2013.

On 13 January 2013, the RAF deployed two C-17s from RAF Brize Norton to the French Évreux Air Base, transporting French armored vehicles to the Malian capital of Bamako during the French intervention in Mali. In June 2015, an RAF C-17 was used to medically evacuate four victims of the 2015 Sousse attacks from Tunisia. On 13 September 2022, C-17 ZZ177 carried the body of Queen Elizabeth II from Edinburgh Airport to RAF Northolt in London. She had been lying in state at St Giles' Cathedral in Edinburgh, Scotland.

Royal Australian Air Force

The Royal Australian Air Force (RAAF) began investigating an acquisition of strategic transport aircraft in 2005. In late 2005, the then Minister for Defence Robert Hill stated that such aircraft were being considered due to the limited availability of strategic airlift aircraft from partner nations and air freight companies. The C-17 was considered to be favored over the A400M as it was a "proven aircraft" and in production. One major RAAF requirement was the ability to airlift the Army's M1 Abrams tanks; another requirement was immediate delivery. Though unstated, commonality with the USAF and the RAF was also considered advantageous. RAAF aircraft were ordered directly from the USAF production run and are identical to American C-17s even in paint scheme, the only difference being the national markings, allowing deliveries to commence within nine months of commitment to the program.

On 2 March 2006, the Australian government announced the purchase of three aircraft and one option with an entry into service date of 2006. In July 2006, Boeing was awarded a fixed price contract to deliver four C-17s for US$780M (A$1bn). Australia also signed a US$80.7M contract to join the global 'virtual fleet' C-17 sustainment program; RAAF C-17s receive the same upgrades as the USAF's fleet.

The RAAF took delivery of its first C-17 in a ceremony at Boeing's plant at Long Beach, California on 28 November 2006. Several days later the aircraft flew from Hickam Air Force Base, Hawaii to Defence Establishment Fairbairn, Canberra, arriving on 4 December 2006. The aircraft was formally accepted in a ceremony at Fairbairn shortly after arrival. The second aircraft was delivered to the RAAF on 11 May 2007 and the third was delivered on 18 December 2007. The fourth Australian C-17 was delivered on 19 January 2008. All the Australian C-17s are operated by No. 36 Squadron and are based at RAAF Base Amberley in Queensland.

On 18 April 2011, Boeing announced that Australia had signed an agreement with the U.S. government to acquire a fifth C-17 due to an increased demand for humanitarian and disaster relief missions. The aircraft was delivered to the RAAF on 14 September 2011. On 23 September 2011, Australian Minister for Defence Materiel Jason Clare announced that the government was seeking information from the U.S. about the price and delivery schedule for a sixth Globemaster. In November 2011, Australia requested a sixth C-17 through the U.S. Foreign Military Sales program; it was ordered in June 2012, and was delivered on 1 November 2012.

In August 2014, Defence Minister David Johnston announced the intention to purchase one or two additional C-17s. On 3 October 2014, Johnston announced the government's approval to buy two C-17s at a total cost of US$770M (A$1bn). The United States Congress approved the sale under the Foreign Military Sales program. Prime Minister Tony Abbott confirmed in April 2015 that two additional aircraft were to be ordered, with both delivered by 4 November 2015; these added to the six C-17s it had.

Royal Canadian Air Force

The Canadian Armed Forces had a long-standing need for strategic airlift for military and humanitarian operations around the world. It had followed a pattern similar to the German Air Force in leasing Antonovs and Ilyushins for many requirements, including deploying the Disaster Assistance Response Team to tsunami-stricken Sri Lanka in 2005; the Canadian Forces had relied entirely on leased An-124 Ruslan for a Canadian Army deployment to Haiti in 2003. A combination of leased Ruslans, Ilyushins and USAF C-17s was also used to move heavy equipment to Afghanistan. In 2002, the Canadian Forces Future Strategic Airlifter Project began to study alternatives, including long-term leasing arrangements.

On 5 July 2006, the Canadian government issued a notice of intent to negotiate with Boeing to procure four airlifters for the Canadian Forces Air Command (Royal Canadian Air Force after August 2011). On 1 February 2007, Canada awarded a contract for four C-17s with delivery beginning in August 2007. Like Australia, Canada was granted airframes originally slated for the USAF to accelerate delivery. The official Canadian designation is CC-177 Globemaster III.

On 23 July 2007, the first Canadian C-17 made its initial flight. It was turned over to Canada on 8 August, and participated at the Abbotsford International Airshow on 11 August prior to arriving at its new home base at 8 Wing, CFB Trenton, Ontario on 12 August. Its first operational mission was to deliver disaster relief to Jamaica following Hurricane Dean that month. The last of the initial four aircraft was delivered in April 2008. On 19 December 2014, it was reported that Canada intended to purchase one more C-17. On 30 March 2015, Canada's fifth C-17 arrived at CFB Trenton. The aircraft are assigned to 429 Transport Squadron based at CFB Trenton.

On 14 April 2010, a Canadian C-17 landed for the first time at CFS Alert, the world's most northerly airport. Canadian Globemasters have been deployed in support of numerous missions worldwide, including Operation Hestia after the earthquake in Haiti, providing airlift as part of Operation Mobile and support to the Canadian mission in Afghanistan. After Typhoon Haiyan hit the Philippines in 2013, Canadian C-17s established an air bridge between the two nations, deploying Canada's DART and delivering humanitarian supplies and equipment. In 2014, they supported Operation Reassurance and Operation Impact.

Strategic Airlift Capability program

At the 2006 Farnborough Airshow, a number of NATO member nations signed a letter of intent to jointly purchase and operate several C-17s within the Strategic Airlift Capability (SAC). SAC members are Bulgaria, Estonia, Hungary, Lithuania, the Netherlands, Norway, Poland, Romania, Slovenia, the U.S., along with two Partnership for Peace countries Finland and Sweden as of 2010. The purchase was for two C-17s, and a third was contributed by the U.S. On 14 July 2009, Boeing delivered the first C-17 under the SAC program. The second and third C-17s were delivered in September and October 2009.

The SAC C-17s are based at Pápa Air Base, Hungary. The Heavy Airlift Wing is hosted by Hungary, which acts as the flag nation. The aircraft are crewed in similar fashion as the NATO E-3 AWACS aircraft. The C-17 flight crew are multi-national, but each mission is assigned to an individual member nation based on the SAC's annual flight hour share agreement. The NATO Airlift Management Programme Office (NAMPO) provides management and support for the Heavy Airlift Wing. NAMPO is a part of the NATO Support Agency (NSPA). In September 2014, Boeing stated that the three C-17s supporting SAC missions had achieved a readiness rate of nearly 94 percent over the last five years and supported over 1,000 missions.

Indian Air Force
In June 2009, the Indian Air Force (IAF) selected the C-17 for its Very Heavy Lift Transport Aircraft requirement to replace several types of transport aircraft. In January 2010, India requested 10 C-17s through the U.S.'s Foreign Military Sales program, the sale was approved by Congress in June 2010. On 23 June 2010, the IAF successfully test-landed a USAF C-17 at the Gaggal Airport, India to complete the IAF's C-17 trials. In February 2011, the IAF and Boeing agreed terms for the order of 10 C-17s with an option for six more; the US$4.1 billion order was approved by the Indian Cabinet Committee on Security on 6 June 2011. Deliveries began in June 2013 and were to continue to 2014. In 2012, the IAF reportedly finalized plans to buy six more C-17s in its five-year plan for 2017–2022.

It provides strategic airlift, the ability to deploy special forces, and to operate in diverse terrain – from Himalayan air bases in North India at 13000 ft to Indian Ocean bases in South India. The C-17s are based at Hindon Air Force Station and are operated by No. 81 Squadron IAF Skylords. The first C-17 was delivered in January 2013 for testing and training; it was officially accepted on 11 June 2013. The second C-17 was delivered on 23 July 2013 and put into service immediately. IAF Chief of Air Staff Norman AK Browne called it "a major component in the IAF's modernization drive" while taking delivery of the aircraft at Boeing's Long Beach factory. On 2 September 2013, the Skylords squadron with three C-17s officially entered IAF service.

The Skylords regularly fly missions within India, such as to high-altitude bases at Leh and Thoise. The IAF first used the C-17 to transport an infantry battalion's equipment to Port Blair on Andaman Islands on 1 July 2013. Foreign deployments to date include Tajikistan in August 2013, and Rwanda to support Indian peacekeepers. One C-17 was used for transporting relief materials during Cyclone Phailin.

The sixth aircraft was received in July 2014. In June 2017, the U.S. Department of State approved the potential sale of one C-17 to India under a proposed $366 million (~$ in ) U.S. Foreign Military Sale. This aircraft, the last C-17 produced, increased the IAF's fleet to 11 C-17s. In March 2018, a contract was awarded for completion by 22 August 2019.

On 7 February 2023, an IAF C-17 delivered humanitarian aid packages for earthquake victims in Turkey and Syria by taking a detour around Pakistan's airspace in the aftermath of 2021 Taliban takeover of Afghanistan.

An IAF C-17 executed a precision airdrop of two Combat Rubberised Raiding Craft along with a platoon of 8 MARCOS commandos in an operation to rescue the ex-MV Ruen, a Maltese-flagged cargo ship hijacked by Somali pirates in December 2023. The mission was conducted on 16 March 2024 in a 10 hour round trip mission to an area 2600 km away from the Indian coast. The ship was being used as a mothership for piracy. In a joint operation carried out with the Indian Navy assets such as P-8I Neptune maritime patrol aircraft, SeaGuardian drones, destroyer INS Kolkata and patrol vessel INS Subhadra, the IAF C-17 airdropped Navy's MARCOS commandos, who boarded the hijacked ship, rescued 17 sailors and disarmed 35 pirates in the operation.

Qatar

Boeing delivered Qatar's first C-17 on 11 August 2009 and the second on 10 September 2009 for the Qatar Emiri Air Force. Qatar received its third C-17 in 2012, and fourth C-17 was received on 10 December 2012. In June 2013, The New York Times reported that Qatar was allegedly using its C-17s to ship weapons from Libya to the Syrian opposition during the civil war via Turkey. On 15 June 2015, it was announced at the Paris Airshow that Qatar agreed to order four additional C-17s from the five remaining "white tail" C-17s to double Qatar's C-17 fleet. One Qatari C-17 bears the civilian markings of government-owned Qatar Airways, although the airplane is owned and operated by the Qatar Emiri Air Force. The head of Qatar's airlift selection committee, Ahmed Al-Malki, said the paint scheme was "to build awareness of Qatar's participation in operations around the world."

United Arab Emirates
In February 2009, the United Arab Emirates Air Force agreed to buy four C-17s. In January 2010, a contract was signed for six C-17s. In May 2011, the first C-17 was handed over and the final was received in June 2012.

Kuwait

Kuwait requested the purchase of one C-17 in September 2010 and a second in April 2013 through the U.S.'s Foreign Military Sales (FMS) program. The nation ordered two C-17s; the first was delivered on 13 February 2014.

Proposed operators
In 2015, New Zealand's Minister of Defence, Gerry Brownlee, was considering the purchase of two C-17s for the Royal New Zealand Air Force at an estimated cost of $600 million as a heavy air transport option. However, the New Zealand Government eventually decided not to acquire the C-17.

Variants

 * C-17A: Initial military airlifter version.
 * C-17A "ER": Unofficial name for C-17As with extended range due to the addition of the center wing tank. This upgrade was incorporated in production beginning in 2001 with Block 13 aircraft.
 * Block 16: This software/hardware upgrade was a major improvement of the improved Onboard Inert Gas-Generating System (OBIGGS II), a new weather radar, an improved stabilizer strut system and other avionics.
 * Block 21: Adds ADS-B capability, IFF modification, communication/navigation upgrades and improved flight management.
 * C-17B: A proposed tactical airlifter version with double-slotted flaps, an additional main landing gear on the center fuselage, more powerful engines, and other systems for shorter landing and take-off distances. Boeing offered the C-17B to the U.S. military in 2007 for carrying the Army's Future Combat Systems (FCS) vehicles and other equipment.
 * KC-17: Proposed tanker variant of the C-17.
 * MD-17: Proposed variant for US airlines participating in the Civil Reserve Air Fleet, later redesignated as BC-17X after 1997 merger.

Operators

 * Royal Australian Air Force – 8 C-17A ERs in service as of Jan. 2018.
 * No. 36 Squadron
 * Royal Canadian Air Force – 5 CC-177 (C-17A ER) aircraft in use as of Jan. 2018.
 * 429 Transport Squadron, CFB Trenton
 * Indian Air Force – 11 C-17s as of Aug. 2019.
 * No. 81 Squadron (Skylords), Hindon AFS
 * Kuwait Air Force – 2 C-17s as of Jan. 2018
 * 🇪🇺 Europe
 * The multi-nation Strategic Airlift Capability Heavy Airlift Wing – 3 C-17s in service as of Jan. 2018, including 1 C-17 contributed by the USAF; based at Pápa Air Base, Hungary.
 * Qatar Emiri Air Force – 8 C-17As in use as of Jan. 2018
 * United Arab Emirates Air Force – 8 C-17As in operation as of Jan. 2018
 * Royal Air Force – 8 C-17A ERs in use as of Jan. 2018
 * No. 99 Squadron, RAF Brize Norton
 * United States Air Force – 222 C-17s in service (157 Active, 47 Air National Guard, 18 Air Force Reserve)
 * 60th Air Mobility Wing – Travis Air Force Base, California
 * 21st Airlift Squadron
 * 62d Airlift Wing – McChord AFB, Washington
 * 4th Airlift Squadron
 * 7th Airlift Squadron
 * 8th Airlift Squadron
 * 10th Airlift Squadron - (2003–2016)
 * 305th Air Mobility Wing – McGuire Air Force Base, New Jersey
 * 6th Airlift Squadron
 * 385th Air Expeditionary Group – Al Udeid Air Base, Qatar
 * 816th Expeditionary Airlift Squadron
 * 436th Airlift Wing – Dover Air Force Base, Delaware
 * 3d Airlift Squadron
 * 437th Airlift Wing – Charleston Air Force Base, South Carolina
 * 14th Airlift Squadron
 * 15th Airlift Squadron
 * 16th Airlift Squadron
 * 17th Airlift Squadron - (1993–2015)
 * 3d Wing – Elmendorf Air Force Base, Alaska
 * 517th Airlift Squadron (Associate)
 * 15th Wing – Hickam Air Force Base, Hawaii
 * 535th Airlift Squadron
 * 97th Air Mobility Wing – Altus AFB, Oklahoma
 * 58th Airlift Squadron
 * 412th Test Wing – Edwards AFB, California
 * 418th Flight Test Squadron
 * Air Force Reserve
 * 315th Airlift Wing (Associate) – Charleston AFB, South Carolina
 * 300th Airlift Squadron
 * 317th Airlift Squadron
 * 701st Airlift Squadron
 * 349th Air Mobility Wing (Associate) – Travis AFB, California
 * 301st Airlift Squadron
 * 445th Airlift Wing – Wright-Patterson AFB, Ohio
 * 89th Airlift Squadron
 * 446th Airlift Wing (Associate) – McChord AFB, Washington
 * 97th Airlift Squadron
 * 313th Airlift Squadron
 * 728th Airlift Squadron
 * 452d Air Mobility Wing – March ARB, California
 * 729th Airlift Squadron
 * 507th Air Refueling Wing – Tinker AFB, Oklahoma
 * 730th Air Mobility Training Squadron (Altus AFB)
 * 512th Airlift Wing (Associate) – Dover AFB, Delaware
 * 326th Airlift Squadron
 * 514th Air Mobility Wing (Associate) – McGuire AFB, New Jersey
 * 732d Airlift Squadron
 * 911th Airlift Wing – Pittsburgh Air Reserve Station, Pennsylvania
 * 758th Airlift Squadron
 * Air National Guard
 * 105th Airlift Wing – Stewart ANGB, New York
 * 137th Airlift Squadron
 * 145th Airlift Wing – Charlotte Air National Guard Base, North Carolina
 * 156th Airlift Squadron
 * 154th Wing – Hickam AFB, Hawaii
 * 204th Airlift Squadron (Associate)
 * 164th Airlift Wing – Memphis ANGB, Tennessee
 * 155th Airlift Squadron
 * 167th Airlift Wing – Shepherd Field ANGB, West Virginia
 * 167th Airlift Squadron
 * 172d Airlift Wing – Allen C. Thompson Field ANGB, Mississippi
 * 183d Airlift Squadron
 * 176th Wing – Elmendorf AFB, Alaska
 * 144th Airlift Squadron
 * 164th Airlift Wing – Memphis ANGB, Tennessee
 * 155th Airlift Squadron
 * 167th Airlift Wing – Shepherd Field ANGB, West Virginia
 * 167th Airlift Squadron
 * 172d Airlift Wing – Allen C. Thompson Field ANGB, Mississippi
 * 183d Airlift Squadron
 * 176th Wing – Elmendorf AFB, Alaska
 * 144th Airlift Squadron

Accidents and notable incidents

 * On 10 September 1998, a USAF C-17 (AF Serial No.96-0006) delivered Keiko the whale to Vestmannaeyjar, Iceland, a 3800 ft runway, and suffered a landing gear failure during landing. There were no injuries, but the landing gear sustained major damage.
 * On 10 December 2003, a USAF C-17 (AF Serial No. 98-0057) was hit by a surface-to-air missile after take-off from Baghdad, Iraq. One engine was disabled and the aircraft returned for a safe landing. It was repaired and returned to service.
 * On 6 August 2005, a USAF C-17 (AF Serial No. 01-0196) ran off the runway at Bagram Air Base in Afghanistan while attempting to land, destroying its nose and main landing gear. After two months making it flightworthy, a test pilot flew the aircraft to Boeing's Long Beach facility as the temporary repairs imposed performance limitations. In October 2006, it returned to service following repairs.
 * On 30 January 2009, a USAF C-17 (AF Serial No. 96-0002 – "Spirit of the Air Force") made a gear-up landing at Bagram Air Base. It was ferried from Bagram AB, making several stops along the way, to Boeing's Long Beach plant for extensive repairs. The USAF Aircraft Accident Investigation Board concluded the cause was the crew's failure to follow the pre-landing checklist and lower the landing gear.
 * On 28 July 2010, a USAF C-17 (AF Serial No. 00-0173 – "Spirit of the Aleutians") crashed at Elmendorf Air Force Base, Alaska, while practicing for the 2010 Arctic Thunder Air Show, killing all four aboard. It crashed near a railroad, disrupting rail operations. A military investigation found pilot error caused a stall. This is the C-17's only fatal crash and the only hull loss accident.
 * On 23 January 2012, a USAF C-17 (AF Serial No. 07-7189), assigned to the 437th Airlift Wing, Joint Base Charleston, South Carolina, landed on runway 34R at Forward Operating Base Shank, Afghanistan. The crew did not realize the required stopping distance exceeded the runway's length thus were unable to stop. It came to rest approximately 700 feet from the runway's end upon an embankment, causing major structural damage but no injuries. After 9 months of repairs to make it airworthy, the C-17 flew to Long Beach. It returned to service at a reported cost of $69.4 million.
 * On 20 July 2012, a USAF C-17 of the 305th Air Mobility Wing, flying from McGuire AFB, New Jersey, to MacDill Air Force Base in Tampa, Florida, mistakenly landed at nearby Peter O. Knight Airport, a small municipal field without a control tower, with Gen. Jim Mattis, then commander of CENTCOM on board. After a few hours, the Globemaster took off from the airport's 3580 ft runway without incident and made the short trip to MacDill AFB. The mistaken landing followed an extended duration flight from Europe to Southwest Asia to embark military passengers before returning to the U.S. The USAF investigation attributed the incident to fatigue leading to pilot error, as both airfields' main runways share the same magnetic heading and are only four miles apart along the shore of Tampa Bay.
 * On 9 April 2021, USAF C-17 10-0223 suffered a fire in its undercarriage after landing at Charleston AFB following a flight from RAF Mildenhall, UK. The fire spread to the fuselage before it was extinguished.

title: Boeing C-17 Globemaster III


text:
Caber

Caber can refer to:

 * Caber toss, a sport

Places

 * Caber, Çivril, a village in Çivril District, Denizli Province, Turkey
 * Caber, Sarayköy, a village in Sarayköy District, Denizli Province, Turkey
 * Çabër, a village in Zubin Potok, Mitrovica district, Kosovo

Other uses

 * CaBER, Capillary Breakup Extensional Rheometer
 * Caber (comics), a deity in Marvel Comics
 * Caber Music, a British record label founded in 1998 by Tom Bancroft

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Caber


text:
Talk:Garbage collection (computer science)

List of Garbage Collection features, by Language
So I was hunting around the internet looking for an exhaustive list of programming languages and a description of their Garbage Collection scheme, and perhaps a list showing which languages do and do not have GC.

Being acclimated to the instantaneous gratification that the internet often brings, I expected that I would, in a few minutes be staring at one of those fancy colored tables on a wikipedia article.

Alas, after an hour my search never bore fruit. This table does not exist!

I can't even find it on websites, let alone wikipedia. Now several hours into this research, I have come away with a little more knowledge than I had before. This new knowledge has convinced me that such a colored table is needed more than ever before. I may end up being the author myself.

It would be unfortunate for someone to work through the first six chapters of a book on R, only to find out in chapter 7 (surprise mothef___r), R has GC, but it is not automatic, and has to be implemented by linking a third-party library and calling bizarre library functions in this complicated way ( as detailed by this blog in the dark recesses of the internet. )

Compile-time quasi-GC
Then there is this issue with the language Rust. Rust has no GC in the manner of say, Java, where it runs as a separate thread doing magic behind the scenes. Rust therefore allows pointers, call-by-reference, and allocation and free-ing a la C. However, the compiler does some manner of sorcery that checks for plausible memory leaks and dangling pointers right at compile time. If a table existed, the entry on Rust would say red "NO" to "has GC". In the column for "compile-time pointer checks" it would say "YES", in green.

Stop The World
JVM languages (Clojure, Scala, et al) have a GC which will halt all threads while the GC comes in and sweeps the RAM clean -- a task often referred to as a "Stop-the-World" event. STWs happen erratically and the application programmer has no control over when, if, or how long the stop takes to complete before handing control back to the program threads. While most of us could care less, this would be a deal-breaker for a realtime game. (imagine a stubborn STW event kicks in, in the middle of a cutscene. The player would think he has crashed, and would reach for the reset button on his Playstation 4. ) Anyways, on our theoretical table, there would be a column labelled "GC has Stop-the-World events" and there would be a green "YES" on Java's entry.

Sweep and Reference Counting
Some GCs utilized Mark-and-sweep, while other GCs use "Reference counting", or perhaps "Tracing without MandS". There may even exist languages that use a strange mixture of these (Python?). Our theoretical table should contain a column for these kinds, with YES's and NO's in appropriate places.

In the meanwhile, here is the existing wikipedia article on Garbage Collection. There is no table there, and no link to any other article with such a useful table.

It is time to write one. Let's do this. 7 December 2018 Miloserdia (talk) 23:33, 7 December 2018 (UTC)

syntactical vs. semantical garbage
In Reachability of an object both terms (syntactical and semantical garbage) are used, which might be confusing the reader. If both mean the same, the alternative term could be introduced.

split
This article has a garbage collection (computer science) section. I see that another article has a section on real-time garbage collection ( tracing garbage collection ). Would it be better to create a stand-alone "real-time garbage collection" article to discuss such algorithms, and convert both of those sections to short WP:SUMMARY of that article? --DavidCary (talk) 17:38, 8 December 2015 (UTC)

Move vailable location
Can you add "The grabage collector move the memory locations of variables, so it causes limitations using pointers, which cause inconvenience for programming." into the Disadventage section? --60.251.198.234 (talk) 02:01, 24 May 2016 (UTC)


text:
Garbage collection (computer science)

In computer science, garbage collection (GC) is a form of automatic memory management. The garbage collector attempts to reclaim memory that was allocated by the program, but is no longer referenced; such memory is called garbage. Garbage collection was invented by American computer scientist John McCarthy around 1959 to simplify manual memory management in Lisp.

Garbage collection relieves the programmer from doing manual memory management, where the programmer specifies what objects to de-allocate and return to the memory system and when to do so. Other, similar techniques include stack allocation, region inference, and memory ownership, and combinations thereof. Garbage collection may take a significant proportion of a program's total processing time, and affect performance as a result.

Resources other than memory, such as network sockets, database handles, windows, file descriptors, and device descriptors, are not typically handled by garbage collection, but rather by other methods (e.g. destructors). Some such methods de-allocate memory also.

Overview
Many programming languages require garbage collection, either as part of the language specification (e.g., RPL, Java, C#, D, Go, and most scripting languages) or effectively for practical implementation (e.g., formal languages like lambda calculus). These are said to be garbage-collected languages. Other languages, such as C and C++, were designed for use with manual memory management, but have garbage-collected implementations available. Some languages, like Ada, Modula-3, and C++/CLI, allow both garbage collection and manual memory management to co-exist in the same application by using separate heaps for collected and manually managed objects. Still others, like D, are garbage-collected but allow the user to manually delete objects or even disable garbage collection entirely when speed is required.

Although many languages integrate GC into their compiler and runtime system, post-hoc GC systems also exist, such as Automatic Reference Counting (ARC). Some of these post-hoc GC systems do not require recompilation.

Advantages
GC frees the programmer from manually de-allocating memory. This helps avoid some kinds of errors:

 * Dangling pointers, which occur when a piece of memory is freed while there are still pointers to it, and one of those pointers is dereferenced. By then the memory may have been reassigned to another use, with unpredictable results.
 * Double free bugs, which occur when the program tries to free a region of memory that has already been freed, and perhaps already been allocated again.
 * Certain kinds of memory leaks, in which a program fails to free memory occupied by objects that have become unreachable, which can lead to memory exhaustion.

Disadvantages
GC uses computing resources to decide which memory to free. Therefore, the penalty for the convenience of not annotating object lifetime manually in the source code is overhead, which can impair program performance. A peer-reviewed paper from 2005 concluded that GC needs five times the memory to compensate for this overhead and to perform as fast as the same program using idealized explicit memory management. The comparison however is made to a program generated by inserting deallocation calls using an oracle, implemented by collecting traces from programs run under a profiler, and the program is only correct for one particular execution of the program. Interaction with memory hierarchy effects can make this overhead intolerable in circumstances that are hard to predict or to detect in routine testing. The impact on performance was given by Apple as a reason for not adopting garbage collection in iOS, despite it being the most desired feature.

The moment when the garbage is actually collected can be unpredictable, resulting in stalls (pauses to shift/free memory) scattered throughout a session. Unpredictable stalls can be unacceptable in real-time environments, in transaction processing, or in interactive programs. Incremental, concurrent, and real-time garbage collectors address these problems, with varying trade-offs.

Tracing
Tracing garbage collection is the most common type of garbage collection, so much so that "garbage collection" often refers to tracing garbage collection, rather than other methods such as reference counting. The overall strategy consists of determining which objects should be garbage collected by tracing which objects are reachable by a chain of references from certain root objects, and considering the rest as garbage and collecting them. However, there are a large number of algorithms used in implementation, with widely varying complexity and performance characteristics.

Reference counting
Reference counting garbage collection is where each object has a count of the number of references to it. Garbage is identified by having a reference count of zero. An object's reference count is incremented when a reference to it is created and decremented when a reference is destroyed. When the count reaches zero, the object's memory is reclaimed.

As with manual memory management, and unlike tracing garbage collection, reference counting guarantees that objects are destroyed as soon as their last reference is destroyed, and usually only accesses memory which is either in CPU caches, in objects to be freed, or directly pointed to by those, and thus tends to not have significant negative side effects on CPU cache and virtual memory operation.

There are a number of disadvantages to reference counting; this can generally be solved or mitigated by more sophisticated algorithms:

 * Cycles: If two or more objects refer to each other, they can create a cycle whereby neither will be collected as their mutual references never let their reference counts become zero. Some garbage collection systems using reference counting (like the one in CPython) use specific cycle-detecting algorithms to deal with this issue. Another strategy is to use weak references for the "backpointers" which create cycles. Under reference counting, a weak reference is similar to a weak reference under a tracing garbage collector. It is a special reference object whose existence does not increment the reference count of the referent object. Furthermore, a weak reference is safe in that when the referent object becomes garbage, any weak reference to it lapses, rather than being permitted to remain dangling, meaning that it turns into a predictable value, such as a null reference.

 * Space overhead (reference count): Reference counting requires space to be allocated for each object to store its reference count. The count may be stored adjacent to the object's memory or in a side table somewhere else, but in either case, every single reference-counted object requires additional storage for its reference count. Memory space with the size of an unsigned pointer is commonly used for this task, meaning that 32 or 64 bits of reference count storage must be allocated for each object. On some systems, it may be possible to mitigate this overhead by using a tagged pointer to store the reference count in unused areas of the object's memory. Often, an architecture does not actually allow programs to access the full range of memory addresses that could be stored in its native pointer size; a certain number of high bits in the address is either ignored or required to be zero. If an object reliably has a pointer at a certain location, the reference count can be stored in the unused bits of the pointer. For example, each object in Objective-C has a pointer to its class at the beginning of its memory; on the ARM64 architecture using iOS 7, 19 unused bits of this class pointer are used to store the object's reference count.

 * Speed overhead (increment/decrement): In naive implementations, each assignment of a reference and each reference falling out of scope often require modifications of one or more reference counters. However, in a common case when a reference is copied from an outer scope variable into an inner scope variable, such that the lifetime of the inner variable is bounded by the lifetime of the outer one, the reference incrementing can be eliminated. The outer variable "owns" the reference. In the programming language C++, this technique is readily implemented and demonstrated with the use of  references. Reference counting in C++ is usually implemented using "smart pointers" whose constructors, destructors, and assignment operators manage the references. A smart pointer can be passed by reference to a function, which avoids the need to copy-construct a new smart pointer (which would increase the reference count on entry into the function and decrease it on exit). Instead, the function receives a reference to the smart pointer which is produced inexpensively. The Deutsch-Bobrow method of reference counting capitalizes on the fact that most reference count updates are in fact generated by references stored in local variables. It ignores these references, only counting references in the heap, but before an object with reference count zero can be deleted, the system must verify with a scan of the stack and register that no other reference to it still exists. A further substantial decrease in the overhead on counter updates can be obtained by update coalescing introduced by Levanoni and Petrank.  Consider a pointer that in a given interval of the execution is updated several times. It first points to an object , then to an object  , and so forth until at the end of the interval it points to some object  . A reference counting algorithm would typically execute  ,  ,  ,  ,  , ...,  . But most of these updates are redundant. In order to have the reference count properly evaluated at the end of the interval it is enough to perform   and  . Levanoni and Petrank measured an elimination of more than 99% of the counter updates in typical Java benchmarks.

 * Requires atomicity: When used in a multithreaded environment, these modifications (increment and decrement) may need to be atomic operations such as compare-and-swap, at least for any objects which are shared, or potentially shared among multiple threads. Atomic operations are expensive on a multiprocessor, and even more expensive if they have to be emulated with software algorithms. It is possible to avoid this issue by adding per-thread or per-CPU reference counts and only accessing the global reference count when the local reference counts become or are no longer zero (or, alternatively, using a binary tree of reference counts, or even giving up deterministic destruction in exchange for not having a global reference count at all), but this adds significant memory overhead and thus tends to be only useful in special cases (it is used, for example, in the reference counting of Linux kernel modules). Update coalescing by Levanoni and Petrank can be used to eliminate all atomic operations from the write-barrier. Counters are never updated by the program threads in the course of program execution. They are only modified by the collector which executes as a single additional thread with no synchronization. This method can be used as a stop-the-world mechanism for parallel programs, and also with a concurrent reference counting collector.

 * Not real-time: Naive implementations of reference counting do not generally provide real-time behavior, because any pointer assignment can potentially cause a number of objects bounded only by total allocated memory size to be recursively freed while the thread is unable to perform other work. It is possible to avoid this issue by delegating the freeing of unreferenced objects to other threads, at the cost of extra overhead.

Escape analysis
Escape analysis is a compile-time technique that can convert heap allocations to stack allocations, thereby reducing the amount of garbage collection to be done. This analysis determines whether an object allocated inside a function is accessible outside of it. If a function-local allocation is found to be accessible to another function or thread, the allocation is said to "escape" and cannot be done on the stack. Otherwise, the object may be allocated directly on the stack and released when the function returns, bypassing the heap and associated memory management costs.

Availability
Generally speaking, higher-level programming languages are more likely to have garbage collection as a standard feature. In some languages lacking built-in garbage collection, it can be added through a library, as with the Boehm garbage collector for C and C++.

Most functional programming languages, such as ML, Haskell, and APL, have garbage collection built in. Lisp is especially notable as both the first functional programming language and the first language to introduce garbage collection.

Other dynamic languages, such as Ruby and Julia (but not Perl 5 or PHP before version 5.3, which both use reference counting), JavaScript and ECMAScript also tend to use GC. Object-oriented programming languages such as Smalltalk, RPL and Java usually provide integrated garbage collection. Notable exceptions are C++ and Delphi, which have destructors.

BASIC
BASIC and Logo have often used garbage collection for variable-length data types, such as strings and lists, so as not to burden programmers with memory management details. On the Altair 8800, programs with many string variables and little string space could cause long pauses due to garbage collection. Similarly the Applesoft BASIC interpreter's garbage collection algorithm repeatedly scans the string descriptors for the string having the highest address in order to compact it toward high memory, resulting in $O(n^2)$ performance and pauses anywhere from a few seconds to a few minutes. A replacement garbage collector for Applesoft BASIC by Randy Wigginton identifies a group of strings in every pass over the heap, reducing collection time dramatically. BASIC.SYSTEM, released with ProDOS in 1983, provides a windowing garbage collector for BASIC that is many times faster.

Objective-C
While the Objective-C traditionally had no garbage collection, with the release of OS X 10.5 in 2007 Apple introduced garbage collection for Objective-C 2.0, using an in-house developed runtime collector. However, with the 2012 release of OS X 10.8, garbage collection was deprecated in favor of LLVM's automatic reference counter (ARC) that was introduced with OS X 10.7. Furthermore, since May 2015 Apple even forbade the usage of garbage collection for new OS X applications in the App Store. For iOS, garbage collection has never been introduced due to problems in application responsivity and performance; instead, iOS uses ARC.

Limited environments
Garbage collection is rarely used on embedded or real-time systems because of the usual need for very tight control over the use of limited resources. However, garbage collectors compatible with many limited environments have been developed. The Microsoft .NET Micro Framework, .NET nanoFramework and Java Platform, Micro Edition are embedded software platforms that, like their larger cousins, include garbage collection.

Java
Garbage collectors available in Java JDKs include:
 * G1
 * Parallel
 * Concurrent mark sweep collector (CMS)
 * Serial
 * C4 (Continuously Concurrent Compacting Collector)
 * Shenandoah
 * ZGC

Compile-time use
Compile-time garbage collection is a form of static analysis allowing memory to be reused and reclaimed based on invariants known during compilation.

This form of garbage collection has been studied in the Mercury programming language, and it saw greater usage with the introduction of LLVM's automatic reference counter (ARC) into Apple's ecosystem (iOS and OS X) in 2011.

Real-time systems
Incremental, concurrent, and real-time garbage collectors have been developed, for example by Henry Baker and by Henry Lieberman.

In Baker's algorithm, the allocation is done in either half of a single region of memory. When it becomes half full, a garbage collection is performed which moves the live objects into the other half and the remaining objects are implicitly deallocated. The running program (the 'mutator') has to check that any object it references is in the correct half, and if not move it across, while a background task is finding all of the objects.

Generational garbage collection schemes are based on the empirical observation that most objects die young. In generational garbage collection, two or more allocation regions (generations) are kept, which are kept separate based on the object's age. New objects are created in the "young" generation that is regularly collected, and when a generation is full, the objects that are still referenced from older regions are copied into the next oldest generation. Occasionally a full scan is performed.

Some high-level language computer architectures include hardware support for real-time garbage collection.

Most implementations of real-time garbage collectors use tracing. Such real-time garbage collectors meet hard real-time constraints when used with a real-time operating system.

title: Garbage collection (computer science)


text:
Canidae

Canidae (from Latin, canis, "dog") is a biological family of dog-like carnivorans, colloquially referred to as dogs, and constitutes a clade. A member of this family is also called a canid. The family includes three subfamilies: the Caninae, and the extinct Borophaginae and Hesperocyoninae. The Caninae are known as canines, and include domestic dogs, wolves, coyotes, foxes, jackals and other species.

Canids are found on all continents except Antarctica, having arrived independently or accompanied by human beings over extended periods of time. Canids vary in size from the 2 m gray wolf to the 24 cm fennec fox. The body forms of canids are similar, typically having long muzzles, upright ears, teeth adapted for cracking bones and slicing flesh, long legs, and bushy tails. They are mostly social animals, living together in family units or small groups and behaving co-operatively. Typically, only the dominant pair in a group breeds and a litter of young are reared annually in an underground den. Canids communicate by scent signals and vocalizations. One canid, the domestic dog, originated from a symbiotic relationship with Upper Paleolithic humans and today remains one of the most widely kept domestic animals.

Taxonomy
In the history of the carnivores, the family Canidae is represented by the two extinct subfamilies designated as Hesperocyoninae and Borophaginae, and the extant subfamily Caninae. This subfamily includes all living canids and their most recent fossil relatives. All living canids as a group form a dental monophyletic relationship with the extinct borophagines, with both groups having a bicuspid (two points) on the lower carnassial talonid, which gives this tooth an additional ability in mastication. This, together with the development of a distinct entoconid cusp and the broadening of the talonid of the first lower molar, and the corresponding enlargement of the talon of the upper first molar and reduction of its parastyle distinguish these late Cenozoic canids and are the essential differences that identify their clade.

The cat-like feliformia and dog-like Caniforms emerged within the Carnivoramorpha around 45–42 Mya (million years ago). The Canidae first appeared in North America during the Late Eocene (37.8-33.9 Mya). They did not reach Eurasia until the Late Miocene or to South America until the Late Pliocene.

Phylogenetic relationships
This cladogram shows the phylogenetic position of canids within Caniformia, based on fossil finds:

Evolution
The Canidae today includes a diverse group of some 37 species ranging in size from the maned wolf with its long limbs to the short-legged bush dog. Modern canids inhabit forests, tundra, savannahs, and deserts throughout tropical and temperate parts of the world. The evolutionary relationships between the species have been studied in the past using morphological approaches, but more recently, molecular studies have enabled the investigation of phylogenetics relationships. In some species, genetic divergence has been suppressed by the high level of gene flow between different populations and where the species have hybridized, large hybrid zones exist.

Eocene epoch
Carnivorans evolved after the extinction of the non-avian dinosaurs 66 million years ago. Around 50 million years ago, or earlier, in the Paleocene, the carnivorans split into two main divisions: caniforms (dog-like) and feliforms (cat-like). By 40 Mya, the first identifiable member of the dog family had arisen. Named Prohesperocyon wilsoni, its fossilized remains have been found in what is now the southwestern part of Texas. The chief features which identify it as a canid include the loss of the upper third molar (part of a trend toward a more shearing bite), and the structure of the middle ear which has an enlarged bulla (the hollow bony structure protecting the delicate parts of the ear). Prohesperocyon probably had slightly longer limbs than its predecessors, and also had parallel and closely touching toes which differ markedly from the splayed arrangements of the digits in bears.

The canid family soon subdivided into three subfamilies, each of which diverged during the Eocene: Hesperocyoninae (about 39.74–15 Mya), Borophaginae (about 34–32 Mya), and Caninae (about 34–30 Mya). The Caninae are the only surviving subfamily and all present-day canids, including wolves, foxes, coyotes, jackals, and domestic dogs. Members of each subfamily showed an increase in body mass with time and some exhibited specialized hypercarnivorous diets that made them prone to extinction.

Oligocene epoch
By the Oligocene, all three subfamilies of canids (Hesperocyoninae, Borophaginae, and Caninae) had appeared in the fossil records of North America. The earliest and most primitive branch of the Canidae was the Hesperocyoninae lineage, which included the coyote-sized Mesocyon of the Oligocene (38–24 Mya). These early canids probably evolved for the fast pursuit of prey in a grassland habitat; they resembled modern viverrids in appearance. Hesperocyonines eventually became extinct in the middle Miocene. One of the early members of the Hesperocyonines, the genus Hesperocyon, gave rise to Archaeocyon and Leptocyon. These branches led to the borophagine and canine radiations.

Miocene epoch
Around 8 Mya, the Beringian land bridge allowed members of the genus Eucyon a means to enter Asia from North America and they continued on to colonize Europe.

Pliocene epoch
The Canis, Urocyon, and Vulpes genera developed from canids from North America, where the canine radiation began. The success of these canines was related to the development of lower carnassials that were capable of both mastication and shearing. Around 5 million years ago, some of the Old World Eucyon evolved into the first members of Canis, During the Pliocene, around 4–5 Mya, Canis lepophagus appeared in North America. This was small and sometimes coyote-like. Others were wolf-like in characteristics. C. latrans (the coyote) is theorized to have descended from C. lepophagus.

The formation of the Isthmus of Panama, about 3 Mya, joined South America to North America, allowing canids to invade South America, where they diversified. However, the most recent common ancestor of the South American canids lived in North America some 4 Mya and more than one incursion across the new land bridge is likely given the fact that more than one lineage is present in South America. Two North American lineages found in South America are the gray fox (Urocyon cinereoargentus) and the now-extinct dire wolf (Aenocyon dirus). Besides these, there are species endemic to South America: the maned wolf (Chrysocyon brachyurus), the short-eared dog (Atelocynus microtis), the bush dog (Speothos venaticus), the crab-eating fox (Cerdocyon thous), and the South American foxes (Lycalopex spp.). The monophyly of this group has been established by molecular means.

Pleistocene epoch
During the Pleistocene, the North American wolf line appeared, with Canis edwardii, clearly identifiable as a wolf, and Canis rufus appeared, possibly a direct descendant of C. edwardii. Around 0.8 Mya, Canis ambrusteri emerged in North America. A large wolf, it was found all over North and Central America and was eventually supplanted by the dire wolf, which then spread into South America during the Late Pleistocene.

By 0.3 Mya, a number of subspecies of the gray wolf (C. lupus) had developed and had spread throughout Europe and northern Asia. The gray wolf colonized North America during the late Rancholabrean era across the Bering land bridge, with at least three separate invasions, with each one consisting of one or more different Eurasian gray wolf clades. MtDNA studies have shown that there are at least four extant C. lupus lineages. The dire wolf shared its habitat with the gray wolf, but became extinct in a large-scale extinction event that occurred around 11,500 years ago. It may have been more of a scavenger than a hunter; its molars appear to be adapted for crushing bones and it may have gone extinct as a result of the extinction of the large herbivorous animals on whose carcasses it relied.

In 2015, a study of mitochondrial genome sequences and whole-genome nuclear sequences of African and Eurasian canids indicated that extant wolf-like canids have colonized Africa from Eurasia at least five times throughout the Pliocene and Pleistocene, which is consistent with fossil evidence suggesting that much of African canid fauna diversity resulted from the immigration of Eurasian ancestors, likely coincident with Plio-Pleistocene climatic oscillations between arid and humid conditions. When comparing the African and Eurasian golden jackals, the study concluded that the African specimens represented a distinct monophyletic lineage that should be recognized as a separate species, Canis anthus (African golden wolf). According to a phylogeny derived from nuclear sequences, the Eurasian golden jackal (Canis aureus) diverged from the wolf/coyote lineage 1.9 Mya, but the African golden wolf separated 1.3 Mya. Mitochondrial genome sequences indicated the Ethiopian wolf diverged from the wolf/coyote lineage slightly prior to that.

Wild canids are found on every continent except Antarctica, and inhabit a wide range of different habitats, including deserts, mountains, forests, and grasslands. They vary in size from the fennec fox, which may be as little as 24 cm in length and weigh 0.6 kg, to the gray wolf, which may be up to 160 cm long, and can weigh up to 79 kg. Only a few species are arboreal—the gray fox, the closely related island fox and the raccoon dog habitually climb trees.

All canids have a similar basic form, as exemplified by the gray wolf, although the relative length of muzzle, limbs, ears, and tail vary considerably between species. With the exceptions of the bush dog, the raccoon dog and some domestic dog breeds, canids have relatively long legs and lithe bodies, adapted for chasing prey. The tails are bushy and the length and quality of the pelage vary with the season. The muzzle portion of the skull is much more elongated than that of the cat family. The zygomatic arches are wide, there is a transverse lambdoidal ridge at the rear of the cranium and in some species, a sagittal crest running from front to back. The bony orbits around the eye never form a complete ring and the auditory bullae are smooth and rounded. Females have three to seven pairs of mammae.

All canids are digitigrade, meaning they walk on their toes. The tip of the nose is always naked, as are the cushioned pads on the soles of the feet. These latter consist of a single pad behind the tip of each toe and a more-or-less three-lobed central pad under the roots of the digits. Hairs grow between the pads and in the Arctic fox the sole of the foot is densely covered with hair at some times of the year. With the exception of the four-toed African wild dog (Lycaon pictus), five toes are on the forefeet, but the pollex (thumb) is reduced and does not reach the ground. On the hind feet are four toes, but in some domestic dogs, a fifth vestigial toe, known as a dewclaw, is sometimes present, but has no anatomical connection to the rest of the foot. In some species, slightly curved nails are non-retractile and more-or-less blunt while other species have sharper, partially-retractile claws.

The canine penis contains a baculum and a structure called the bulbus glandis that expands during copulation, forming a copulatory tie that lasts for up to an hour. Young canids are born blind, with their eyes opening a few weeks after birth. All living canids (Caninae) have a ligament analogous to the nuchal ligament of ungulates used to maintain the posture of the head and neck with little active muscle exertion; this ligament allows them to conserve energy while running long distances following scent trails with their nose to the ground. However, based on skeletal details of the neck, at least some of the Borophaginae (such as Aelurodon) are believed to have lacked this ligament.

Dentition
Dentition relates to the arrangement of teeth in the mouth, with the dental notation for the upper-jaw teeth using the upper-case letters I to denote incisors, C for canines, P for premolars, and M for molars, and the lower-case letters i, c, p and m to denote the mandible teeth. Teeth are numbered using one side of the mouth and from the front of the mouth to the back. In carnivores, the upper premolar P4 and the lower molar m1 form the carnassials that are used together in a scissor-like action to shear the muscle and tendon of prey.

Canids use their premolars for cutting and crushing except for the upper fourth premolar P4 (the upper carnassial) that is only used for cutting. They use their molars for grinding except for the lower first molar m1 (the lower carnassial) that has evolved for both cutting and grinding depending on the canid's dietary adaptation. On the lower carnassial, the trigonid is used for slicing and the talonid is used for grinding. The ratio between the trigonid and the talonid indicates a carnivore's dietary habits, with a larger trigonid indicating a hypercarnivore and a larger talonid indicating a more omnivorous diet. Because of its low variability, the length of the lower carnassial is used to provide an estimate of a carnivore's body size.

A study of the estimated bite force at the canine teeth of a large sample of living and fossil mammalian predators, when adjusted for their body mass, found that for placental mammals the bite force at the canines was greatest in the extinct dire wolf (163), followed among the modern canids by the four hypercarnivores that often prey on animals larger than themselves: the African wild dog (142), the gray wolf (136), the dhole (112), and the dingo (108). The bite force at the carnassials showed a similar trend to the canines. A predator's largest prey size is strongly influenced by its biomechanical limits.

Most canids have 42 teeth, with a dental formula of:. The bush dog has only one upper molar with two below, the dhole has two above and two below. and the bat-eared fox has three or four upper molars and four lower ones. The molar teeth are strong in most species, allowing the animals to crack open bone to reach the marrow. The deciduous, or baby teeth, formula in canids is, molars being completely absent.

Social behavior

Almost all canids are social animals and live together in groups. In general, they are territorial or have a home range and sleep in the open, using their dens only for breeding and sometimes in bad weather. In most foxes, and in many of the true dogs, a male and female pair work together to hunt and to raise their young. Gray wolves and some of the other larger canids live in larger groups called packs. African wild dogs have packs which may consist of 20 to 40 animals and packs of fewer than about seven individuals may be incapable of successful reproduction. Hunting in packs has the advantage that larger prey items can be tackled. Some species form packs or live in small family groups depending on the circumstances, including the type of available food. In most species, some individuals live on their own. Within a canid pack, there is a system of dominance so that the strongest, most experienced animals lead the pack. In most cases, the dominant male and female are the only pack members to breed.

Communication
Canids communicate with each other by scent signals, by visual clues and gestures, and by vocalizations such as growls, barks, and howls. In most cases, groups have a home territory from which they drive out other conspecifics. Canids use urine scent marks to mark their food caches or warn trespassing individuals. Social behavior is also mediated by secretions from glands on the upper surface of the tail near its root and from the anal glands, preputial glands, and supracaudal glands.

Reproduction
Canids as a group exhibit several reproductive traits that are uncommon among mammals as a whole. They are typically monogamous, provide paternal care to their offspring, have reproductive cycles with lengthy proestral and dioestral phases and have a copulatory tie during mating. They also retain adult offspring in the social group, suppressing the ability of these to breed while making use of the alloparental care they can provide to help raise the next generation of offspring. Most canid species are spontaneous ovulators, although maned wolves are induced ovulators.

During the proestral period, increased levels of estradiol make the female attractive to the male. There is a rise in progesterone during the estral phase when female is receptive. Following this, the level of estradiol fluctuates and there is a lengthy dioestrous phase during which the female is pregnant. Pseudo-pregnancy frequently occurs in canids that have ovulated but failed to conceive. A period of anestrus follows pregnancy or pseudo-pregnancy, there being only one oestral period during each breeding season. Small and medium-sized canids mostly have a gestation period of 50 to 60 days, while larger species average 60 to 65 days. The time of year in which the breeding season occurs is related to the length of day, as has been demonstrated in the case of several species that have been translocated across the equator to the other hemisphere and experiences a six-month shift of phase. Domestic dogs and certain small canids in captivity may come into oestrus more frequently, perhaps because the photoperiod stimulus breaks down under conditions of artificial lighting. Canids have an oestrus period of 1 to 20 days, lasting one week in most species.

The size of a litter varies, with from one to 16 or more pups being born. The young are born small, blind and helpless and require a long period of parental care. They are kept in a den, most often dug into the ground, for warmth and protection. When the young begin eating solid food, both parents, and often other pack members, bring food back for them from the hunt. This is most often vomited up from the adult's stomach. Where such pack involvement in the feeding of the litter occurs, the breeding success rate is higher than is the case where females split from the group and rear their pups in isolation. Young canids may take a year to mature and learn the skills they need to survive. In some species, such as the African wild dog, male offspring usually remain in the natal pack, while females disperse as a group and join another small group of the opposite sex to form a new pack.

Canids and humans

One canid, the domestic dog, entered into a partnership with humans a long time ago. The dog was the first domesticated species. The archaeological record shows the first undisputed dog remains buried beside humans 14,700 years ago, with disputed remains occurring 36,000 years ago. These dates imply that the earliest dogs arose in the time of human hunter-gatherers and not agriculturists.

The fact that wolves are pack animals with cooperative social structures may have been the reason that the relationship developed. Humans benefited from the canid's loyalty, cooperation, teamwork, alertness and tracking abilities, while the wolf may have benefited from the use of weapons to tackle larger prey and the sharing of food. Humans and dogs may have evolved together.

Among canids, only the gray wolf has widely been known to prey on humans. Nonetheless, at least two records of coyotes killing humans have been published, and at least two other reports of golden jackals killing children. Human beings have trapped and hunted some canid species for their fur and some, especially the gray wolf, the coyote and the red fox, for sport. Canids such as the dhole are now endangered in the wild because of persecution, habitat loss, a depletion of ungulate prey species and transmission of diseases from domestic dogs.

title: Canidae


text:
Subspecies of Canis lupus

There are 38 subspecies of Canis lupus listed in the taxonomic authority Mammal Species of the World (2005, 3rd edition). These subspecies were named over the past 250 years, and since their naming, a number of them have gone extinct. The nominate subspecies is the Eurasian wolf (Canis lupus lupus).

Taxonomy
In 1758, the Swedish botanist and zoologist Carl Linnaeus published in his Systema Naturae the binomial nomenclature – or the two-word naming – of species. Canis is the Latin word meaning "dog", and under this genus he listed the dog-like carnivores including domestic dogs, wolves, and jackals. He classified the domestic dog as Canis familiaris, and on the next page he classified the wolf as Canis lupus. Linnaeus considered the dog to be a separate species from the wolf because of its head, body, and cauda recurvata – its upturning tail – which is not found in any other canid.

In 1999, a study of mitochondrial DNA indicated that the domestic dog may have originated from multiple wolf populations, with the dingo and New Guinea singing dog "breeds" having developed at a time when human populations were more isolated from each other. In the third edition of Mammal Species of the World published in 2005, the mammalogist W. Christopher Wozencraft listed under the wolf Canis lupus some 36 wild subspecies, and proposed two additional subspecies: familiaris Linnaeus, 1758 and dingo Meyer, 1793. Wozencraft included hallstromi – the New Guinea singing dog – as a taxonomic synonym for the dingo. Wozencraft referred to the mDNA study as one of the guides in forming his decision, and listed the 38 subspecies under the biological common name of "wolf", with the nominate subspecies being the Eurasian wolf (Canis lupus lupus) based on the type specimen that Linnaeus studied in Sweden. However, the classification of several of these canines as either species or subspecies has recently been challenged.

List of extant subspecies
Living subspecies recognized by MSW3 and divided into Old World and New World:

Eurasia and Australasia
Sokolov and Rossolimo (1985) recognised nine Old World subspecies of wolf. These were C. l. lupus, C. l. albus, C. l. pallipes, C. l. cubanensis, C. l. campestris, C. l. chanco, C. l. desortorum, C. l. hattai, and C. l. hodophilax. In his 1995 statistical analysis of skull morphometrics, mammalogist Robert Nowak recognized the first four of those subspecies, synonymized campestris, chanco and desortorum with C. l. lupus, but did not examine the two Japanese subspecies. In addition, he recognized C. l. communis as a subspecies distinct from C. l. lupus. In 2003, Nowak also recognized the distinctiveness of C. l. arabs, C. l. hattai, C. l. italicus, and C. l. hodophilax. In 2005, MSW3 included C. l. filchneri. In 2003, two forms were distinguished in southern China and Inner Mongolia as being separate from C. l. chanco and C. l. filchneri and have yet to be named.

North America

For North America, in 1944 the zoologist Edward Goldman recognized as many as 23 subspecies based on morphology. In 1959, E. Raymond Hall proposed that there had been 24 subspecies of lupus in North America. In 1970, L. David Mech proposed that there was "probably far too many subspecific designations...in use", as most did not exhibit enough points of differentiation to be classified as separate subspecies. The 24 subspecies were accepted by many authorities in 1981 and these were based on morphological or geographical differences, or a unique history. In 1995, the American mammologist Robert M. Nowak analyzed data on the skull morphology of wolf specimens from around the world. For North America, he proposed that there were only five subspecies of the wolf. These include a large-toothed Arctic wolf named C. l. arctos, a large wolf from Alaska and western Canada named C. l. occidentalis, a small wolf from southeastern Canada named C. l. lycaon, a small wolf from the southwestern U.S. named C. l. baileyi and a moderate-sized wolf that was originally found from Texas to Hudson Bay and from Oregon to Newfoundland named C. l. nubilus.

The taxonomic classification of Canis lupus in Mammal Species of the World (3rd edition, 2005) listed 27 subspecies of North American wolf, corresponding to the 24 Canis lupus subspecies and the three Canis rufus subspecies of Hall (1981). The table below shows the extant subspecies, with the extinct ones listed in the following section.

List of extinct subspecies
Subspecies recognized by MSW3 which have gone extinct over the past 150 years:

Subspecies discovered since the publishing of MSW3 in 2005 which have gone extinct over the past 150 years:

Global
In 2019, a workshop hosted by the IUCN/SSC Canid Specialist Group considered the New Guinea singing dog and the dingo to be feral dogs (Canis familiaris). In 2020, a literature review of canid domestication stated that modern dogs were not descended from the same Canis lineage as modern wolves, and proposed that dogs may be descended from a Pleistocene wolf closer in size to a village dog. In 2021, the American Society of Mammalogists also considered dingos a feral dog (Canis familiaris) population.

Italian wolf
The Italian wolf (or Apennine wolf) was first recognised as a distinct subspecies (Canis lupus italicus) in 1921 by zoologist Giuseppe Altobello. Altobello's classification was later rejected by several authors, including Reginald Innes Pocock, who synonymised C. l. italicus with C. l. lupus. In 2002, the noted paleontologist R.M. Nowak reaffirmed the morphological distinctiveness of the Italian wolf and recommended the recognition of Canis lupus italicus. A number of DNA studies have found the Italian wolf to be genetically distinct. In 2004, the genetic distinction of the Italian wolf subspecies was supported by analysis which consistently assigned all the wolf genotypes of a sample in Italy to a single group. This population also showed a unique mitochondrial DNA control-region haplotype, the absence of private alleles and lower heterozygosity at microsatellite loci, as compared to other wolf populations. In 2010, a genetic analysis indicated that a single wolf haplotype (w22) unique to the Apennine Peninsula and one of the two haplotypes (w24, w25), unique to the Iberian Peninsula, belonged to the same haplogroup as the prehistoric wolves of Europe. Another haplotype (w10) was found to be common to the Iberian peninsula and the Balkans. These three populations with geographic isolation exhibited a near lack of gene flow and spatially correspond to three glacial refugia.

The taxonomic reference Mammal Species of the World (3rd edition, 2005) does not recognize Canis lupus italicus; however, NCBI/Genbank publishes research papers under that name.

Iberian wolf
The Iberian wolf was first recognised as a distinct subspecies (Canis lupus signatus) in 1907 by zoologist Ángel Cabrera. The wolves of the Iberian peninsula have morphologically distinct features from other Eurasian wolves and each are considered by their researchers to represent their own subspecies.

The taxonomic reference Mammal Species of the World (3rd edition, 2005) does not recognize Canis lupus signatus; however, NCBI/Genbank does list it.

Himalayan wolf
The Himalayan wolf is distinguished by its mitochondrial DNA, which is basal to all other wolves. The taxonomic name of this wolf is disputed, with the species Canis himalayensis being proposed based on two limited DNA studies. In 2017, a study of mitochondrial DNA, X-chromosome (maternal lineage) markers and Y-chromosome (male lineage) markers found that the Himalayan wolf was genetically basal to the Holarctic grey wolf and has an association with the African golden wolf.

In 2019, a workshop hosted by the IUCN/SSC Canid Specialist Group noted that the Himalayan wolf's distribution included the Himalayan range and the Tibetan Plateau. The group recommends that this wolf lineage be known as the "Himalayan wolf" and classified as Canis lupus chanco until a genetic analysis of the holotypes is available. In 2020, further research on the Himalayan wolf found that it warranted species-level recognition under the Unified Species Concept, the Differential Fitness Species Concept, and the Biological Species Concept. It was identified as an Evolutionary Significant Unit that warranted assignment onto the IUCN Red List for its protection.

Indian plains wolf
The Indian plains wolf is a proposed clade within the Indian wolf (Canis lupus pallipes) that is distinguished by its mitochondrial DNA, which is basal to all other wolves except for the Himalayan wolf. The taxonomic status of this wolf clade is disputed, with the separate species Canis indica being proposed based on two limited DNA studies. The proposal has not been endorsed because it relied on a limited number of museum and zoo samples that may not have been representative of the wild population, and a call for further fieldwork has been made.

The taxonomic reference Mammal Species of the World (3rd edition, 2005) does not recognize Canis indica; however, NCBI/Genbank lists it as a new subspecies, Canis lupus indica.

Southern Chinese wolf
In 2017, a comprehensive study found that the gray wolf was present across all of mainland China, both in the past and today. It exists in southern China, which refutes claims made by some researchers in the Western world that the wolf had never existed in southern China. This wolf has not been taxonomically classified.

In 2019, a genomic study on the wolves of China included museum specimens of wolves from southern China that were collected between 1963 and 1988. The wolves in the study formed three clades: northern Asian wolves that included those from northern China and eastern Russia, Himalayan wolves from the Tibetan Plateau, and a unique population from southern China. One specimen from Zhejiang Province in eastern China shared gene flow with the wolves from southern China; however, its genome was 12–14 percent admixed with a canid that may be the dhole or an unknown canid that predates the genetic divergence of the dhole. The wolf population from southern China is believed to still exist in that region.

Coastal wolves
A study of the three coastal wolves indicates a close phylogenetic relationship across regions that are geographically and ecologically contiguous, and the study proposed that Canis lupus ligoni (the Alexander Archipelago wolf), Canis lupus columbianus (the British Columbian wolf), and Canis lupus crassodon (the Vancouver Coastal Sea wolf) should be recognized as a single subspecies of Canis lupus, synonymized as Canis lupus crassodon. They share the same habitat and prey species, and form one study's six identified North American ecotypes – a genetically and ecologically distinct population separated from other populations by their different types of habitat.

Eastern wolf
The eastern wolf has two proposals over its origin. One is that the eastern wolf is a distinct species (C. lycaon) that evolved in North America, as opposed to the gray wolf that evolved in the Old World, and is related to the red wolf. The other is that it is derived from admixture between gray wolves, which inhabited the Great Lakes area and coyotes, forming a hybrid that was classified as a distinct species by mistake.

The taxonomic reference Mammal Species of the World (3rd edition, 2005) does not recognize Canis lycaon; however, NCBI/Genbank does list it. In 2021, the American Society of Mammalogists also considered Canis lycaon a valid species.

Red wolf
The red wolf is an enigmatic taxon, of which there are two proposals over its origin. One is that the red wolf is a distinct species (C. rufus) that has undergone human-influenced admixture with coyotes. The other is that it was never a distinct species but was derived from past admixture between coyotes and gray wolves, due to the gray wolf population being eliminated by humans.

The taxonomic reference Mammal Species of the World (3rd edition, 2005) does not recognize Canis rufus; however, NCBI/Genbank does list it. In 2021, the American Society of Mammalogists also considered Canis rufus a valid species.

title: Subspecies of Canis lupus