* Futures down: Dow 111 pts, S&P 12.25 pts, Nasdaq 14 pts
By Abhiram Nandakumar
Dec 18 (Reuters) - U.S. stock index futures were lower on Friday as a selloff in commodity markets showed no signs of easing.
* Trading could be more volatile than usual on "quadruple-witching" Friday, when options on stocks and indexes as well as futures on indexes and single stocks expire.
* Crude oil prices were heading for their third straight weekly loss as global oversupply continued to pile pressure. Exxon and Chevron shares were down 0.6 percent in premarket trading.
* Global markets fell on Friday, with investors turning wary about the impact of a stronger dollar and weakening commodity prices on the global economy. U.S. stocks closed lower on Thursday, led by declines in energy and materials shares.
* The dollar index, however, shed some of its post-Fed rate hike gains on Friday, after the Bank of Japan reorganized its stimulus program.
* Carnival Corp shares were up 1.2 percent at $51.06, ahead of its quarterly results before the bell.
* Qihoo 360 was up 2.8 percent at $73.80 after the Chinese mobile security software maker said it would be taken private for about $9.3 billion, including about $1.6 billion in debt.
* Red Hat was up 4.6 percent at $82.50 after it raised its full-year revenue forecast for a second time.
* Richmond Fed President Jeffrey Lacker is scheduled to speak at 12:30 p.m. ET.
Futures snapshot at 7:21 a.m. ET:
* Dow e-minis were down 111 points, or 0.64 percent, with 42,565 contracts changing hands.
* S&P 500 e-minis were down 12.25 points, or 0.61 percent, with 255,726 contracts traded.
* Nasdaq 100 e-minis were down 14 points, or 0.31 percent, on volume of 39,432 contracts. (Reporting by Abhiram Nandakumar and Aastha Agnihotri in Bengaluru; Editing by Don Sebastian)