According to the C99 standard [0], an omitted optional expression-2 in a for loop is replaced by a non-zero constant. Thus, for(;;) is equivalent to for(;1;) which is effectively equivalent to while(1) as there are no other declarations or expressions. And, for those who are counting, it also saves a character.

[0] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf (See 6.8.5.3)