Dennis Ritchie, C's creator, along with Ken Thompson, Unix' creator, and other Bell Labs people from the Computing Science Research Center thought the C standardization committee was wrong, so the Plan9 C compiler[1,2] is not ANSI compatible. Not only the standard library is nothing alike, the language itself is slightly changed in an incompatible way.

[1] http://cm.bell-labs.com/sys/doc/comp.html

[2] http://cm.bell-labs.com/sys/doc/compiler.html