Here are some pointers about C programming on Plan 9.
/sys/doc/comp.ps
/sys/doc/compiler.ps
2c, 6c, 8c, etc all refer to the same compiler suite, but each compiles for a different target architecture. The compiler suite is sometimes referred to as kencc. The language itself is sometimes referred to as 9c.
If an ANSI C compiler is needed, use pcc.
/sys/doc/ape.ps