This family of functions is used to do low level port input and output. They are primarily designed for internal kernel use, but can be used from user space, given the following information in addition to that given in outb(9)?
You compile with -O or -O2 or similar. The functions are defined as inline macros, and will not be substituted in without optimization enabled, causing unresolved references at link time.
outb and friends are hardware specific. The port and value arguments are in the opposite order to most DOS implementations.