inb_p(2) -- Linux man page
NAMEoutb, outw, outl, outsb, outsw, outsl, inb, inw, inl, insb, insw, insl, outb_p, outw_p, outl_p, inb_p, inw_p, inl_p - port I/O
DESCRIPTIONThis family of functions is used to do low level port input and output. The out* functions do port output, the in* functions do port input; the b-suffix functions are byte-width and the w-suffix functions word-width; the _p-suffix functions pause until the I/O completes.
They are primarily designed for internal kernel use, but can be used from user space.
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.
You use ioperm(2) or alternatively iopl(2) to tell the kernel to allow the user space application to access the I/O ports in question. Failure to do this will cause the application to receive a segmentation fault.
CONFORMING TOoutb and friends are hardware specific. The port and value arguments are in the opposite order from most DOS implementations.
SEE ALSOioperm(2), iopl(2)