Routines for accessing I/O ports are in /usr/include/asm/io.h (or linux/include/asm-i386/io.h in the kernel source distribution). The routines there are inline macros, so it is enough to #include ; you do not need any additional libraries.
Before you access any ports, you must give your program permission to do so by calling the ioperm() function (declared in unistd.h) before any I/O port accesses. The syntax is ioperm(from, num, turn_on), where from is the first port number to give access to, and num the number of consecutive ports to give access to. The last argument is a Boolean value specifying whether to give access to the program to the ports (true (1)) or to remove access (false (0)). ioperm() can only give access to ports 0x000 through 0x3ff; for higher ports, you need to use iopl() (which gives you access to all ports at once).
Accessing the ports
To input a byte (8 bits) from a port, call inb(port), it returns the byte it got. To output a byte, call outb(value, port) (please note the order of the parameters).
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