2. Using I/O ports in C programs
2.1 The normal method
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 <asm/io.h>; you do not need any additional
libraries.
Because of a limitation in gcc (present in all versions I know of,
including egcs), you have to compile any source code that uses
these routines with optimisation turned on (gcc -O1 or higher),
or alternatively use #define extern static before you
#include <asm/io.h> (remember to #undef
externafterwards).
For debugging, you can use gcc -g -O (at least with modern
versions of gcc), though optimisation can sometimes make the debugger
behave a bit strangely. If this bothers you, put the routines that use
I/O port access in a separate source file and compile only that with
optimisation turned on.
Permissions
Before you access any ports, you must give your program permission to
do so. This is done by calling the ioperm() function (declared
in unistd.h, and defined in the kernel) somewhere near the start
of your program (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. For example, ioperm(0x300, 5, 1) would give
access to ports 0x300 through 0x304 (a total of 5 ports). 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)). You
can call ioperm() multiple times to enable multiple
non-consecutive ports. See the ioperm(2) manual page for details
on the syntax.
The ioperm() call requires your program to have root privileges;
thus you need to either run it as the root user, or make it setuid
root. You can drop the root privileges after you have called
ioperm() to enable the ports you want to use. You are not
required to explicitly drop your port access privileges with
ioperm(..., 0) at the end of your program; this is done
automatically as the process exits.
A setuid() to a non-root user does not disable the port access
granted by ioperm(), but a fork() does (the child process
does not get access, but the parent retains it).
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). Use the level argument 3 (i.e., iopl(3)) to
give your program access to all I/O ports (so be careful ---
accessing the wrong ports can do all sorts of nasty things to your
computer). Again, you need root privileges to call iopl(). See
the iopl(2) manual page for details.
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). To input a word (16 bits) from
ports x and x+1 (one byte from each to form the word, using
the assembler instruction inw), call inw(x). To output a
word to the two ports, use outw(value, x). If you're unsure of
which port instructions (byte or word) to use, you probably want
inb() and outb() --- most devices are designed for bytewise
port access. Note that all port access instructions take at least
about a microsecond to execute.
The inb_p(), outb_p(), inw_p(), and outw_p()
macros work otherwise identically to the ones above, but they do an
additional short (about one microsecond) delay after the port access;
you can make the delay about four microseconds with #define
REALLY_SLOW_IO before you #include <asm/io.h>. These
macros normally (unless you #define SLOW_IO_BY_JUMPING, which
is probably less accurate) use a port output to port 0x80 for their
delay, so you need to give access to port 0x80 with ioperm()
first (outputs to port 0x80 should not affect any part of the
system). For more versatile methods of delaying, read on.
There are manual pages for ioperm(2), iopl(2), and the above
macros in reasonably recent releases of the Linux manual page
collection.
2.2 An alternate method: /dev/port
Another way to access I/O ports is to open() /dev/port (a
character device, major number 1, minor 4) for reading and/or writing
(the stdio f*() functions have internal buffering, so avoid
them). Then lseek() to the appropriate byte in the file (file
position 0 = port 0x00, file position 1 = port 0x01, and so on), and
read() or write() a byte or word from or to it.
Naturally, for this to work your program needs read/write access to
/dev/port. This method is probably slower than the normal
method above, but does not need compiler optimisation nor
ioperm(). It doesn't need root access either, if you give a
non-root user or group access to /dev/port --- but this is a
very bad thing to do in terms of system security, since it is possible
to hurt the system, perhaps even gain root access, by using
/dev/port to access hard disks, network cards, etc. directly.
You cannot use select(2) or poll(2) to read /dev/port,
because the hardware does not have a facility for notifying the CPU
when a value in an input port changes.
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