iopl - change I/O privilege level
#include <unistd.h> /* for libc5 */
#include <sys/io.h> /* for glibc */
int iopl(int level);
iopl changes the I/O privilege level of the current pro
cess, as specified in level.
This call is necessary to allow 8514-compatible X servers
to run under Linux. Since these X servers require access
to all 65536 I/O ports, the ioperm call is not sufficient.
In addition to granting unrestricted I/O port access, run
ning at a higher I/O privilege level also allows the pro
cess to disable interrupts. This will probably crash the
system, and is not recommended.
Permissions are inherited by fork and exec.
The I/O privilege level for a normal process is 0.
On success, zero is returned. On error, -1 is returned,
and errno is set appropriately.
EINVAL level is greater than 3.
EPERM The current user is not the super-user.
NOTES FROM THE KERNEL SOURCE
iopl has to be used when you want to access the I/O ports
beyond the 0x3ff range: to get the full 65536 ports
bitmapped you'd need 8kB of bitmaps/process, which is a
iopl is Linux specific and should not be used in processes
intended to be portable.
Libc5 treats it as a system call and has a prototype in
<unistd.h>. Glibc1 does not have a prototype. Glibc2 has
a prototype both in <sys/io.h> and in <sys/perm.h>. Avoid
the latter, it is available on i386 only.