iopl - change I/O privilege level
iopl(2) changes the I/O privilege level of the current process, 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(2) call is not sufficient.
In addition to granting unrestricted I/O port access, running at a higher I/O privilege level also allows the process 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.
iopl(2) 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 bit excessive.
iopl(2) is Linux specific and should not be used in processes intended to be portable.
Under libc5, the prototype for iopl() was given 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.