IOPL

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 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 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.

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.

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 bit excessive.

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.

ioperm(2)