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RedHat 9 (Linux i386) - man page for iopl (redhat section 2)

IOPL(2) 						     Linux Programmer's Manual							   IOPL(2)

iopl - change I/O privilege level
#include <sys/io.h> 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 inter- rupts. 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 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) Linux 0.99.11 1993-07-24 IOPL(2)