IOPL(2) Linux Programmer's Manual IOPL(2)
iopl - change I/O privilege level
int iopl(int level);
iopl() changes the I/O privilege level of the calling process, as spec-
ified 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 inter-
rupts. This will probably crash the system, and is not recommended.
Permissions are inherited by fork(2) and execve(2).
The I/O privilege level for a normal process is 0.
This call is mostly for the i386 architecture. On many other architec-
tures it does not exist or will always return an error.
On success, zero is returned. On error, -1 is returned, and errno is
EINVAL level is greater than 3.
ENOSYS This call is unimplemented.
EPERM The calling process has insufficient privilege to call iopl();
the CAP_SYS_RAWIO capability is required.
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
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2004-05-27 IOPL(2)