.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
.\" Portions extracted from linux/kernel/ioport.c (no copyright notice).
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified Tue Aug  1 16:47    1995 by Jochen Karrer
.\"                              <cip307@cip.physik.uni-wuerzburg.de>
.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Fri Nov 27 14:50:36 CET 1998 by Andries Brouwer <aeb@cwi.nl>
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\"
.TH iopl 2 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
iopl \- change I/O privilege level
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/io.h>
.P
.BI "[[deprecated]] int iopl(int " level );
.fi
.SH DESCRIPTION
.BR iopl ()
changes the I/O privilege level of the calling thread,
as specified by the two least significant bits in
.IR level .
.P
The I/O privilege level for a normal thread is 0.
Permissions are inherited from parents to children.
.P
This call is deprecated, is significantly slower than
.BR ioperm (2),
and is only provided for older X servers which require
access to all 65536 I/O ports.
It is mostly for the i386 architecture.
On many other architectures it does not exist or will always
return an error.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
.I level
is greater than 3.
.TP
.B ENOSYS
This call is unimplemented.
.TP
.B EPERM
The calling thread has insufficient privilege to call
.BR iopl ();
the
.B CAP_SYS_RAWIO
capability is required to raise the I/O privilege level
above its current value.
.SH VERSIONS
.\" Libc5 treats it as a system call and has a prototype in
.\" .IR <unistd.h> .
.\" glibc1 does not have a prototype.
glibc2 has a prototype both in
.I <sys/io.h>
and in
.IR <sys/perm.h> .
Avoid the latter, it is available on i386 only.
.SH STANDARDS
Linux.
.SH HISTORY
Prior to Linux 5.5
.BR iopl ()
allowed the thread to disable interrupts while running
at a higher I/O privilege level.
This will probably crash the system, and is not recommended.
.P
Prior to Linux 3.7,
on some architectures (such as i386), permissions
.I were
inherited by the child produced by
.BR fork (2)
and were preserved across
.BR execve (2).
This behavior was inadvertently changed in Linux 3.7,
and won't be reinstated.
.SH SEE ALSO
.BR ioperm (2),
.BR outb (2),
.BR capabilities (7)