Section: Linux Programmer's Manual (2)
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iopl - change I/O privilege level
int iopl(int level);
changes the I/O privilege level of the calling process,
as specified by the two least significant bits in
This call is necessary to allow 8514-compatible X servers to run under
Since these X servers require access to all 65536 I/O ports, the
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 not inherited by the child process created by
and are not preserved across
(but see NOTES).
The I/O privilege level for a normal process is 0.
This call is mostly for the i386 architecture.
On many other architectures it does not exist or will always
return an error.
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
is greater than 3.
This call is unimplemented.
The calling process has insufficient privilege to call
capability is required to raise the I/O privilege level
above its current value.
is Linux-specific and should not be used in programs that are
intended to be portable.
Glibc2 has a prototype both in
Avoid the latter, it is available on i386 only.
Prior to Linux 3.7,
on some architectures (such as i386), permissions
inherited by the child produced by
and were preserved across
This behavior was inadvertently changed in Linux 3.7,
and won't be reinstated.
This page is part of release 4.16 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at