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 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
(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 interrupts. This will
probably crash the system, and is not recommended.
Permissions are not inherited by the child process created by fork
are not preserved across execve
(2) (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 errno
- level is greater than 3.
- This call is unimplemented.
- The calling process has insufficient privilege to call
iopl(); the CAP_SYS_RAWIO 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 <sys/io.h>
. 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 fork
(2) and were
preserved across execve
(2). This behavior was inadvertently changed in
Linux 3.7, and won't be reinstated.
This page is part of release 4.13 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at