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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 calling thread, as specified by the two least significant bits in level.

The I/O privilege level for a normal thread is 0. Permissions are inherited from parents to children.

This call is deprecated, is significantly slower than 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.

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

EINVAL
level is greater than 3.
ENOSYS
This call is unimplemented.
EPERM
The calling thread has insufficient privilege to call iopl(); the CAP_SYS_RAWIO capability is required to raise the I/O privilege level above its current value.

iopl() 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> and in <sys/perm.h>. Avoid the latter, it is available on i386 only.

Prior to Linux 5.5 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.

Prior to Linux 3.7, on some architectures (such as i386), permissions were 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.

ioperm(2), outb(2), capabilities(7)

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2020-08-13 Linux