IOPERM(2) Linux Programmer's Manual IOPERM(2)
ioperm - set port input/output permissions
#include <unistd.h> /* for libc5 */
#include <sys/io.h> /* for glibc */
int ioperm(unsigned long from, unsigned long num, int turn_on);
ioperm() sets the port access permission bits for the calling thread for num bits starting
from port address from. If turn_on is nonzero, then permission for the specified bits is
enabled; otherwise it is disabled. If turn_on is nonzero, the calling thread must be
Before Linux 2.6.8, only the first 0x3ff I/O ports could be specified in this manner. For
more ports, the iopl(2) system call had to be used (with a level argument of 3). Since
Linux 2.6.8, 65,536 I/O ports can be specified.
Permissions are not inherited by the child created by fork(2); following a fork(2) the
child must turn on those permissions that it needs. Permissions are preserved across
execve(2); this is useful for giving port access permissions to unprivileged programs.
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 is set appropriately.
EINVAL Invalid values for from or num.
EIO (on PowerPC) This call is not supported.
ENOMEM Out of memory.
EPERM The calling thread has insufficient privilege.
ioperm() is Linux-specific and should not be used in programs intended to be portable.
The /proc/ioports file shows the I/O ports that are currently allocated on the system.
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 i386 only.
iopl(2), outb(2), capabilities(7)
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Linux 2013-03-12 IOPERM(2)