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RedHat 9 (Linux i386) - man page for ioperm (redhat section 2)

IOPERM(2)						     Linux Programmer's Manual							 IOPERM(2)

NAME
ioperm - set port input/output permissions
SYNOPSIS
#include <unistd.h> /* for libc5 */ #include <sys/io.h> /* for glibc */ int ioperm(unsigned long from, unsigned long num, int turn_on);
DESCRIPTION
Ioperm sets the port access permission bits for the process for num bytes starting from port address from to the value turn_on. The use of ioperm requires root privileges. Only the first 0x3ff I/O ports can be specified in this manner. For more ports, the iopl function must be used. Permissions are not inherited on fork, but on exec they are. This is useful for giving port access permissions to non-privileged tasks.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
CONFORMING TO
ioperm is Linux specific and should not be used in programs intended to be portable.
NOTES
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.
SEE ALSO
iopl(2) Linux 1993-01-21 IOPERM(2)