IOPL Section: Linux Programmer's Manual (2) Updated: 24 July 2025 Index   NAME iopl - change I/O privilege level   SYNOPSIS #include <unistd.h> /* for libc5 */ #include <sys/io.h> /* for glibc */ int iopl(int level);   DESCRIPTION iopl changes the I/O privilege level of the current process, as specified 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 ioperm 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 inherited by fork and exec. The I/O privilege level for a normal process is 0.   RETURN VALUE On success, zero is returned. On error, -1 is returned, and errno is set appropriately.   ERRORS EINVAL level is greater than 3. EPERM The current user is not the super-user.   NOTES FROM THE KERNEL SOURCE iopl has to be used when you want to access the I/O ports beyond the 0x3ff range: to get the full 65536 ports bitmapped you'd need 8kB of bitmaps/process, which is a bit excessive.   CONFORMING TO iopl is Linux specific and should not be used in processes 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   Index NAME SYNOPSIS DESCRIPTION RETURN VALUE ERRORS NOTES FROM THE KERNEL SOURCE CONFORMING TO NOTES SEE ALSO This document was created by using the manual pages. Time: 11:56:06 GMT, March 24, 2025