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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 current process, as spec- ified 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 inter- rupts. 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. This call is mostly for the i386 architecture. On many other architec- tures 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 process has insufficient privilege to call iopl(); the CAP_SYS_RAWIO capability is required.


iopl() is Linux specific and should not be used in processes intended to be portable.


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.


ioperm(2), capabilities(7) Linux 2.6.6 2004-05-27 IOPL(2)

1994 Man-cgi 1.15, Panagiotis Christias <>