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Test Your Knowledge in Computers #599
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For modern C programming, defining functions is a two step process. First, you declare a function and its signature using a prototype. Then you define the function by providing a function body that defines what the function does.
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iopl(2) [linux man page]

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

NAME
iopl - change I/O privilege level SYNOPSIS
#include <sys/io.h> int iopl(int level); DESCRIPTION
iopl() changes the I/O privilege level of the calling 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(2) 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(2) and execve(2). The I/O privilege level for a normal process is 0. This call is mostly for the i386 architecture. On many other architectures it does not exist or will always return an error. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
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. 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
ioperm(2), capabilities(7) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2004-05-27 IOPL(2)

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

NAME
iopl - change I/O privilege level SYNOPSIS
#include <sys/io.h> int iopl(int level); DESCRIPTION
iopl() changes the I/O privilege level of the calling process, as specified by the two least significant bits 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(2) 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 not inherited by the child process created by fork(2) and are not preserved across execve(2) (but see NOTES). The I/O privilege level for a normal process is 0. This call is mostly for the i386 architecture. On many other architectures it does not exist or will always return an error. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
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 to raise the I/O privilege level above its current value. CONFORMING TO
iopl() is Linux-specific and should not be used in programs that are intended to be portable. NOTES
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 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. SEE ALSO
ioperm(2), outb(2), capabilities(7) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2017-09-15 IOPL(2)