int iopl(int level);
通常のスレッドの I/O 特権レベルは 0 である。許可 (permissions) は親から子に継承される。
This call is deprecated, is significantly slower than ioperm(2), and is only provided for older X servers which require access to all 65536 I/O ports. It is mostly for the i386 architecture. On many other architectures it does not exist or will always return an error.
Prior to Linux 5.5 iopl() allowed the thread to disable interrupts while running at a higher I/O privilege level. This will probably crash the system, and is not recommended.
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.
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