mirror of https://github.com/mkerrisk/man-pages
182 lines
5.1 KiB
Groff
182 lines
5.1 KiB
Groff
.\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Written 11 April 1996 by Andries Brouwer <aeb@cwi.nl>
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.\" 960412: Added comments from Stephen Tweedie
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.\" Modified Tue Oct 22 22:28:41 1996 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified Mon Jan 5 20:31:04 1998 by aeb.
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.\"
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.TH SYSCTL 2 2017-09-15 "Linux" "Linux Programmer's Manual"
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.SH NAME
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sysctl \- read/write system parameters
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.B #include <linux/sysctl.h>
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.PP
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.BI "int _sysctl(struct __sysctl_args *" args );
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.fi
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.PP
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.IR Note :
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There is no glibc wrapper for this system call; see NOTES.
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.SH DESCRIPTION
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.B Do not use this system call!
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See NOTES.
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.PP
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The
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.BR _sysctl ()
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call reads and/or writes kernel parameters.
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For example, the hostname,
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or the maximum number of open files.
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The argument has the form
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.PP
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.in +4n
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.EX
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struct __sysctl_args {
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int *name; /* integer vector describing variable */
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int nlen; /* length of this vector */
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void *oldval; /* 0 or address where to store old value */
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size_t *oldlenp; /* available room for old value,
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overwritten by actual size of old value */
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void *newval; /* 0 or address of new value */
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size_t newlen; /* size of new value */
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};
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.EE
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.in
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.PP
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This call does a search in a tree structure, possibly resembling
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a directory tree under
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.IR /proc/sys ,
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and if the requested item is found calls some appropriate routine
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to read or modify the value.
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.SH RETURN VALUE
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Upon successful completion,
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.BR _sysctl ()
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returns 0.
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Otherwise, a value of \-1 is returned and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.BR EACCES ", " EPERM
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No search permission for one of the encountered "directories",
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or no read permission where
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.I oldval
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was nonzero, or no write permission where
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.I newval
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was nonzero.
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.TP
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.B EFAULT
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The invocation asked for the previous value by setting
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.I oldval
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non-NULL, but allowed zero room in
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.IR oldlenp .
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.TP
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.B ENOTDIR
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.I name
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was not found.
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.SH CONFORMING TO
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This call is Linux-specific, and should not be used in programs
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intended to be portable.
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A
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.BR sysctl ()
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call has been present in Linux since version 1.3.57.
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It originated in
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4.4BSD.
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Only Linux has the
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.I /proc/sys
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mirror, and the object naming schemes differ between Linux and 4.4BSD,
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but the declaration of the
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.BR sysctl ()
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function is the same in both.
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.SH NOTES
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Glibc does not provide a wrapper for this system call; call it using
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.BR syscall (2).
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Or rather...
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.I don't
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call it:
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use of this system call has long been discouraged,
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and it is so unloved that
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\fBit is likely to disappear in a future kernel version\fP.
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.\" See http://lwn.net/Articles/247243/
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Since Linux 2.6.24,
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uses of this system call result in warnings in the kernel log.
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.\" Though comments in suggest that it is needed by old glibc binaries,
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.\" so maybe it's not going away.
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Remove it from your programs now; use the
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.I /proc/sys
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interface instead.
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.PP
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This system call is available only if the kernel was configured with the
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.B CONFIG_SYSCTL_SYSCALL
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option.
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.SH BUGS
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The object names vary between kernel versions,
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making this system call worthless for applications.
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.PP
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Not all available objects are properly documented.
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.PP
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It is not yet possible to change operating system by writing to
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.IR /proc/sys/kernel/ostype .
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.SH EXAMPLE
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.EX
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#define _GNU_SOURCE
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#include <unistd.h>
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#include <sys/syscall.h>
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#include <string.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <linux/sysctl.h>
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int _sysctl(struct __sysctl_args *args );
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#define OSNAMESZ 100
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int
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main(void)
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{
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struct __sysctl_args args;
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char osname[OSNAMESZ];
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size_t osnamelth;
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int name[] = { CTL_KERN, KERN_OSTYPE };
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memset(&args, 0, sizeof(struct __sysctl_args));
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args.name = name;
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args.nlen = sizeof(name)/sizeof(name[0]);
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args.oldval = osname;
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args.oldlenp = &osnamelth;
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osnamelth = sizeof(osname);
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if (syscall(SYS__sysctl, &args) == \-1) {
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perror("_sysctl");
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exit(EXIT_FAILURE);
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}
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printf("This machine is running %*s\\n", osnamelth, osname);
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exit(EXIT_SUCCESS);
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}
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.EE
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.SH SEE ALSO
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.BR proc (5)
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