mirror of https://github.com/mkerrisk/man-pages
151 lines
4.5 KiB
Groff
151 lines
4.5 KiB
Groff
.\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
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.\"
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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.\" manual page may be incorrect or out-of-date.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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.\"
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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 1996-04-11 "Linux 1.3.85" "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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.br
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.B #include <linux/unistd.h>
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.br
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.B #include <linux/sysctl.h>
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.br
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.B #include <errno.h>
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.sp
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.B _syscall1(int, _sysctl, struct __sysctl_args *, args)
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/* Using \fBsyscall\fP(2) may be preferable; see \fBintro\fP(2) */
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.sp
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.BI "int _sysctl(struct __sysctl_args *" args );
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.fi
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.SH DESCRIPTION
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The
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.BR _sysctl ()
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call reads and/or writes kernel parameters. For example, the hostname,
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or the maximum number of open files. The argument has the form
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.PP
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.nf
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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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.fi
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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 EXAMPLE
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.nf
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#include <linux/unistd.h>
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#include <linux/types.h>
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#include <linux/sysctl.h>
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_syscall1(int, _sysctl, struct __sysctl_args *, args);
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int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
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void *newval, size_t newlen)
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{
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struct __sysctl_args args={name,nlen,oldval,oldlenp,newval,newlen};
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return _sysctl(&args);
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}
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#define SIZE(x) sizeof(x)/sizeof(x[0])
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#define OSNAMESZ 100
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char osname[OSNAMESZ];
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int osnamelth;
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int name[] = { CTL_KERN, KERN_OSTYPE };
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main(){
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osnamelth = sizeof(osname);
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if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
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perror("sysctl");
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else
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printf("This machine is running %*s\en", osnamelth, osname);
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return 0;
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}
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.fi
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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. 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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.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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.TP
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.B 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 non-zero, or no write permission where
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.I newval
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was non-zero.
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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. It originated in
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4.4BSD. 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 (2)
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function is the same in both.
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.SH BUGS
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The object names vary between kernel versions.
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THIS MAKES THIS SYSTEM CALL WORTHLESS FOR APPLICATIONS.
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Use the
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.I /proc/sys
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interface instead.
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.br
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Not all available objects are properly documented.
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.br
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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 "SEE ALSO"
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.BR proc (5)
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