man-pages/man2/sysctl.2

.\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
.\"
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.\" Written 11 April 1996 by Andries Brouwer <aeb@cwi.nl>
.\" 960412: Added comments from Stephen Tweedie
.\" Modified Tue Oct 22 22:28:41 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Mon Jan  5 20:31:04 1998 by aeb.
.\"
.TH SYSCTL 2 2012-12-21 "Linux" "Linux Programmer's Manual"
.SH NAME
sysctl \- read/write system parameters
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.br
.B #include <linux/sysctl.h>
.sp
.BI "int _sysctl(struct __sysctl_args *" args );
.fi

.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.B Do not use this system call!
See NOTES.

The
.BR _sysctl ()
call reads and/or writes kernel parameters.
For example, the hostname,
or the maximum number of open files.
The argument has the form
.PP
.in +4n
.nf
struct __sysctl_args {
    int    *name;    /* integer vector describing variable */
    int     nlen;    /* length of this vector */
    void   *oldval;  /* 0 or address where to store old value */
    size_t *oldlenp; /* available room for old value,
                        overwritten by actual size of old value */
    void   *newval;  /* 0 or address of new value */
    size_t  newlen;  /* size of new value */
};
.fi
.in
.PP
This call does a search in a tree structure, possibly resembling
a directory tree under
.IR /proc/sys ,
and if the requested item is found calls some appropriate routine
to read or modify the value.
.SH "RETURN VALUE"
Upon successful completion,
.BR _sysctl ()
returns 0.
Otherwise, a value of \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
The invocation asked for the previous value by setting
.I oldval
non-NULL, but allowed zero room in
.IR oldlenp .
.TP
.B ENOTDIR
.I name
was not found.
.TP
.B EPERM
No search permission for one of the encountered "directories",
or no read permission where
.I oldval
was nonzero, or no write permission where
.I newval
was nonzero.
.SH "CONFORMING TO"
This call is Linux-specific, and should not be used in programs
intended to be portable.
A
.BR sysctl ()
call has been present in Linux since version 1.3.57.
It originated in
4.4BSD.
Only Linux has the
.I /proc/sys
mirror, and the object naming schemes differ between Linux and 4.4BSD,
but the declaration of the
.BR sysctl ()
function is the same in both.
.SH NOTES
Glibc does not provide a wrapper for this system call; call it using
.BR syscall (2).
Or rather...
.I don't
call it:
use of this system call has long been discouraged,
and it is so unloved that
\fBit is likely to disappear in a future kernel version\fP.
.\" See http://lwn.net/Articles/247243/
Since Linix 2.6.24,
uses of this system call result in warnings in the kernel log.
.\" Though comments in suggest that it is needed by old glibc binaries,
.\" so maybe it's not going away.
Remove it from your programs now; use the
.I /proc/sys
interface instead.
.SH BUGS
The object names vary between kernel versions,
making this system call worthless for applications.
.PP
Not all available objects are properly documented.
.PP
It is not yet possible to change operating system by writing to
.IR /proc/sys/kernel/ostype .
.SH EXAMPLE
.nf
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <linux/sysctl.h>

int _sysctl(struct __sysctl_args *args );

#define OSNAMESZ 100

int
main(void)
{
    struct __sysctl_args args;
    char osname[OSNAMESZ];
    size_t osnamelth;
    int name[] = { CTL_KERN, KERN_OSTYPE };

    memset(&args, 0, sizeof(struct __sysctl_args));
    args.name = name;
    args.nlen = sizeof(name)/sizeof(name[0]);
    args.oldval = osname;
    args.oldlenp = &osnamelth;

    osnamelth = sizeof(osname);

    if (syscall(SYS__sysctl, &args) == \-1) {
        perror("_sysctl");
        exit(EXIT_FAILURE);
    }
    printf("This machine is running %*s\\n", osnamelth, osname);
    exit(EXIT_SUCCESS);
}
.fi
.SH "SEE ALSO"
.BR proc (5)