.\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 2023-03-30 "Linux man-pages 6.04"
.SH NAME
sysctl \- read/write system parameters
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.B #include <linux/sysctl.h>
.PP
.BI "[[deprecated]] int _sysctl(struct __sysctl_args *" args );
.fi
.SH DESCRIPTION
.B This system call no longer exists on current kernels!
See NOTES.
.PP
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
.EX
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 */
};
.EE
.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
.BR EACCES ", " 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.
.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.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 1.3.57.
Removed in Linux 5.5, glibc 2.32.
.PP
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
Use of this system call was long discouraged:
since Linux 2.6.24,
uses of this system call result in warnings in the kernel log,
and in Linux 5.5, the system call was finally removed.
Use the
.I /proc/sys
interface instead.
.PP
Note that on older kernels where this system call still exists,
it is available only if the kernel was configured with the
.B CONFIG_SYSCTL_SYSCALL
option.
Furthermore, glibc does not provide a wrapper for this system call,
necessitating the use of
.BR syscall (2).
.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 EXAMPLES
.\" SRC BEGIN (sysctl.c)
.EX
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>

#include <linux/sysctl.h>

#define ARRAY_SIZE(arr)  (sizeof(arr) / sizeof((arr)[0]))

int _sysctl(struct __sysctl_args *args);

#define OSNAMESZ 100

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

    memset(&args, 0, sizeof(args));
    args.name = name;
    args.nlen = ARRAY_SIZE(name);
    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\en", (int) osnamelth, osname);
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR proc (5)