sysctl(2) — Linux manual pages

Linux manual pages	Section 2

Name

sysctl — read/write system parameters

Synopsis

#include <unistd.h>
#include <linux/sysctl.h>

int _sysctl( struct __sysctl_args *args);

	Note
	There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

Do not use this system call! See NOTES.

The _sysctl() call reads and/or writes kernel parameters. For example, the hostname, or the maximum number of open files. The argument has the form

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 */

};

This call does a search in a tree structure, possibly resembling a directory tree under /proc/sys, and if the requested item is found calls some appropriate routine to read or modify the value.

RETURN VALUE

Upon successful completion, _sysctl() returns 0. Otherwise, a value of −1 is returned and errno is set to indicate the error.

ERRORS

EFAULT: The invocation asked for the previous value by setting oldval non-NULL, but allowed zero room in oldlenp.
ENOTDIR: name was not found.
EACCES, EPERM: No search permission for one of the encountered "directories", or no read permission where oldval was nonzero, or no write permission where newval was nonzero.

CONFORMING TO

This call is Linux-specific, and should not be used in programs intended to be portable. A sysctl() call has been present in Linux since version 1.3.57. It originated in 4.4BSD. Only Linux has the /proc/sys mirror, and the object naming schemes differ between Linux and 4.4BSD, but the declaration of the sysctl() function is the same in both.

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2). Or rather... don't call it: use of this system call has long been discouraged, and it is so unloved that it is likely to disappear in a future kernel version. Since Linux 2.6.24, uses of this system call result in warnings in the kernel log. Remove it from your programs now; use the /proc/sys interface instead.

This system call is available only if the kernel was configured with the CONFIG_SYSCTL_SYSCALL option.

BUGS

The object names vary between kernel versions, making this system call worthless for applications.

Not all available objects are properly documented.

It is not yet possible to change operating system by writing to /proc/sys/kernel/ostype.

EXAMPLE

#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);
}

COLOPHON

This page is part of release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.

  Copyright (C) 1996 Andries Brouwer (aebcwi.nl)

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Written 11 April 1996 by Andries Brouwer <aebcwi.nl>
960412: Added comments from Stephen Tweedie
Modified Tue Oct 22 22:28:41 1996 by Eric S. Raymond <esrthyrsus.com>
Modified Mon Jan  5 20:31:04 1998 by aeb.

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 */
};