sysctl(2) — Linux manual pages

DESCRIPTION

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.
EPERM: No search permission for one of the encountered `directories', or no read permission where oldval was non-zero, or no write permission where newval was non-zero.

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(2) function is the same in both.

BUGS

The object names vary between kernel versions. THIS MAKES THIS SYSTEM CALL WORTHLESS FOR APPLICATIONS. Use the /proc/sys interface instead.

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);
}

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2).

  Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)

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Permission is granted to copy and distribute modified versions of this
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Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
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the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

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.

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