NAME
sysctl - read/write system parameters
SYNOPSIS
#include <unistd.h>
#include <linux/sysctl.h>
int _sysctl(struct __sysctl_args *args);
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.
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. Remove it from your programs now; use the
/proc/sys interface instead.
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);
}
SEE ALSO
proc(5)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.