NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON
SYSCTL(2) Linux Programmer's Manual SYSCTL(2)
sysctl - read/write system parameters
#include <unistd.h>
#include <linux/sysctl.h>
int _sysctl(struct __sysctl_args *args);
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.
Upon successful completion, _sysctl() returns 0. Otherwise, a value of -1 is
returned and errno is set to indicate the error.
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.
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.
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.
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.
#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);
}
proc(5)
This page is part of release 3.23 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/.
Linux 2008-11-20 SYSCTL(2)