SYSCTL
NAME
sysctl - read/write system parameters
SYNOPSIS
#include <unistd.h>
#include <linux/sysctl.h>
I int _sysctl(struct __sysctl_args * args );
DESCRIPTION
The
R _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
R /proc/sys ,
and if the requested item is found calls some appropriate routine
to read or modify the value.
RETURN VALUE
Upon successful completion,
R _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
R 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
R 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.
NOTES
Glibc does not provide a wrapper for this system call; call it using
syscall(2).
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
R /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
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