SYSCTL (2)
read/write system parameters
SYNOPSIS
#include <unistd.h>
#include <linux/unistd.h>
#include <linux/sysctl.h>
_syscall1(int, _sysctl, struct __sysctl_args *, args);
int _sysctl(struct __sysctl_args * args );
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.
EXAMPLE
#include <linux/unistd.h>
#include <linux/types.h>
#include <linux/sysctl.h>
_syscall1(int, _sysctl, struct __sysctl_args *, args);
int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
void *newval, size_t newlen)
{
struct __sysctl_args args={name,nlen,oldval,oldlenp,newval,newlen};
return _sysctl(&args);
}
#define SIZE(x) sizeof(x)/sizeof(x[0])
#define OSNAMESZ 100
char osname[OSNAMESZ];
int osnamelth;
int name[] = { CTL_KERN, KERN_OSTYPE };
main(){
osnamelth = SIZE(osname);
if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
perror("sysctl");
else
printf("This machine is running %*s\en", osnamelth, osname);
return 0;
}
RETURN VALUES
Upon successful completion,
_sysctl
returns 0. Otherwise, a value of -1 is returned and
errno
is set to indicate the error.
ERRORS
ENOTDIR
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.
EFAULT
The invocation asked for the previous value by setting
oldval
non-NULL, but allowed zero room in
oldlenp .
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 BSD 4.4,
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 .
SEE ALSO
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