NAME
sysctl - 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 resem-
bling 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\n", 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 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.
EFAULT The invocation asked for the previous value by set-
ting oldval non-NULL, but allowed zero room in
oldlenp.
CONFORMING TO
This call is Linux-specific, and should not be used in pro-
grams 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
proc(5).