SYSCTL(2)

NAME
sysctl - read/write system parameters

SYNOPSIS
#include <unistd.h>
#include <linux/unistd.h>
#include <linux/sysctl.h>
#include <errno.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 = sizeof(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 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 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 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.

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)