SYSCTL(2)

NAME
sysctl - read/write system parameters

SYNOPSIS
#include <unistd.h>
#include <linux/sysctl.h>

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.

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.

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 /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
proc(5)