SYSCTL

sysctl ? read/write system parameters

#include <unistd.h>

#include <linux/unistd.h>

#include <linux/sysctl.h>

_syscall1(int, _sysctl, struct __sysctl_args *, args);

int _sysctl(struct __sysctl_args *args);

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.

#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;
}

Upon successful completion, _sysctl returns 0. Otherwise, a value of ?1 is returned and errno is set to indicate the error.

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 setting oldval non-NULL, but allowed zero room in oldlenp.

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.

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.

proc(5)