用户空间和内核空间数据交换方式-sysctl

cat /proc/sys/net/ipv4/ip_forward 

echo 1 > /proc/sys/net/ipv4/ip_forward 

# sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0
# sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1
# sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 1 

{
	.ctl_name = 0
}

# insmod ./sysctl-exam-kern.ko
# cat /proc/sys/mysysctl/myint
0
# cat /proc/sys/mysysctl/mystring
# ./sysctl-exam-user
mysysctl.myint =0
mysysctl.mystring =""
# ./sysctl-exam-user 100"Hello, World"
old value: mysysctl.myint =0
newvalue: mysysctl.myint =100
old vale: mysysctl.mystring =""
newvalue: mysysctl.mystring ="Hello, World"
# cat /proc/sys/mysysctl/myint
100
# cat /proc/sys/mysysctl/mystring
Hello, World
#

struct ctl_table
{
	int ctl_name;            /* Binary ID */
	const char *procname;        /* Text ID for /proc/sys, or zero */
	void *data;
	int maxlen;
	mode_t mode;
	struct ctl_table *child;
	struct ctl_table *parent;    /* Automatically set */
	proc_handler *proc_handler;    /* Callback for text formatting */
	ctl_handler *strategy;        /* Callback function for all r/w */
	void *extra1;
	void *extra2;
};

//header: sysctl-exam.h
#ifndef _SYSCTL_EXAM_H
#define _SYSCTL_EXAM_H
#include <linux/sysctl.h>
#define MY_ROOT (CTL_CPU + 10)
#define MY_MAX_SIZE 256
enum {
	MY_INT_EXAM = 1,
	MY_STRING_EXAM = 2,
};
#endif

//kernel module: sysctl-exam-kern.c
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/sysctl.h>
#include "sysctl-exam.h"

static char mystring[256];
static int myint;

static struct ctl_table my_sysctl_exam[] = {
	{
		.ctl_name       = MY_INT_EXAM,
		.procname       = "myint",
		.data           = &myint,
		.maxlen         = sizeof(int),
		.mode           = 0666,
		.proc_handler   = &proc_dointvec,
	},
	{
		.ctl_name       = MY_STRING_EXAM,
		.procname       = "mystring",
		.data           = mystring,
		.maxlen         = MY_MAX_SIZE,
		.mode           = 0666,
		.proc_handler   = &proc_dostring,
		.strategy       = &sysctl_string,
	}，
	{
		.ctl_name = 0
	}
};

static struct ctl_table my_root = {
	.ctl_name       = MY_ROOT,
	.procname       = "mysysctl",
	.mode           = 0555,
	.child          = my_sysctl_exam,
};

static struct ctl_table_header * my_ctl_header;

static int __init sysctl_exam_init(void)
{
	my_ctl_header = register_sysctl_table(&my_root, 0);

	return 0;
}

static void __exit sysctl_exam_exit(void)
{
	unregister_sysctl_table(my_ctl_header);
}

module_init(sysctl_exam_init);
module_exit(sysctl_exam_exit);
MODULE_LICENSE("GPL");

//application: sysctl-exam-user.c
#include <linux/unistd.h>
#include <linux/types.h>
#include <linux/sysctl.h>
#include "sysctl-exam.h"
#include <stdio.h>
#include <errno.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

int oldmyint;
int oldmyintlen;

int newmyint;
int newmyintlen;

char oldmystring[MY_MAX_SIZE];
int oldmystringlen;

char newmystring[MY_MAX_SIZE];
int newmystringlen;

int myintctl[] = {MY_ROOT, MY_INT_EXAM};
int mystringctl[] = {MY_ROOT, MY_STRING_EXAM};

int main(int argc, char ** argv)
{
	if (argc < 2) 
	{
		oldmyintlen = sizeof(int);
		if (sysctl(myintctl, SIZE(myintctl), &oldmyint, &oldmyintlen, 0, 0)) {
			perror("sysctl");
			exit(-1);
		}
		else {
			printf("mysysctl.myint = %d\n", oldmyint);
		}

		oldmystringlen = MY_MAX_SIZE;
		if (sysctl(mystringctl, SIZE(mystringctl), oldmystring, &oldmystringlen, 0, 0)) {
			perror("sysctl");
			exit(-1);
		}
		else {
			printf("mysysctl.mystring = \"%s\"\n", oldmystring);
		}
	}
	else if (argc != 3) 
	{
		printf("Usage:\n");
		printf("\tsysctl-exam-user\n");
		printf("Or\n");
		printf("\tsysctl-exam-user aint astring\n");
	}
	else 
	{
		newmyint = atoi(argv[1]);
		newmyintlen = sizeof(int);
		oldmyintlen = sizeof(int);

		strcpy(newmystring, argv[2]);
		newmystringlen = strlen(newmystring);
		oldmystringlen = MY_MAX_SIZE;

		if (sysctl(myintctl, SIZE(myintctl), &oldmyint, &oldmyintlen, &newmyint, newmyintlen)) {
			perror("sysctl");
			exit(-1);
		}
		else {
			printf("old value: mysysctl.myint = %d\n", oldmyint);
			printf("new value: mysysctl.myint = %d\n", newmyint);
		}


		if (sysctl(mystringctl, SIZE(mystringctl), oldmystring, &oldmystringlen, newmystring, newmystringlen)) {
			perror("sysctl");
			exit(-1);
		}
		else {
			printf("old vale: mysysctl.mystring = \"%s\"\n", oldmystring);
			printf("new value: mysysctl.mystring = \"%s\"\n", newmystring);
		}
	}
	exit(0);
}