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Old 08-14-2011, 06:05 PM   #1
LQ Newbie
Registered: Aug 2011
Posts: 7

Rep: Reputation: Disabled

Following is a simple sysfs code that creates the kobject test_test and adds a new attribute attr1 to it.

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/kobject.h>
#include <linux/sysfs.h>

static ssize_t attr1_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
	printk("goldeneagle: attr1_show\n");
	return 0;

static ssize_t attr1_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t cnt)
	printk("goldeneagle: attr1_store\n");
	return 0;

struct kobject *kobj;
const struct kobj_attribute kobj_attr =
	__ATTR(attr1, 0666, attr1_show, attr1_store);

static int test_char_init(void)
	int ret = 0;

	printk("\ngoldeneagle:%s\n", __func__);

	kobj = kobject_create_and_add("test_test", kernel_kobj);
	if (!kobj) {
		printk("goldeneagle: kobj could not be created and added\n");
		ret = -ENOMEM;
		goto fail;

	ret = sysfs_create_file(kobj, &kobj_attr.attr);
	if (ret) {
		printk("goldeneagle: Failed to add new attribute\n");
		goto err;
	return 0;
	sysfs_remove_file(kobj, &kobj_attr.attr);
	return ret;

static void test_char_exit(void)
	printk("\ngoldeneagle:%s\n", __func__);
	sysfs_remove_file(kobj, &kobj_attr.attr);


When I try to write something to attr1 via "echo 0 > attr1"
I see attr1_store getting called infinite no. of time.

Please suggest what might be causing this issue !!!
Old 08-16-2011, 04:38 AM   #2
Registered: Jul 2011
Posts: 113

Rep: Reputation: Disabled
warning * i'm not very good at following other peoples code

first thing i see right off is you are using printk
printk uses a cyclic buffer to hold messages managed by klogd and spit to terminal or logs
you should have something like

// create a file in your sysfs subdirectory to write to named for example "example"
static int example;

static ssize_t attr1_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
// however you print a string with sprintf i think it's %s
	 return sprintf(buf, "%s\n", example);

static ssize_t attr1_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t cnt)
//and then for this one
	 sscanf(buf, "%su", &example);
         return cnt;



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