无法打印从用户空间C应用程序发送到linux内核模块的消息

Question

我已经开发了一个简单的linux内核模块：

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>


ssize_t exer_open(struct inode *pinode, struct file *pfile) {

    return 0;
}

ssize_t exer_read(struct file *pfile, char __user *buffer, size_t length, loff_t *offset) {

    return 0;
}

ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {

    return length;
}

ssize_t exer_close(struct inode *pinode, struct file *pfile) {

    return 0;
}

struct file_operations exer_file_operations = { 
    .owner = THIS_MODULE,
    .open = exer_open,
    .read = exer_read,
    .write = exer_write,
    .release = exer_close,
};

int exer_simple_module_init(void) {

    printk(KERN_ALERT "Inside the %s function\n", __FUNCTION__);
    register_chrdev(240, "Simple Char Drv", &exer_file_operations);
    return 0;
}

void exer_simple_module_exit(void) {

    unregister_chrdev(240, "Simple Char Drv");
}

module_init(exer_simple_module_init);
module_exit(exer_simple_module_exit);

我使用insmod命令将这个模块插入内核没有任何问题。

我想使用此模块来打印由我也开发的用户空间程序发送给它的消息：

#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<fcntl.h>
#include<string.h>
#include<unistd.h>


int main()

{

int ret, fd;
char stringToSend[] = "Hello World !";

fd = open("/dev/char_device", O_RDWR);             // Open the device with read/write access

if (fd < 0)
    {
            perror("Failed to open the device...");
            return errno;
    }

ret = write(fd, stringToSend, strlen(stringToSend)); // Send the string to the LKM

if (ret < 0)
    {
            perror("Failed to write the message to the device.");
            return errno;
    }

return 0;

}

当我执行该程序并使用tail -f /var/log/messages命令检查内核日志时，我可以看到：user.alert kernel: Inside the exer_read function但是我看不到消息“ Hello World！”。

我不知道我在这里缺少什么，特别是我仍然是开发模块和使用它的初学者。请帮帮我！

Answer 1

对于仍然找不到解决方案的人，我有一个答案。

这是模块：

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>  
#include <linux/kernel.h>
#include <linux/uaccess.h>


MODULE_LICENSE("GPL");      
MODULE_AUTHOR("Gaston");  
MODULE_DESCRIPTION("A simple Linux char driver"); 
MODULE_VERSION("0.1"); 

#define MAX 256

static char message[MAX] ="";           ///< Memory for the string that is passed from userspace


ssize_t exer_open(struct inode *pinode, struct file *pfile) {

    printk(KERN_INFO "Device has been opened\n");
    return 0;
}


ssize_t exer_read(struct file *pfile, char __user *buffer, size_t length, loff_t *offset) {

    return 0;
}


ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {
    if (length > MAX)
        return -EINVAL;

    if (copy_from_user(message, buffer, length) != 0)
        return -EFAULT;

    printk(KERN_INFO "Received %s characters from the user\n", message);
    return 0;

}   


ssize_t exer_close(struct inode *pinode, struct file *pfile) {

    printk(KERN_INFO "Device successfully closed\n");
    return 0;
}


struct file_operations exer_file_operations = { 
    .owner = THIS_MODULE,
    .open = exer_open,
    .read = exer_read,
    .write = exer_write,
    .release = exer_close,
};


int exer_simple_module_init(void) {

    printk(KERN_INFO "Initializing the LKM\n");
    register_chrdev(240, "Simple Char Drv", &exer_file_operations);
    return 0;
}


void exer_simple_module_exit(void) {

    unregister_chrdev(240, "Simple Char Drv");
}

module_init(exer_simple_module_init);
module_exit(exer_simple_module_exit);

这是应用程序：

#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<fcntl.h>
#include<string.h>
#include<unistd.h>


#define BUFFER_LENGTH 256 

int main()

{

int ret, fd;
char stringToSend[BUFFER_LENGTH];


fd = open("/dev/char_device", O_RDWR);             // Open the device with read/write access

if (fd < 0)
    {
            perror("Failed to open the device...");
            return errno;
    }


printf("Type in a short string to send to the kernel module:\n");

scanf("%s", stringToSend);                // Read in a string (with spaces)

printf("Writing message to the device [%s].\n", stringToSend);

ret = write(fd, stringToSend, strlen(stringToSend)); // Send the string to the LKM

if (ret < 0)
    {
            perror("Failed to write the message to the device.");
            return errno;
    }

return 0;

}

你会发现这会很好地工作。