Get Character String Written to Custom /dev/ file

I’m currently working with a custom device driver that I installed. I’m very new to this and having trouble understanding how to get the data that I write into it from the command line.

For example, I write data to the file like so:

echo -n "test" > /dev/custom

And then I merely want to get the string here, so I can use the characters within:

static ssize_t custom_write (struct file *f, const char *buf, size_t len, loff_t *offset)
{
  printk(KERN_ALERT "Testn");
    return len;
}

Answer

The string is in buf, but it’s in user memory. You need to use copy_from_user() to copy to kernel memory.

static ssize_t custom_write (struct file *f, const char *buf, size_t len, loff_t *offset)
{
    char custom_buffer[MAX_BUFFER_SIZE];
    size_t custom_buffer_size = len;
    if (custom_buffer_size > MAX_BUFFER_SIZE) {
        custom_buffer_size = MAX_BUFFER_SIZE;
    }
    if (copy_from_user(custom_buffer, buf, custom_buffer_size) != 0) {
        return -EFAULT;
    }
    printk(KERN_ALERT "Test %.*sn", (int)custom_buffer_size, custom_buffer);
    return custom_buffer_size;
}