如何写到/ proc内核模块

Question

How can I get data when a user writes to proc? (Ie: echo "foo" > /proc/myproc)

I've seen reading from proc (ie user does a "cat /proc/myproc") as detailed from this site: http://www.linux.com/learn/linux-training/39972-kernel-debugging-with-proc-qsequenceq-files-part-2-of-3

That seems to work well enough, but I haven't seen the analogous method for writing. Also, I have checked out TLDP, but their article seems way out of date: http://tldp.org/LDP/lkmpg/2.6/html/x769.html

Answer 1

Actually the kernel API for dealing with /proc and /sys may vary between different kernel versions. Good news is that kernel itself is quite self explanatory. Means you can easily find an example of code just exploring sources. Just look for some file in /proc which supports write operation:

$ ls -l /proc | grep w

For example sysrq-trigger does:

--w-------  1 root root  0 Mar 12 00:41 sysrq-trigger

Then find the corresponding string in kernel sources:

$ find . -name '*.[ch]' -exec grep -Hn '"sysrq-trigger"' {} \;

Which gives you:

drivers/tty/sysrq.c:1105:   if (!proc_create("sysrq-trigger", S_IWUSR, NULL,

Now look into code in found file (look here for version 3.18):

static ssize_t write_sysrq_trigger(struct file *file, const char __user *buf,
                                   size_t count, loff_t *ppos)
{
        if (count) {
                char c;

                if (get_user(c, buf))
                        return -EFAULT;
                __handle_sysrq(c, false);
        }

        return count;
}

static const struct file_operations proc_sysrq_trigger_operations = {
        .write          = write_sysrq_trigger,
        .llseek         = noop_llseek,
};

static void sysrq_init_procfs(void)
{
        if (!proc_create("sysrq-trigger", S_IWUSR, NULL,
                         &proc_sysrq_trigger_operations))
                pr_err("Failed to register proc interface\n");
}

The same you can do for some sysfs file. For faster search use some code indexer (like cscope for vim, or built-in code indexer in Eclipse).