Why printk doesn't print message in kernel log(dmesg)

Question

I wrote small kernel module code as mentioned below, I am testing it in ubuntu 14.04

#include <linux/module.h> 
#include <linux/version.h>
#include <linux/kernel.h>
#include <linux/init.h>

int init_mod_func(void)
{
    printk(KERN_INFO "My module inserted\n ");
    return 0;
}

void cleanup_mod_func(void)
{
    printk(KERN_INFO "My module removed\n ");
}

module_init(init_mod_func);
module_exit(cleanup_mod_func);


MODULE_AUTHOR("Ankur");
MODULE_DESCRIPTION("TEST MODULE");
MODULE_LICENSE("GPL");

Now when I compile and insert above module using insmod I don't see printk message in the dmesg. However after module removal using rmmod I see both the printk messages.

With closure look, I found out that it is happening because of space after \n in the printk.
However I don't get why it is like that.

ankur:~/temp/tmp$ 
ankur:~/temp/tmp$ 
ankur:~/temp/tmp$ sudo dmesg -C /dev/null
ankur:~/temp/tmp$ 
ankur:~/temp/tmp$ 
ankur:~/temp/tmp$ sudo insmod testmod.ko 
ankur:~/temp/tmp$ dmesg
ankur:~/temp/tmp$ 
ankur:~/temp/tmp$ sudo rmmod testmod
ankur:~/temp/tmp$ dmesg
[ 4062.140441] My module inserted
[ 4062.140441]  
[ 4073.324994] My module removed
[ 4073.324994]

Cowherd

Answer 1

The kernel log ring buffer behaves as if it were line-buffered, as it can be seen in the implementation:

• in vprintk_emit you can see how it marks a buffer to be flushed/buffered based on the existence of a trailing newline

• in log_output you can see the actual code which takes care of flushing or buffering the message

As it is evident, your messages do not contain trailing newlines. Had you deleted the trailing space, you would see the messages in the kernel log after they had been flushed in the printk call(s).

Delphinedelphinia

Thank you for reply, Is there any way to flush buffer outside of code. E.g. when you are debugging driver and don't see any messages from printk, can we trigger buffer flush from commandline? – Cowherd

Not explicitly, but calls to [v]printk or printk_deferred might flush your own module's messages as well. As the comment in line 1724, says "An earlier newline was missing, or another task also prints continuation lines...". – Delphinedelphinia