why my kernel log is not showing the latest output

Question

I'm coding a simple kernel module, in linux ubuntu 17.04, that takes a string and prints it in the kernel log.

#include<linux/module.h>
#include<linux/init.h>
#include<linux/moduleparam.h>
char* mystring = "hello world";
module_param(mystring ,charp ,S_IRUSR | S_IWUSR);

void display(void){
printk(KERN_ALERT "%s" ,mystring);
}
static int hello(void){
//printk(KERN_ALERT "hello module");
display();
return 0;
} 
static void bye(void){
printk(KERN_ALERT "bye");
}
module_init(hello);
module_exit(bye);

I run command make and then when I run insmod test.ko mystring="blahblahblah", the module will be inserted correctly but when I run dmesg it doesn't show the "blahblahblah". after I run rmmod test.ko and dmseg the expression "blahblahblah" will appear in the terminal. when I run insmod test.ko mystring="blahblahblah" again and then dmesg the "blahblahblah" will be printed. what is the problem exactly? is it my problem or the system?

Answer 1

Sometimes printk may defer output (that is, message is stored in the intrenal buffer, but not in kernel log). For avoid such behavior, always add newline (\n) at the end of the string printed:

printk(KERN_ALERT "%s\n" ,mystring);