dmesg is not showing printk statement

Question

I'm trying to create a proc entry. My init_module function is as below

int init_module()
{
printk(KERN_INFO "proc2:Module Loaded\n");
proc_entry=proc_create_data(proc_name,0644,NULL,&fops,NULL);
if(proc_entry==NULL)
{
    printk(KERN_INFO "proc2:Error registering proc entry");
}
else
{
    printk(KERN_INFO "proc2:Proc Entry Created");
}
return 0;
}

following is cleanup method

void cleanup_module()
{
printk(KERN_INFO "proc2:module unloaded");
remove_proc_entry(proc_name,proc_entry);
}

rest of program include variable definition and callback functions.

when i compile this program it compiles well. when i use insmod it doesn't reply me prompt. lsmod lists my module and shows used by one (don't know what). dmesg shows none of the above printk messages.

can you tell me what's wrong here?

Answer 1

Try echo "7" > /proc/sys/kernel/printk to enable all console log levels.

The numbers are corresponding to below:

#define KERN_EMERG "<0>" /* system is unusable*/
#define KERN_ALERT "<1>" /* action must be taken immediately*/
#define KERN_CRIT "<2>" /* critical conditions*/
#define KERN_ERR "<3>" /* error conditions*/
#define KERN_WARNING "<4>" /* warning conditions*/
#define KERN_NOTICE "<5>" /* normal but significant condition*/
#define KERN_INFO "<6>" /* informational*/
#define KERN_DEBUG "<7>" /* debug-level messages*/

The default number is 4, which allows console to show messages only at least in KERN_WARNING. That's why you cannot see log in KERN_INFO level.

Answer 2

Even after using echo "7" > /proc/sys/kernel/printk, printk didn't worked for me.

https://lwn.net/Articles/487437/ says about modified or more enhanced version of printk called pr_** api's (pr_info,pr_emerg).

I used pr_info instead of printk and it solved my problem.

Answer 3

In addition to Wayne's Answer, the kernel source says

This macro expands to dynamic_pr_debug() if CONFIG_DYNAMIC_DEBUG is set. Otherwise, if DEBUG is defined, it's equivalent to a printk with KERN_DEBUG loglevel. If DEBUG is not defined it does nothing.

Which means you'd need to either define DEBUG or use dynamic debug. I was able to successfully load the module with debug printk via

$ sudo insmod module-name.ko dyndbg