Adding printf() to observe semaphore behavior?

Philip Semanchuk philip at
Tue Feb 23 02:48:15 UTC 2016

Hi all,
I’m trying to debug a potential kernel problem with POSIX semaphores. I’m trying to build a custom kernel with some added printf() statements and I’m not seeing any output, neither to /var/log/messages nor to the console. I would appreciate any help at getting the kernel to talk to me. 

Here’s what I’ve already done on my PC BSD 10.2 running inside a VM —  
- Downloaded the source to /usr/src
- Added a printf(“Hello world\n”) in each implementation of sem_open() (both in lib/libc/gen/sem.c and sem_new.c) and one inside the implementation of ksem_open() (in sys/kern/uipc_sem.c) for good measure. I did not #include <stdio.h> which I’ve read is the correct thing to do when one wants to use the kernel’s printf(). 
- cd /usr/src
- sudo make buildkernel 
- sudo make installkernel
- reboot
- Create a POSIX semaphore. It works, but I don’t see my “Hello world” message.

Any suggestions on what I’m doing wrong would be appreciated.


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