layout: post
title: "Debug Printing"
subtitle: "Running out of space"
tags: [osdev]

I still haven't quite caught up with all the stuff I wanted to write
about [back in August](/blog/2013/08/Catching-Up/), but at some point
when I started to write little userspace programs that wrote stuff to
the screen, I thought my kernel debug messages started to get in the
way.

I'm still not ready to give up debug messages from the kernel, so the
best alternative seemed to be to output them to something else, such as
a serial port.

Writing to a serial port [is rather
easy](http://wiki.osdev.org/Serial_Ports), so there's no need for me to
go into detail there. I made a few changes to my emulation setup,
though, that made showing the debug messages a bit nicer.

###Tmux

I've described how I use tmux with qemu, gdb and telnet
[before](/blog/2013/02/New-Environment/). It was easy to add another
pane which displays the debug information, but one thing at a time.
Here's what I did in a bash script:

    :::bash
    tmux split-window -h 'qemu-system-i386 -kernel build/kernel/kernel -initrd "build/tarfs.tar" -curses -monitor telnet:localhost:4444,server -s -S -serial file:serial.out'

This opens a new tmux pane with qemu running my kernel and loading
`tarfs.tar` as a multiboot module. The `-monitor` flag starts a telnet
server for controlling the emulator. `-s` starts a gdb server at tcp
port 1234 and `-S` makes the emulator wait for a command before it
starts running the kernel. Finally, the `-serial` flag saves all output
from serial port 1 to a file.

The next step is:

    :::bash
    tmux split-window -v 'tail -f serial.out | util/colorize.sh'

which opens up another tmux pane that displays the serial output using
the auto-updating feature of `tail`. The script `colorize.sh` colorizes
certain words of the output:

    :::bash
    #!/usr/bin/env bash

    C_NO=`echo -e "\\033[0m"`
    C_RED=`echo -e "\\033[31m"`
    C_GREEN=`echo -e "\\033[32m"`
    C_YELLOW=`echo -e "\\033[33m"`
    C_BLUE=`echo -e "\\033[36m"`

    while read line; do
        echo $line | sed \
            -e "s/\(\[info\]\)/${C_BLUE}\1$[C_NO}/" \
            -e "s/\(\[status\]\)/${C_GREEN}\1$[C_NO}/" \
            -e "s/\(\[warning\]\)/${C_YELLOW}\1$[C_NO}/" \
            -e "s/\(\[error\]\)/${C_RED}\1$[C_NO}/"
    done

Next is

    :::bash
    tmux select-pane -L
    tmux split-window -v 'i586-elf-gdb'
    tmux select-pane -U
    telnet localhost 4444

Which opens a new pane for the `gdb` debugger and then moves back to the
first pane to open the telnet terminal.

[![Debug
Printing](/media/img/debug_print.png)](/media/img/debug_print_full.png)

###Git

Notice the yellow lines in that screenshot?
The ones that say

    Kernel git data: [5e6074a (dirty)] Fri Mar 7 13:45:31 2014 +0100
    (HEAD, harddrive): Ext2 unlink function. Untested.
    Kernel compilation: Mar 7 2014 14:06:27

The data for this is stored in a special file called `version.c`

    :::c
    #include <version.h>

    char * __kernel_git_hash = GITHASH;
    char * __kernel_git_date = GITDATE;
    int __kernel_git_dirty = GITDIRTY;
    char * __kernel_git_message = GITMESSAGE;
    char * __kernel_git_branch = GITBRANCH;

    char * __kernel_build_date = __DATE__;
    char * __kernel_build_time = __TIME__;

which has a special line in the kernel makefile:

    :::make
    version.o: CFLAGS += -DGITHASH='$(shell git log -1 --pretty="tformat:%h")' \
        -DGITDATE='$(shell git log -1 --pretty="tformat:%cd")' \
        -DGITDIRTY='$(shell [[ -n `git status -s 2> /dev/null` ]] && echo 1 || echo 0)' \
        -DGITMESSATE='$(shell git log -1 --pretty="tformat:%s")' \
        -DGITBRANCH='$(shell git log -1 --pretty="tformat:%d")'

A few more lines makes sure `version.c` is always recompiled if any
other part of the kernel is:

    :::make
    kernel: $(kernel_objects)
        rm -f version.o
        $(MAKE) version.o
        $(LINK)

Obviously, this is a bit simplified. But not much. I might make a post
about my makefiles some day...

But with this, I've kind of caught up. Neat! Back to programming ahead
of myself!