merc/doc/hacker.txt

Merc Release 2.1
Sunday 01 August 1993

Furey	mec@shell.portal.com
Hatchet	hatchet@uclink.berkeley.edu
Kahn	michael@uclink.berkeley.edu



=== 'I'm running a Mud so I can learn C programming!'

Yeah, right.

The purpose of this document is to record some of our knowledge, experience and
philosophy.  No matter what your level, we hope that this document will help
you become a better software engineer.

Remember that engineering is work, and NO document will substitute for your
own thinking, learning and experimentation.



=== How to Learn in the First Place

(1) Play with something.
(2) Read the documentation on it.
(3) Play with it some more.
(4) Read documentation again.
(5) Play with it some more.
(6) Read documentation again.
(7) Play with it some more.
(8) Read documentation again.
(9) Get the idea?

The idea is that your mind can accept only so much 'new data' in a single
session.  Playing with something doesn't introduce very much new data, but it
does transform data in your head from the 'new' category to the 'familiar'
category.  Reading documentation doesn't make anything 'familiar', but it
refills your 'new' hopper.

Most people, if they even read documentation in the first place, never return
to it.  They come to a certain minimum level of proficiency and then never
learn any more.  But modern operating systems, languages, networks, and even
applications simply cannot be learned in a single session.  You have to work
through the two-step learning cycle MANY times to master it.



=== The Environment

Computer: the big or little box that you're using to run Merc.  Computers come
    from a _manufacturer_ and have a _model_ name.  Here is a list of common
    manufacturers and models that you're likely to encounter:

	Manufacturer	Model
	------------	-----

	Sun		Sun-2
	Sun		Sun-3
	Sun		Sun-4
	DEC		Vax 5000
	DEC		Vax 5900
	IBM		RS/6000
	NeXT		NextCube
	Sequent		Symmetry
	Sequent		Balance

    As far as hardware goes, Merc will run on any 32-bit hardware.

Operating system: the lowest level program running on your computer.  Most
    common computers run Unix or some variant of it, such as SunOS, Ultrix,
    AIX, Mach, or Dynix.  Notice that many of these variants end in 'IX'.

    The two major 'families' of Unix are Berkeley Unix (developed at the
    illustrious University of California, Berkeley) and System 5 Unix
    (developed by Bell Laboratories, the progenitors of Unix).

    The most common non-Unix operating system is VMS (a proprietary operating
    system from DEC for their VAX computers).  In the personal computer world,
    you'll find MS-DOS, OS/2 for IBM PC's and compatibles, and MacOS for Apple
    Macintosh'es.

    GET THIS STRAIGHT: 'VAX' IS NOT AN OPERATING SYSTEM.  It's the name of a
    family of computers from DEC.  There are plenty of Vax'es running VMS, and
    there are even more Vax'es running Berkeley Unix or Ultrix.  The Vax'es
    running Unix have a lot more in common with other machines running
    Unix than they have with Vax'es running VMS.

    As far as operating systems go, Merc will run on Unix or Unix variants with
    TCP/IP networking compatible with Berkeley Unix.  It will also run, in
    single-user mode only, on MS-DOS.  With a reasonable amount of work, Merc
    can be ported to any operating system that provides TCP service for telnet
    connections.

Languages: Merc is written in C.  ANSI (the American National Standards
    Institute) has a specification for the C language, and Merc is written in
    Ansi Standard C.

    The most popular compiler for Ansi Standard C is the Gnu 'gcc' compiler
    produced by the Free Software Foundation.  It's available by anonymous
    ftp from prep.ai.mit.edu.  Merc compiles just fine with Gcc 1.38, so
    you can probably use 1.42 and skip the much larger 2.X versions.

    You don't have to use gcc.  IBM RS/6000's running the AIX operating system
    come with an Ansi C compiler already.  So do NeXT machines (the standard
    'cc' on NeXT happens to be the Gnu C compiler).  Any Ansi compiler will
    work.

    Unfortunately, there are still many machines out there without an Ansi
    standard C compiler.  (Sun is the worst offender in this regard).  You
    can attempt to compile Merc with a non-Ansi (traditional) C compiler by
    using the 'mktrad' script.  See trad.txt for details.

If you don't know what the manufacturer and model of your computer is, as well
as its operating system, and whether the C compiler is Ansi or non-Ansi, then
you need to find out.



=== Basic Unix Tools

'man'	-- gives you online manual pages

'grep'	-- stands for 'global regular expression print'

'vi'
'emacs'
'jove'	-- use whatever editor floats your boat
	   but learn the hell out of it
	   you should know EVERY command in your editor

'ctags'	-- makes 'tags' for your editor
	   allows you to goto functions by name in any source file

'>'
'>>'
'<'
'|'	-- input and output redirection
	   get someone to show you, or dig it out of 'man csh'

These are the basic day-in day-out development tools.  Developing without
knowing how to use ALL of these well is like driving a car without knowing how
to change gears.



=== Debugging: Theory

Debugging is a science.  You formulate a hypothesis, make predictions based on
the hypothesis, run the program and provide it experimental input, observe its
behavior, and confirm or refute the hypothesis.

A good hypothesis is one which makes surprising predictions which then come
true; predictions that other hypotheses don't make.

The first step in debugging is not to write bugs in the first place.  This
sounds obvious, but sadly, is all too often ignored.

If you build a program, and you get ANY errors or ANY warnings, you should fix
them before continuing.  C was designed so that many buggy ways of writing code
are legal, but will draw warnings from a suitably smart compiler (such as 'gcc'
with the '-Wall' flag enabled).  It takes only minutes to check your warnings
and to fix the code that generates them, but it takes hours to find bugs
otherwise.

'Desk checking' (proof reading) is almost a lost art in 1993.  Too bad.  You
should desk check your code before even compiling it, and desk-check it again
periodically to keep it fresh in mind and find new errors.  If you have someone
in your group whose ONLY job it is to desk-check other people's code, that
person will find and fix more bugs than everyone else combined.

One can desk-check several hundred lines of code per hour.  A top-flight
software engineer will write, roughly, 99% accurate code on the first pass,
which still means one bug per hundred lines.  And you are not top flight.
So ... you will find several bugs per hour by desk checking.  This is a very
rapid bug fixing technique.  Compare that to all the hours you spend screwing
around with broken programs trying to find ONE bug at a time.

The next technique beyond desk-checking is the time-honored technique of
inserting 'print' statements into the code, and then watching the logged
values.  Within Merc code, you can call 'printf' or 'fprintf' to dump
interesting values at interesting times.  Where and when to dump these values
is an art, which you will learn only with practice.

If you don't already know how to redirect output in your operating system, now
is the time to learn.  On Unix, type the command 'man csh', and read the part
about the '>' operator.  You should also learn the difference between
'standard output' (e.g. output from 'printf') and 'error output' (e.g. output
from 'fprintf').

Ultimately, you cannot fix a program unless you understand how it's operating
in the first place.  Powerful debugging tools will help you collect data, but
they can't interpret it, and they can't fix the underlying problems.  Only you
can do that.

When you find a bug ... your first impulse will be to change the code, kill the
manifestation of the bug, and declare it fixed.  Not so fast!  The bug you
observe is often just the symptom of a deeper bug.  You should keep pursuing
the bug, all the way down.  You should grok the bug and cherish it in fullness
before causing its discorporation.

Also, when finding a bug, ask yourself two questions: 'what design and
programming habits led to the introduction of the bug in the first place?'
And: 'what habits would systematically prevent the introduction of bugs like
this?'



=== Debugging: Tools

When a Unix process accesses an invalid memory location, or (more rarely)
executes an illegal instruction, or (even more rarely) something else goes
wrong, the Unix operating system takes control.  The process is incapable of
further execution and must be killed.  Before killing the process, however, the
operating system does something for you: it opens a file named 'core' and
writes the entire data space of the process into it.

Thus, 'dumping core' is not a cause of problems, or even an effect of problems.
It's something the operating system does to help you find fatal problems which
have rendered your process unable to continue.

One reads a 'core' file with a debugger.  The two most popular debuggers on
Unix are 'adb' and 'gdb', although occasionally one finds 'dbx'.  Typically
one starts a debugger like this: 'adb merc' or 'gdb merc core'.

The first thing, and often the only thing, you need to do inside the debugger
is take a stack trace.   In 'adb', the command for this is '$c'.  In gdb,
the command is 'backtrace'.  The stack trace will tell you what function your
program was in when it crashed, and what functions were calling it.  The
debugger will also list the arguments to these functions.  Interpreting these
arguments, and using more advanced debugger features, requires a fair amount of
knowledge about assembly language programming.

If you have access to a program named 'Purify' ... learn how to use it.



=== Profiling

Here is how to profile a program:

(1) Remove all the .o files and the 'merc' executable:

	rm *.o 'merc'

(2) Edit your makefile, and change the PROF= line:

	PROF    = -p

(3) Remake merc:

	make

(4) Run merc as usual.  Shutdown the game with shutdown when you have run long
    enough to get a good profiling base.  If you crash the game, or kill the
    process externally, you won't get profiling information.

(5) Run the 'prof' command:

	prof merc > prof.out

(6) Read prof.out.  Run 'man prof' to understand the format of the output.

For advanced profiling, you can use 'PROF = -pg' in step (2), and use the
'gprof' command in step 5.  The 'gprof' form of profiling gives you a report
which lists exactly how many times any function calls any other function.  This
information is valuable for debugging as well as performance analysis.

Availability of 'prof' and 'gprof' varies from system to system.  Almost every
Unix system has 'prof'.  Only some systems have 'gprof'.



=== Schedule versus Features versus Quality

Now for a few words on project management.

Sooner or later, almost any project faces a trade-off between schedule,
features, and quality.  Consider a student writing a term paper on the last
night.  He has three unpalatable choices: he can turn it in late (miss the
schedule).  He can turn in a shorter paper that doesn't cover everything
(reduce the features).  Or he can churn out gibberish (lower the quality).

Similarly in a software project, one often has a choice between making the
release date, or dropping features, or shipping everything on time and
hoping that it works (it usually doesn't).

The most important thing to realize about this decision is that it IS a
decision.  One can't get out of it by hoping that some miracle will occur.
If you don't react consciously, then external circumstances will drive the
decision.

Ok, so suppose you are faced with the trade-off and go for a schedule slip.
Don't take a small slip ... take a big impressive slip.  If you say
'I'll just fix this one problem and finish ASAP', then likely you will
wish you had taken just a little more time later.  If you say 'I think I
need another day, so I'll slip by a week', then it's much more likely
that what you'll have at the end of the week will do the job.  It's better
to slip a large block of time once then to slip day-by-day or hour-by-hour
repeatedly.

If you go for dropping features, again, carve off a big hunk.  Don't be
timid and pretend that you're going to do that work 'if you just get a
little spare time.'  That feature of your project is GONE, exploit the
lessened requirements for all the savings you can!

I can't offer much advise on how to reduce quality, because that's always
my last choice for what to drop on a project.



=== Sleeping

Simple and obvious, but true ... engineering takes an alert mind.

It's very easy, very seductive, to throw a lot of consecutive hours at a
problem.  One can get into a 'flow' state where one's mind becomes filled
with the problem, and the work just pours out, hour after hour.  Many
writers report that they watch a story take place, and just transcribe
what they see, pounding out page after page of text.  Many software
engineers have experienced a similar feeling, where the code appears
to arise spontaneously as they watch themselves type.

I believe most real work gets done in this state.

My experience, however, is that the 'flow' period can end subtly and
gradually.  Without ever noticing a change, I notice that new work isn't
flowing out of my hands anymore, that I'm spending lots of time fixing
up mistakes I made just a few moments ago.  Instead of ideas flashing
confidently through my mind, doubts and questions arise.

At this point there is a temptation to throw some more hours at the problem.
'I'm here, and I was getting a lot of work done, why don't I just stay all
night until I figure this out?'  This is a trap!  Don't do it!

Instead, I suggest: go home, eat, shower, sleep, put yourself back together
again.  Resume the next day.  While you sleep, your mind will work on the
problem anyways, and you'll probably wake up with new ideas.  You'll get
more done between 10:00 am and 2:00 pm the next day, then if you stayed up
between midnight and 10:00 am.

There is a problem with this strategy: remotivating yourself in the morning.
If the project is one of your choice, that's usually not a problem.  If it's
something you have to do but don't enjoy, you have to balance the remotivation
problem versus the very low productivity of working without sleep.



=== Books for Serious Programmers

Out of all the thousands of books out there, three stand out:

Kernighan and Plaugher, _The Elements of Programming Style_.

Kernighan and Ritchie, _The C Programming Language_.

Brooks, _The Mythical Man Month_