New section on booleans. Elaborate thread safety.

[empserver] / doc / coding

                Guidelines for Writing Empire Code

             compiled and edited by Markus Armbruster

Preface
-------

Empire is an old program, and the age shows in places.  It took a lot
of people a lot of effort to write it.  Chances are that some of them
were smarter than you and me.  Treat the code with the respect it
deserves.

These guidelines are just that: guidelines.  Not law.  There's no
Empire Coding Police.  People hacked on Empire code long before I knew
it existed, and I hope they still will when I fade into Empire
history.  But there's substantial experience behind these guidelines,
so don't dismiss them cavalierly.

Here's our goal: algorithmic clarity.  All the rules and guidelines
are subservient to it.  If you'd have to obfuscate your code to
satisfy the letter of some guideline, don't!

Mind, I said `clarity', not `self-realization' or `expression of your
individuality'.  Dave Pare wrote:

  Be invisible.  When you make a change, think not about marking your
  place in history, or about showing off how much nicer your two-space
  tabs are than those old, icky eight-space tabs that the stupid
  empire hackers of old used, but think instead about the aesthetics
  of the whole.  The resulting lines of code should flow smoothly from
  the beginning of the procedure to the end.  Empire is 60,000 lines
  of code.  If you're the general case, you're only changing fifty
  lines, so be aware of that.

Some guidelines are more serious than others.  When I use words like
`is', `shall' or `must', I mean it.  When I write `should', it's a
recommendation.

Many guidelines concern style, i.e. they involve matters of taste.
That's how it has to be.  Uniformity of style is important for
clarity.

These guidelines don't attempt to be exhaustive.  More complete
guidelines that are mostly compatible with Empire can be found at
http://www.jetcafe.org/~jim/c-style.html


Source tree organization
------------------------

FIXME


Code formatting
---------------

    In My Egotistical Opinion, most people's C programs should be
    indented six feet downward and covered with dirt."
        -- Blair P. Houghton

Over the years, enough Empire coders lacked the good taste to preserve
the style of the original Empire code in their changes, and thus
turned the source code into an unreadable mess.  In 2003, we fed it to
GNU indent.

We tried to restore things to the original style, mostly.  There is
one noteable change: basic indentation is now four spaces.  Restoring
the original eight spaces would have resulted in many more long lines,
which would have to be broken by indent.  Since indent isn't good at
breaking lines tastefully, we reluctantly chose four instead.

FIXME mention src/scripts/intend-emp, even better: convert it into an
indent profile and mention that.

If you use Emacs, `stroustrup' indentation style produces satisfactory
results.  The command `c-set-style', normally bound to `C-c .', lets
you set the style for the current buffer.  Set variable
`c-default-style' to "stroustrup" to switch to this style for all
buffers.

Tab character use

Whether you use tab characters or not doesn't really matter that much,
but tab stops are every eight characters, period.

Indentation, placement of braces, function name

Basic indentation is four spaces.  The opening brace goes on the same
line as `if', `struct', etc.  Put `else' and do loops' `while' one the
same line as the closing brace.  You are encouraged to leave out
syntactically optional braces.  Don't indent case labels.

Example:

        if (is_fubar())
            despair();
        else {
            do {
                char ch = getchar();
                switch (ch) {
                case EOF:
                    return;
                case '\n':
                    break;
                default:
                    grind_along(ch);
                }
            } while (happy());
        }

In a function definition, the return type, function name and the
braces must all start on a new line, unindented, like this:

        type
        name(arguments)
        {
            body
        }

This does not apply to function declarations.

Breaking long lines

Line length should not exceed 75 characters.  Break such lines at a
logical place, preferably before an operator with low precedence.
Line up the continuation line with the innermost unclosed parenthesis,
if there is one, else indent it four spaces.

Example:

        FIXME

Bad example:

        FIXME

Blank lines

Use blank lines to separate different things.  Functions must be
separated by a blank line.  You are encouraged to insert a blank line
between a block's declarations and statements.

Spaces

There is a space after `for', `if', and `while'.  If `for' and `while'
are followed by `;' (empty loop body) on the same line, there is a
space before the `;'.  There is no space between the function name and
the `(', but there is a space after each `,'.  There should be no
space between type cast operator and operand.  There is no extra space
after '(' and before ')'.

Example:

        for (p = s; *p; ++p) ;
        printf("%ld\n", (long)(p-s));

Comments

The function comment describing what a function does goes directly
above the definition.

Comments to the right of code should start in column 32 if possible
(counting from zero).

Comment lines should be indented exactly like the code the belong to.

You are encouraged to format multi-line comments like this:

        /*
         * Please use complete sentences, with proper grammer,
         * capitalization and punctuation.  Use two spaces between
         * sentences.
         */

But please avoid elaborate boxes like this:

        /***********************************************************
         * Such decorations are a waste of time, hard to edit, and *
         * ugly, too.                                              *
         ***********************************************************/

Do not use

        // C++/C99 comments

because they are not portable C89.


Conditional compilation

Unless the conditional code is very short, please comment it like
this:

        #ifdef FOO
        ...
        #endif /* FOO */

        #ifdef BAR
        ...
        #else  /* !BAR */
        ...
        #endif /* !BAR */


Style
-----

Names

DoNotUseStudlyCaps!  Stick to lower_case and use underscores.  Upper
case is for macros and enumeration constants.

File names should not differ in case only, since not all file systems
distinguish case.  Best stick to lower case.  Avoid funny characters
in file names.  This includes space.

Preprocessor

Like many good things, the preprocessor is best used sparingly.
Especially conditional compilation.

Do not use the preprocessor to `improve' the syntax of C!

Macro names should be ALL_CAPS to make them stand out.  Otherwise,
they can be mistaken for objects or functions.  This is bad, because
`interesting' behavior can hide inside macros, like not evaluating
arguments, or changing them.  Exception: if a function-like macro
behaves exactly like a function, then you may use a lower case name.

Parameters that can take expression arguments must be parenthesized in
the expansion.  If the expansion is an expression, it should be
parenthesized as well.

You are encouraged to use enumeration constants instead of macros when
possible.  The existing code doesn't, but it makes sense all the same.

Comments

Every file should have a file comment FIXME

Every function should have a function comment that describes what it
does.  FIXME elaborate.  Writing such comments helps both future
maintainers and yourself: if it's too hard to write a concise function
comment, then your function is likely too complicated and could use a
redesign.

The existing code has very little useful comments, and it'll likely
take us years to fix it.  Please don't make it harder than it already
is.

Declarations

Do not declare system functions yourself; include the appropriate
system header.

Use prototypes, not old-style function declarations.

To get the best use of C's type checking, each function or variable
with external linkage should have exactly one declaration, in some
header file, and that declaration must be in scope at the definition.
No other declarations should exist.  In particular, please include the
appropriate header instead of just declaring stuff again.  The code
used to be full of this kind of junk, and it was quite some work to
clean it up.

Forward declarations of static functions should all go in one place
near the beginning of the file.

If you want a typedef name in addition to a structure or union tag,
give it the same name, like this:

        typedef struct foo foo;

Yes, this is incompatble with C++.  Reducing the number of names for
the same thing is more useful than compatibility to a programming
language we don't use.

Please don't hide away pointers with typedefs, like this:

       typedef struct foo *foop;

When I see `foo *', I *know* what it is.  When I see `foop', I have to
look it up.

Booleans

Do not use increment operators to set a variable to logical true!  It
obfuscates the purpose, and narrow variables could even overflow.
Just assign 1.  A lot of cleanup remains to be done there.

Type casts

Casting pointers to and from `void *' clutters the code and serves no
purpose (repeat after me: C is not C++).  It is also unsafe, because
the cast suppresses type warnings.

Casting function values to void clutters the code and serves little
purpose (repeat after me: C is not Pascal).

Casting an argument to the type of the parameter clutters the code and
serves no purpose (repeat after me: ISO C is not K&R C).

Factor out common code

Do not gratuitiously duplicate code!  Ken Stevens said it well, and
it's as relevant as ever:

  Cut-and-Paste coding is by far the biggest problem that the current
  Empire suffers from.  This is how cut-and-paste coding happens.  Joe
  Shmuck decides that he wants to add a new function to the server.
  So he goes hunting through the server to find some already existing
  code which does something similar to what he wants to do.  This is
  good.  You should always write new code to imitate old code.  What
  is bad is when Joe Shmuck decides to simply "copy" 200 lines of code
  from the old function into his new function, and then just change a
  couple of little things here and there to suit his needs.  This
  method, known as Cut-and-Paste coding is the fastest and easiest way
  to code.  However, it results in a server that is impossible to
  maintain.  What Joe _should_ have done, is "move" the 200 lines of
  code into a new _third_ function which the first two both call.
  This is called writing a "general solution" to handle both cases.
  Most of my work in the Empire2 project consisted in cleaning up
  after a bunch of Joe Shmucks.


Portability
-----------

FIXME C89, POSIX

FIXME sizes, printf formats

FIXME reserved names

FIXME conditional compilation is a last resort

FIXME s_char


Robustness
----------

Error checking

Check error conditions meticulously.  The existing code is bad enough
at this, please don't make it worse.

FIXME what to do on error

Buffer overruns

FIXME

Thread safety

Thread stacks cannot grow.  If you use too much automatic storage, you
can overrun the stack with disastrous consequences.  FIXME elaborate

Empire uses non-preemptive threads: a thread runs until it yields the
processor.  This simplifies programming, but there are still traps to
avoid.  Yielding the processor is an explicit thread operation, and
whether a thread operation yields is documented in empthread.h.
However, the operation may be buried in library code.

In particular, player input may yield.  Player output may yield only
if it goes to the current player, and his last command doesn't have
the C_MOD flag.  You can use this to simplify command code: set C_MOD
if you don't want to worry about yield on output.  This is commonly
done for commands that modify game state.

Be careful with static storage.  Read-only data is fine.  More than
one thread writing static data is problematic, but there are safe
uses.  For instance, a static scratch buffer that is never used across
`yields' is safe.

Yielding the processor invalidates *all* the game state you copied
into variables.  You have to reread and possibly recheck.  See below.

Accessing game state

Game state consists of several sets of objects, e.g. sectors, ships,
bmaps, ...  Each set is fully held in memory and backed by a file in
the data directory.

There are several ways to access an object in set FOO:

* You can get a pointer to the object (not a copy!) with getFOOp().

  This is inherently dangerous, because it allows you to update the
  object in memory without updating the disk file.  It's used mostly
  in update code, where it pays major performance dividends, but there
  are uses in other places.  Whether they are wise is debatable.

  Obviously, the object can change when you let another thread run.
  This is not an issue in update code, because it never yields.

* You can get a copy with getFOO() and write it back with putFOO().

  Yielding the processor invalidates the copy.  In particular, if you
  yield the processor between get and put, and another thread changes
  the game data, then put will clobber that change, possibly resulting
  in a corrupted game.

  Therefore, you have to re-get after a yield, and repeat any checks
  you already made.  If you can afford to bail out when something
  changed at all, use check_FOO_ok().

  Putting updates both the object in memory and the disk file.

  If you write a function that takes a pointer to an object, think
  twice before you put it, and if you do, mention it in the function
  comment.

* Bmaps have special access functions.

  Each country has two bmaps: the working bmap and the true bmap.
  Unfortunately, the bmap code calls the former `bmap' and the latter
  `map'.

  You update bmaps with map_set().  This doesn't write through to the
  file; instead, it returns non-zero when the update changed anything.
  In that case, you must write bmaps to disk with writemap() before
  you yield.  If you only updated the working bmap, then you can call
  writebmap() instead.


CVS
---

Commit related changes together, unrelated changes separately.

See chapter Change Logs in the GNU coding standards for guidelines on
log messages.  Try http://www.gnu.org/prep/standards_40.html#SEC40 or
search the web for `gnu coding standards change logs'.

Don't change whitespace gratuitiously, say just because your editor
screws up tabs.  Such changes make it much harder to figure out who
changed what and when.

\f
Historical guidelines, superseded by the above


Remarks from Dave Pare:

And now, a few editorial remarks on my views on Good Techniques for
Modifying Existing Software:

My safari through the hot and bug-ridden jungle that Empire has become
has given me some new insights on the modification of large software
packages.  My adventure has prompted me to propose some simple coding
guidelines for those dreaming of hacking on the code:

1) Be invisible.  When you make a change, think not about marking your
place in history, or about showing off how much nicer your two-space
tabs are than those old, icky eight-space tabs that the stupid empire
hackers of old used, but think instead about the asethetics of the whole.
The resulting lines of code should flow smoothly from the beginning of the
procedure to the end.  Empire is 60,000 lines of code.  If you're the
general case, you're only changing fifty lines, so be aware of that.

2) Procedurize.  If you find yourself in a triple-nested loop, or a five
hundred line procedure, perhaps it's because you need to split some of
that code off into a procedure.  In general, if you've got more than two
levels of tabs (or indentations), or more than one or two major loops in
a procedure, something may well be amiss.


Sasha Mikheev on indentation:

The empire indentation style can be achived by using 
indent -orig -i8 foo.c

or in old c-mode emacs (versions before 19.29):
;add this to .emacs
(setq c-indent-level 8)
(setq c-continued-statement-offset 8)
(setq c-argdecl-indent 8)
(setq c-brace-offset -8)
(setq c-label-offset -8)


Further comments by Ken Stevens:

1) Global variables
The only variables which should be global are constants.  If you write
a routine which changes a global variable, then you will corrupt the
data when two different players run that routine at the same time.

2) Portability.
The following code:
                char    a;
        #ifdef FOO
                unsigned char b;
        #endif FOO
Should be rewritten as:
                s_char  a;
        #ifdef FOO
                u_char  b;
        #endif /* FOO */

AIX has different conventions for signed chars, and IRIX requires the
/* comments */ after #endif.

3) Cut-and-Paste
Cut-and-Paste coding is by far the biggest problem that the current
Empire suffers from.  This is how cut-and-paste coding happens.  Joe
Shmuck decides that he wants to add a new function to the server.  So
he goes hunting through the server to find some already existing code
which does something similar to what he wants to do.  This is good.
You should always write new code to imitate old code.  What is bad is
when Joe Shmuck decides to simply "copy" 200 lines of code from the old
function into his new function, and then just change a couple of
little things here and there to suit his needs.  This method, known as
Cut-and-Paste coding is the fastest and easiest way to code.  However,
it results in a server that is impossible to maintain.  What Joe
_should_ have done, is "move" the 200 lines of code into a new _third_
function which the first two both call.  This is called writing a
"general solution" to handle both cases.  Most of my work in the
Empire2 project consisted in cleaning up after a bonch of Joe Shmucks.
I took repeated code and "consolidated" it into general function
libraries.

4) Good style.
Just to add to Dave's "Be Invisible" motto, I'd like to give a little
example to illustrate some basic do's and don'ts for coding style:

The following function has bad style:

double att_combat_eff(com,own)
struct combat *com;
natid own;{
  double str;
  double eff=1.0;
  if(com->type==EF_SECTOR)
  {
    eff=com->eff/100.0;
    if(com->own==own){
      str=com->sct_dcp->d_ostr;
    }else{
str=com->sct_dcp->d_dstr;eff=2.0+(str-2.0)*eff;
    }else if(com->type==EF_SHIP&&com->own!=own)
      eff=(1.0+com->shp_mcp->m_armor/100.0);
  return eff;}

Here is the same function written with "good" style:

double
att_combat_eff(com, own)
        struct  combat *com;
        natid   own;
{
        double  eff = 1.0;
        double  str;

        if (com->type == EF_SECTOR) {
                eff = com->eff / 100.0;
                if (com->own == own)
                        str = com->sct_dcp->d_ostr;
                else
                        str = com->sct_dcp->d_dstr;
                eff = 2.0 + (str - 2.0) * eff;
        } else if (com->type == EF_SHIP && com->own != own)
                eff = (1.0 + com->shp_mcp->m_armor / 100.0);

        return eff;
}

These are all the things I fixed when changing the bad to the good:
- Function names should always start a new line (so you can search for them)
- There should always be a space after a ","
- Function arguments should be indented 8 spaces
- There should always be a tab after a type declaration
- Opening function bracket should be on a line by itself
- Indentation should be 8 spaces
- There should always be a space on both sides of every operator
- There should always be a space after if, for, while, switch
- The opening bracket should be on the same line as the if
- There should always be a space on either side of a {
- There should always be a new line after a ;
- The closing function bracket should be on a line by itself