1 Guidelines for Writing Empire Code
3 compiled and edited by Markus Armbruster
8 Empire is an old program, and the age shows in places. It took a lot
9 of people a lot of effort to write it. Chances are that some of them
10 were smarter than you and me. Treat the code with the respect it
13 These guidelines are just that: guidelines. Not law. There's no
14 Empire Coding Police. People hacked on Empire code long before I knew
15 it existed, and I hope they still will when I fade into Empire
16 history. But there's substantial experience behind these guidelines,
17 so don't dismiss them cavalierly.
19 Here's our goal: algorithmic clarity. All the rules and guidelines
20 are subservient to it. If you'd have to obfuscate your code to
21 satisfy the letter of some guideline, don't!
23 Mind, I said `clarity', not `self-realization' or `expression of your
24 individuality'. Dave Pare wrote:
26 Be invisible. When you make a change, think not about marking your
27 place in history, or about showing off how much nicer your two-space
28 tabs are than those old, icky eight-space tabs that the stupid
29 empire hackers of old used, but think instead about the aesthetics
30 of the whole. The resulting lines of code should flow smoothly from
31 the beginning of the procedure to the end. Empire is 60,000 lines
32 of code. If you're the general case, you're only changing fifty
33 lines, so be aware of that.
35 Some guidelines are more serious than others. When I use words like
36 `is', `shall' or `must', I mean it. When I write `should', it's a
39 Many guidelines concern style, i.e. they involve matters of taste.
40 That's how it has to be. Uniformity of style is important for
43 These guidelines don't attempt to be exhaustive. More complete
44 guidelines that are mostly compatible with Empire can be found at
45 <http://www.jetcafe.org/~jim/c-style.html>.
47 See also doc/contributing.
50 Source tree organization
51 ------------------------
59 In My Egotistical Opinion, most people's C programs should be
60 indented six feet downward and covered with dirt."
63 Over the years, enough Empire coders lacked the good taste to preserve
64 the style of the original Empire code in their changes, and thus
65 turned the source code into an unreadable mess. In 2003, we fed it to
68 We tried to restore things to the original style, mostly. There is
69 one notable change: basic indentation is now four spaces. Restoring
70 the original eight spaces would have resulted in many more long lines,
71 which would have to be broken by indent. Since indent isn't good at
72 breaking lines tastefully, we reluctantly chose four instead.
74 FIXME mention src/scripts/intend-emp, even better: convert it into an
75 indent profile and mention that.
77 If you use Emacs, `stroustrup' indentation style produces satisfactory
78 results. The command `c-set-style', normally bound to `C-c .', lets
79 you set the style for the current buffer. Set variable
80 `c-default-style' to "stroustrup" to switch to this style for all
83 Avoid gratuitous space change
85 Don't change whitespace gratuitiously, say just because your editor
86 screws up tabs. Such changes make it much harder to figure out who
87 changed what and when.
91 Whether you use TAB characters or not doesn't really matter that much,
92 but TAB stops are every eight characters, period.
94 Indentation, placement of braces, function name
96 Basic indentation is four spaces. The opening brace goes on the same
97 line as `if', `struct', etc. Put `else' and do loops' `while' one the
98 same line as the closing brace. You are encouraged to leave out
99 syntactically optional braces. Don't indent case labels.
119 In a function definition, the return type, function name and the
120 braces must all start on a new line, unindented, like this:
128 This does not apply to function declarations.
132 Line length should not exceed 75 characters. Break such lines at a
133 logical place, preferably before an operator with low precedence.
134 Line up the continuation line with the innermost unclosed parenthesis,
135 if there is one, else indent it four spaces.
147 Use blank lines to separate different things. Functions must be
148 separated by a blank line. You are encouraged to insert a blank line
149 between a block's declarations and statements.
153 There is a space after `for', `if', and `while'. If `for' and `while'
154 are followed by `;' (empty loop body) on the same line, there is a
155 space before the `;'. There is no space between the function name and
156 the `(', but there is a space after each `,'. There should be no
157 space between type cast operator and operand. There is no extra space
158 after '(' and before ')'.
162 for (p = s; *p; ++p) ;
163 printf("%ld\n", (long)(p-s));
167 The function comment describing what a function does goes directly
168 above the definition.
170 Comments to the right of code should start in column 32 if possible
171 (counting from zero).
173 Comment lines should be indented exactly like the code they belong to.
175 You are encouraged to format multi-line comments like this:
178 * Please use complete sentences, with proper grammar,
179 * capitalization and punctuation. Use two spaces between
183 But please avoid elaborate boxes like this:
185 /***********************************************************
186 * Such decorations are a waste of time, hard to edit, and *
188 ***********************************************************/
194 because they are not portable C89.
196 Conditional compilation
198 Unless the conditional code is very short, please comment it like
217 DoNotUseStudlyCaps! Stick to lower_case and use underscores. Upper
218 case is for macros and enumeration constants.
220 File names should not differ in case only, since not all file systems
221 distinguish case. Best stick to lower case. Avoid funny characters
222 in file names. This includes space.
226 Like many good things, the preprocessor is best used sparingly.
227 Especially conditional compilation.
229 Do not use the preprocessor to `improve' the syntax of C!
231 Macro names should be ALL_CAPS to make them stand out. Otherwise,
232 they can be mistaken for objects or functions. This is bad, because
233 `interesting' behavior can hide inside macros, like not evaluating
234 arguments, or changing them. Exception: if a function-like macro
235 behaves exactly like a function, then you may use a lower case name.
237 Parameters that can take expression arguments must be parenthesized in
238 the expansion. If the expansion is an expression, it should be
239 parenthesized as well.
241 You are encouraged to use enumeration constants instead of macros when
242 possible. The existing code doesn't, but it makes sense all the same.
246 Every file should have a file comment FIXME
248 Every function should have a function comment that describes what it
249 does, unless it's blatantly obvious. If the function is longer than a
250 few lines, it's almost certainly not obvious.
252 The function comment should serve as a contract: state the
253 preconditions, side effects, return values. Make sure to cover error
256 Writing such comments helps both future maintainers and yourself: when
257 writing a concise function comment is too hard, then your function is
258 likely too complicated and could use a redesign.
260 Use @param to refer to parameters. Use func() to refer to functions
261 or function-like macros. Use arr[] to refer to arrays. This is to
262 make the references stand out and to avoid ambiguity.
267 * Read update schedule from file @fname.
268 * Put the first @n-1 updates after @t0 into @sched[] in ascending order,
269 * terminated with a zero.
270 * Use @anchor as initial anchor for anchor-relative times.
271 * Return 0 on success, -1 on failure.
274 read_schedule(char *fname, time_t sched[], int n, time_t t0, time_t anchor)
276 When documenting a struct or union, use @member to refer to its
279 The existing code has very little useful comments, and it'll likely
280 take us years to fix it. Please don't make it harder than it already
285 Do not declare system functions yourself; include the appropriate
288 Use prototypes, not old-style function declarations.
290 To get the best use of C's type checking, each function or variable
291 with external linkage should have exactly one declaration, in some
292 header file, and that declaration must be in scope at the definition.
293 No other declarations should exist. In particular, please include the
294 appropriate header instead of just declaring stuff again. The code
295 used to be full of this kind of junk, and it was quite some work to
298 Forward declarations of static functions should all go in one place
299 near the beginning of the file.
301 If you want a typedef name in addition to a structure or union tag,
302 give it the same name, like this:
304 typedef struct foo foo;
306 Yes, this is incompatible with C++. Reducing the number of names for
307 the same thing is more useful than compatibility to a programming
308 language we don't use.
310 Please don't hide away pointers with typedefs, like this:
312 typedef struct foo *foop;
314 When I see `foo *', I *know* what it is. When I see `foop', I have to
319 Do not use increment operators to set a variable to logical true! It
320 obfuscates the purpose, and narrow variables could even overflow.
321 Just assign 1. A lot of cleanup remains to be done there.
323 Null pointer constant
325 Please use NULL for clarity, not just 0.
329 Casting pointers to and from `void *' clutters the code and serves no
330 purpose (repeat after me: C is not C++). It is also unsafe, because
331 the cast suppresses type warnings.
333 Casting function values to void clutters the code and serves little
334 purpose (repeat after me: C is not Pascal).
336 Casting an argument to the type of the parameter clutters the code and
337 serves no purpose (repeat after me: ISO C is not K&R C).
339 Factor out common code
341 Do not gratuitously duplicate code! Ken Stevens said it well, and
342 it's as relevant as ever:
344 Cut-and-Paste coding is by far the biggest problem that the current
345 Empire suffers from. This is how cut-and-paste coding happens. Joe
346 Shmuck decides that he wants to add a new function to the server.
347 So he goes hunting through the server to find some already existing
348 code which does something similar to what he wants to do. This is
349 good. You should always write new code to imitate old code. What
350 is bad is when Joe Shmuck decides to simply "copy" 200 lines of code
351 from the old function into his new function, and then just change a
352 couple of little things here and there to suit his needs. This
353 method, known as Cut-and-Paste coding is the fastest and easiest way
354 to code. However, it results in a server that is impossible to
355 maintain. What Joe _should_ have done, is "move" the 200 lines of
356 code into a new _third_ function which the first two both call.
357 This is called writing a "general solution" to handle both cases.
358 Most of my work in the Empire2 project consisted in cleaning up
359 after a bunch of Joe Shmucks.
367 FIXME sizes, printf formats
371 FIXME conditional compilation is a last resort
379 Check error conditions meticulously. The existing code is bad enough
380 at this, please don't make it worse.
382 FIXME what to do on error
386 In many places, the code checks for conditions that should not happen,
387 and then tries to recover. This is sound defensive programming.
388 Please use CANT_HAPPEN() and CANT_REACH() for this purpose, because
389 they log the error condition, and optionally abort the program, or
390 write out a core dump. This is called an "oops".
398 Thread stacks cannot grow. If you use too much automatic storage, you
399 can overrun the stack with disastrous consequences. FIXME elaborate
401 Empire uses non-preemptive threads: a thread runs until it yields the
402 processor. This simplifies programming, but there are still traps to
403 avoid. Yielding the processor is an explicit thread operation, and
404 whether a thread operation yields is documented in empthread.h.
405 However, the operation may be buried in library code.
407 In particular, player input may yield. Player output may yield only
408 if it goes to the current player, and his last command doesn't have
409 the C_MOD flag. You can use this to simplify command code: set C_MOD
410 if you don't want to worry about yield on output. This is commonly
411 done for commands that modify game state.
413 Be careful with static storage. Read-only data is fine. More than
414 one thread writing static data is problematic, but there are safe
415 uses. For instance, a static scratch buffer that is never used across
418 Yielding the processor invalidates *all* the game state you copied
419 into variables. You have to reread and possibly recheck. See below.
423 Game state consists of several sets of objects, e.g. sectors, ships,
424 bmaps, ... Each set is fully held in memory and backed by a file in
427 There are several ways to access an object in set FOO:
429 * You can get a pointer to the object (not a copy!) with getFOOp().
431 This is inherently dangerous, because it allows you to update the
432 object in memory without updating the disk file. It's used mostly
433 in update code, where it pays major performance dividends, but there
434 are uses in other places. Whether they are wise is debatable.
436 Obviously, the object can change when you let another thread run.
437 This is not an issue in update code, because it never yields.
439 * You can get a copy with getFOO() and write it back with putFOO().
441 Putting updates both the object in memory and the disk file.
443 Any change to the object invalidates the copy. Putting such an
444 invalid copy will clobber the change(s) that invalidated it,
445 possibly resulting in corrupted game state. The code oopses on such
446 puts, but it can't repair the damage.
448 There are two common ways to invalidate a copy: calling a function
449 that updates the object you copied (unless it does that through your
450 copy), and yielding the processor, which lets other threads update
451 the object you copied.
453 Therefore, you have to re-get after a possible invalidation, and
454 deal with changes. In particular, if you checked whether the object
455 is suitable for a task, you need to check again after re-getting it.
456 If you can afford to bail out when something changed, use
459 Function comments should state what objects the function can update.
460 Unfortunately, they generally don't.
462 It's best to keep puts close to gets, both at runtime and in the
465 * Bmaps have special access functions.
467 Each country has two bmaps: the working bmap and the true bmap.
468 Unfortunately, the bmap code calls the former `bmap' and the latter
471 You update bmaps with map_set(). This doesn't write through to the
472 file; instead, it returns non-zero when the update changed anything.
473 In that case, you must write bmaps to disk with writemap() before
474 you yield. If you only updated the working bmap, then you can call
479 Reading player input can fail, and you must check for that.
480 Neglecting it can break the interrupt feature for players (normally
481 Ctrl-C), and produce extra prompts that could confuse clients. Even
482 worse is neglecting it in a loop that terminates only when input is
485 When reading input fails, you should normally abort the command with
486 status RET_SYN. There are exceptions, e.g. when aborting a pinpoint
487 bombing run over the target.
489 Some functions to read player input return a special error value you
490 can check, e.g. recvclient(), prmptrd() and uprmptrd() return a
491 negative value, getstring() and getstarg() return NULL.
493 Others return the same error value for failed read and for successful
494 read of input that is invalid. Then you need to check
495 player->aborted; if it is set after a read, the read failed.
498 Historical guidelines, superseded by the above
501 Remarks from Dave Pare:
503 And now, a few editorial remarks on my views on Good Techniques for
504 Modifying Existing Software:
506 My safari through the hot and bug-ridden jungle that Empire has become
507 has given me some new insights on the modification of large software
508 packages. My adventure has prompted me to propose some simple coding
509 guidelines for those dreaming of hacking on the code:
511 1) Be invisible. When you make a change, think not about marking your
512 place in history, or about showing off how much nicer your two-space
513 tabs are than those old, icky eight-space tabs that the stupid empire
514 hackers of old used, but think instead about the asethetics of the whole.
515 The resulting lines of code should flow smoothly from the beginning of the
516 procedure to the end. Empire is 60,000 lines of code. If you're the
517 general case, you're only changing fifty lines, so be aware of that.
519 2) Procedurize. If you find yourself in a triple-nested loop, or a five
520 hundred line procedure, perhaps it's because you need to split some of
521 that code off into a procedure. In general, if you've got more than two
522 levels of tabs (or indentations), or more than one or two major loops in
523 a procedure, something may well be amiss.
526 Sasha Mikheev on indentation:
528 The empire indentation style can be achived by using
529 indent -orig -i8 foo.c
531 or in old c-mode emacs (versions before 19.29):
533 (setq c-indent-level 8)
534 (setq c-continued-statement-offset 8)
535 (setq c-argdecl-indent 8)
536 (setq c-brace-offset -8)
537 (setq c-label-offset -8)
540 Further comments by Ken Stevens:
543 The only variables which should be global are constants. If you write
544 a routine which changes a global variable, then you will corrupt the
545 data when two different players run that routine at the same time.
553 Should be rewritten as:
559 AIX has different conventions for signed chars, and IRIX requires the
560 /* comments */ after #endif.
563 Cut-and-Paste coding is by far the biggest problem that the current
564 Empire suffers from. This is how cut-and-paste coding happens. Joe
565 Shmuck decides that he wants to add a new function to the server. So
566 he goes hunting through the server to find some already existing code
567 which does something similar to what he wants to do. This is good.
568 You should always write new code to imitate old code. What is bad is
569 when Joe Shmuck decides to simply "copy" 200 lines of code from the old
570 function into his new function, and then just change a couple of
571 little things here and there to suit his needs. This method, known as
572 Cut-and-Paste coding is the fastest and easiest way to code. However,
573 it results in a server that is impossible to maintain. What Joe
574 _should_ have done, is "move" the 200 lines of code into a new _third_
575 function which the first two both call. This is called writing a
576 "general solution" to handle both cases. Most of my work in the
577 Empire2 project consisted in cleaning up after a bonch of Joe Shmucks.
578 I took repeated code and "consolidated" it into general function
582 Just to add to Dave's "Be Invisible" motto, I'd like to give a little
583 example to illustrate some basic do's and don'ts for coding style:
585 The following function has bad style:
587 double att_combat_eff(com,own)
592 if(com->type==EF_SECTOR)
596 str=com->sct_dcp->d_ostr;
598 str=com->sct_dcp->d_dstr;eff=2.0+(str-2.0)*eff;
599 }else if(com->type==EF_SHIP&&com->own!=own)
600 eff=(1.0+com->shp_mcp->m_armor/100.0);
603 Here is the same function written with "good" style:
606 att_combat_eff(com, own)
613 if (com->type == EF_SECTOR) {
614 eff = com->eff / 100.0;
616 str = com->sct_dcp->d_ostr;
618 str = com->sct_dcp->d_dstr;
619 eff = 2.0 + (str - 2.0) * eff;
620 } else if (com->type == EF_SHIP && com->own != own)
621 eff = (1.0 + com->shp_mcp->m_armor / 100.0);
626 These are all the things I fixed when changing the bad to the good:
627 - Function names should always start a new line (so you can search for them)
628 - There should always be a space after a ","
629 - Function arguments should be indented 8 spaces
630 - There should always be a tab after a type declaration
631 - Opening function bracket should be on a line by itself
632 - Indentation should be 8 spaces
633 - There should always be a space on both sides of every operator
634 - There should always be a space after if, for, while, switch
635 - The opening bracket should be on the same line as the if
636 - There should always be a space on either side of a {
637 - There should always be a new line after a ;
638 - The closing function bracket should be on a line by itself