aboutsummaryrefslogtreecommitdiff
path: root/Documentation/CodingGuidelines
blob: c4cb5ff0d477938b8fd49749c3589c5afbb04221 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
Like other projects, we also have some guidelines to keep to the
code.  For Git in general, a few rough rules are:

 - Most importantly, we never say "It's in POSIX; we'll happily
   ignore your needs should your system not conform to it."
   We live in the real world.

 - However, we often say "Let's stay away from that construct,
   it's not even in POSIX".

 - In spite of the above two rules, we sometimes say "Although
   this is not in POSIX, it (is so convenient | makes the code
   much more readable | has other good characteristics) and
   practically all the platforms we care about support it, so
   let's use it".

   Again, we live in the real world, and it is sometimes a
   judgement call, the decision based more on real world
   constraints people face than what the paper standard says.

 - Fixing style violations while working on a real change as a
   preparatory clean-up step is good, but otherwise avoid useless code
   churn for the sake of conforming to the style.

   "Once it _is_ in the tree, it's not really worth the patch noise to
   go and fix it up."
   Cf. http://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html

Make your code readable and sensible, and don't try to be clever.

As for more concrete guidelines, just imitate the existing code
(this is a good guideline, no matter which project you are
contributing to). It is always preferable to match the _local_
convention. New code added to Git suite is expected to match
the overall style of existing code. Modifications to existing
code is expected to match the style the surrounding code already
uses (even if it doesn't match the overall style of existing code).

But if you must have a list of rules, here they are.

For shell scripts specifically (not exhaustive):

 - We use tabs for indentation.

 - Case arms are indented at the same depth as case and esac lines,
   like this:

	case "$variable" in
	pattern1)
		do this
		;;
	pattern2)
		do that
		;;
	esac

 - Redirection operators should be written with space before, but no
   space after them.  In other words, write 'echo test >"$file"'
   instead of 'echo test> $file' or 'echo test > $file'.  Note that
   even though it is not required by POSIX to double-quote the
   redirection target in a variable (as shown above), our code does so
   because some versions of bash issue a warning without the quotes.

	(incorrect)
	cat hello > world < universe
	echo hello >$world

	(correct)
	cat hello >world <universe
	echo hello >"$world"

 - We prefer $( ... ) for command substitution; unlike ``, it
   properly nests.  It should have been the way Bourne spelled
   it from day one, but unfortunately isn't.

 - If you want to find out if a command is available on the user's
   $PATH, you should use 'type <command>', instead of 'which <command>'.
   The output of 'which' is not machine parseable and its exit code
   is not reliable across platforms.

 - We use POSIX compliant parameter substitutions and avoid bashisms;
   namely:

   - We use ${parameter-word} and its [-=?+] siblings, and their
     colon'ed "unset or null" form.

   - We use ${parameter#word} and its [#%] siblings, and their
     doubled "longest matching" form.

   - No "Substring Expansion" ${parameter:offset:length}.

   - No shell arrays.

   - No strlen ${#parameter}.

   - No pattern replacement ${parameter/pattern/string}.

 - We use Arithmetic Expansion $(( ... )).

 - Inside Arithmetic Expansion, spell shell variables with $ in front
   of them, as some shells do not grok $((x)) while accepting $(($x))
   just fine (e.g. dash older than 0.5.4).

 - We do not use Process Substitution <(list) or >(list).

 - Do not write control structures on a single line with semicolon.
   "then" should be on the next line for if statements, and "do"
   should be on the next line for "while" and "for".

	(incorrect)
	if test -f hello; then
		do this
	fi

	(correct)
	if test -f hello
	then
		do this
	fi

 - We prefer "test" over "[ ... ]".

 - We do not write the noiseword "function" in front of shell
   functions.

 - We prefer a space between the function name and the parentheses,
   and no space inside the parentheses. The opening "{" should also
   be on the same line.

	(incorrect)
	my_function(){
		...

	(correct)
	my_function () {
		...

 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
   [::], [==], or [..]) for portability.

   - We do not use \{m,n\};

   - We do not use -E;

   - We do not use ? or + (which are \{0,1\} and \{1,\}
     respectively in BRE) but that goes without saying as these
     are ERE elements not BRE (note that \? and \+ are not even part
     of BRE -- making them accessible from BRE is a GNU extension).

 - Use Git's gettext wrappers in git-sh-i18n to make the user
   interface translatable. See "Marking strings for translation" in
   po/README.

 - We do not write our "test" command with "-a" and "-o" and use "&&"
   or "||" to concatenate multiple "test" commands instead, because
   the use of "-a/-o" is often error-prone.  E.g.

     test -n "$x" -a "$a" = "$b"

   is buggy and breaks when $x is "=", but

     test -n "$x" && test "$a" = "$b"

   does not have such a problem.


For C programs:

 - We use tabs to indent, and interpret tabs as taking up to
   8 spaces.

 - We try to keep to at most 80 characters per line.

 - As a Git developer we assume you have a reasonably modern compiler
   and we recommend you to enable the DEVELOPER makefile knob to
   ensure your patch is clear of all compiler warnings we care about,
   by e.g. "echo DEVELOPER=1 >>config.mak".

 - We try to support a wide range of C compilers to compile Git with,
   including old ones. That means that you should not use C99
   initializers, even if a lot of compilers grok it.

 - Variables have to be declared at the beginning of the block.

 - NULL pointers shall be written as NULL, not as 0.

 - When declaring pointers, the star sides with the variable
   name, i.e. "char *string", not "char* string" or
   "char * string".  This makes it easier to understand code
   like "char *string, c;".

 - Use whitespace around operators and keywords, but not inside
   parentheses and not around functions. So:

        while (condition)
		func(bar + 1);

   and not:

        while( condition )
		func (bar+1);

 - We avoid using braces unnecessarily.  I.e.

	if (bla) {
		x = 1;
	}

   is frowned upon. But there are a few exceptions:

	- When the statement extends over a few lines (e.g., a while loop
	  with an embedded conditional, or a comment). E.g.:

		while (foo) {
			if (x)
				one();
			else
				two();
		}

		if (foo) {
			/*
			 * This one requires some explanation,
			 * so we're better off with braces to make
			 * it obvious that the indentation is correct.
			 */
			doit();
		}

	- When there are multiple arms to a conditional and some of them
	  require braces, enclose even a single line block in braces for
	  consistency. E.g.:

		if (foo) {
			doit();
		} else {
			one();
			two();
			three();
		}

 - We try to avoid assignments in the condition of an "if" statement.

 - Try to make your code understandable.  You may put comments
   in, but comments invariably tend to stale out when the code
   they were describing changes.  Often splitting a function
   into two makes the intention of the code much clearer.

 - Multi-line comments include their delimiters on separate lines from
   the text.  E.g.

	/*
	 * A very long
	 * multi-line comment.
	 */

   Note however that a comment that explains a translatable string to
   translators uses a convention of starting with a magic token
   "TRANSLATORS: ", e.g.

	/*
	 * TRANSLATORS: here is a comment that explains the string to
	 * be translated, that follows immediately after it.
	 */
	_("Here is a translatable string explained by the above.");

 - Double negation is often harder to understand than no negation
   at all.

 - There are two schools of thought when it comes to comparison,
   especially inside a loop. Some people prefer to have the less stable
   value on the left hand side and the more stable value on the right hand
   side, e.g. if you have a loop that counts variable i down to the
   lower bound,

	while (i > lower_bound) {
		do something;
		i--;
	}

   Other people prefer to have the textual order of values match the
   actual order of values in their comparison, so that they can
   mentally draw a number line from left to right and place these
   values in order, i.e.

	while (lower_bound < i) {
		do something;
		i--;
	}

   Both are valid, and we use both.  However, the more "stable" the
   stable side becomes, the more we tend to prefer the former
   (comparison with a constant, "i > 0", is an extreme example).
   Just do not mix styles in the same part of the code and mimic
   existing styles in the neighbourhood.

 - There are two schools of thought when it comes to splitting a long
   logical line into multiple lines.  Some people push the second and
   subsequent lines far enough to the right with tabs and align them:

        if (the_beginning_of_a_very_long_expression_that_has_to ||
		span_more_than_a_single_line_of ||
		the_source_text) {
                ...

   while other people prefer to align the second and the subsequent
   lines with the column immediately inside the opening parenthesis,
   with tabs and spaces, following our "tabstop is always a multiple
   of 8" convention:

        if (the_beginning_of_a_very_long_expression_that_has_to ||
	    span_more_than_a_single_line_of ||
	    the_source_text) {
                ...

   Both are valid, and we use both.  Again, just do not mix styles in
   the same part of the code and mimic existing styles in the
   neighbourhood.

 - When splitting a long logical line, some people change line before
   a binary operator, so that the result looks like a parse tree when
   you turn your head 90-degrees counterclockwise:

        if (the_beginning_of_a_very_long_expression_that_has_to
	    || span_more_than_a_single_line_of_the_source_text) {

   while other people prefer to leave the operator at the end of the
   line:

        if (the_beginning_of_a_very_long_expression_that_has_to ||
	    span_more_than_a_single_line_of_the_source_text) {

   Both are valid, but we tend to use the latter more, unless the
   expression gets fairly complex, in which case the former tends to
   be easier to read.  Again, just do not mix styles in the same part
   of the code and mimic existing styles in the neighbourhood.

 - When splitting a long logical line, with everything else being
   equal, it is preferable to split after the operator at higher
   level in the parse tree.  That is, this is more preferable:

	if (a_very_long_variable * that_is_used_in +
	    a_very_long_expression) {
		...

   than

	if (a_very_long_variable *
	    that_is_used_in + a_very_long_expression) {
		...

 - Some clever tricks, like using the !! operator with arithmetic
   constructs, can be extremely confusing to others.  Avoid them,
   unless there is a compelling reason to use them.

 - Use the API.  No, really.  We have a strbuf (variable length
   string), several arrays with the ALLOC_GROW() macro, a
   string_list for sorted string lists, a hash map (mapping struct
   objects) named "struct decorate", amongst other things.

 - When you come up with an API, document it.

 - The first #include in C files, except in platform specific compat/
   implementations, must be either "git-compat-util.h", "cache.h" or
   "builtin.h".  You do not have to include more than one of these.

 - A C file must directly include the header files that declare the
   functions and the types it uses, except for the functions and types
   that are made available to it by including one of the header files
   it must include by the previous rule.

 - If you are planning a new command, consider writing it in shell
   or perl first, so that changes in semantics can be easily
   changed and discussed.  Many Git commands started out like
   that, and a few are still scripts.

 - Avoid introducing a new dependency into Git. This means you
   usually should stay away from scripting languages not already
   used in the Git core command set (unless your command is clearly
   separate from it, such as an importer to convert random-scm-X
   repositories to Git).

 - When we pass <string, length> pair to functions, we should try to
   pass them in that order.

 - Use Git's gettext wrappers to make the user interface
   translatable. See "Marking strings for translation" in po/README.

For Perl programs:

 - Most of the C guidelines above apply.

 - We try to support Perl 5.8 and later ("use Perl 5.008").

 - use strict and use warnings are strongly preferred.

 - Don't overuse statement modifiers unless using them makes the
   result easier to follow.

	... do something ...
	do_this() unless (condition);
        ... do something else ...

   is more readable than:

	... do something ...
	unless (condition) {
		do_this();
	}
        ... do something else ...

   *only* when the condition is so rare that do_this() will be almost
   always called.

 - We try to avoid assignments inside "if ()" conditions.

 - Learn and use Git.pm if you need that functionality.

 - For Emacs, it's useful to put the following in
   GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:

    ;; note the first part is useful for C editing, too
    ((nil . ((indent-tabs-mode . t)
                  (tab-width . 8)
                  (fill-column . 80)))
     (cperl-mode . ((cperl-indent-level . 8)
                    (cperl-extra-newline-before-brace . nil)
                    (cperl-merge-trailing-else . t))))

For Python scripts:

 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).

 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.

 - Where required libraries do not restrict us to Python 2, we try to
   also be compatible with Python 3.1 and later.

 - When you must differentiate between Unicode literals and byte string
   literals, it is OK to use the 'b' prefix.  Even though the Python
   documentation for version 2.6 does not mention this prefix, it has
   been supported since version 2.6.0.

Error Messages

 - Do not end error messages with a full stop.

 - Do not capitalize ("unable to open %s", not "Unable to open %s")

 - Say what the error is first ("cannot open %s", not "%s: cannot open")


Externally Visible Names

 - For configuration variable names, follow the existing convention:

   . The section name indicates the affected subsystem.

   . The subsection name, if any, indicates which of an unbounded set
     of things to set the value for.

   . The variable name describes the effect of tweaking this knob.

   The section and variable names that consist of multiple words are
   formed by concatenating the words without punctuations (e.g. `-`),
   and are broken using bumpyCaps in documentation as a hint to the
   reader.

   When choosing the variable namespace, do not use variable name for
   specifying possibly unbounded set of things, most notably anything
   an end user can freely come up with (e.g. branch names).  Instead,
   use subsection names or variable values, like the existing variable
   branch.<name>.description does.


Writing Documentation:

 Most (if not all) of the documentation pages are written in the
 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
 processed into HTML and manpages (e.g. git.html and git.1 in the
 same directory).

 The documentation liberally mixes US and UK English (en_US/UK)
 norms for spelling and grammar, which is somewhat unfortunate.
 In an ideal world, it would have been better if it consistently
 used only one and not the other, and we would have picked en_US
 (if you wish to correct the English of some of the existing
 documentation, please see the documentation-related advice in the
 Documentation/SubmittingPatches file).

 Every user-visible change should be reflected in the documentation.
 The same general rule as for code applies -- imitate the existing
 conventions.

 A few commented examples follow to provide reference when writing or
 modifying command usage strings and synopsis sections in the manual
 pages:

 Placeholders are spelled in lowercase and enclosed in angle brackets:
   <file>
   --sort=<key>
   --abbrev[=<n>]

 If a placeholder has multiple words, they are separated by dashes:
   <new-branch-name>
   --template=<template-directory>

 Possibility of multiple occurrences is indicated by three dots:
   <file>...
   (One or more of <file>.)

 Optional parts are enclosed in square brackets:
   [<extra>]
   (Zero or one <extra>.)

   --exec-path[=<path>]
   (Option with an optional argument.  Note that the "=" is inside the
   brackets.)

   [<patch>...]
   (Zero or more of <patch>.  Note that the dots are inside, not
   outside the brackets.)

 Multiple alternatives are indicated with vertical bars:
   [-q | --quiet]
   [--utf8 | --no-utf8]

 Parentheses are used for grouping:
   [(<rev> | <range>)...]
   (Any number of either <rev> or <range>.  Parens are needed to make
   it clear that "..." pertains to both <rev> and <range>.)

   [(-p <parent>)...]
   (Any number of option -p, each with one <parent> argument.)

   git remote set-head <name> (-a | -d | <branch>)
   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
   brackets) be provided.)

 And a somewhat more contrived example:
   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
   Here "=" is outside the brackets, because "--diff-filter=" is a
   valid usage.  "*" has its own pair of brackets, because it can
   (optionally) be specified only when one or more of the letters is
   also provided.

  A note on notation:
   Use 'git' (all lowercase) when talking about commands i.e. something
   the user would type into a shell and use 'Git' (uppercase first letter)
   when talking about the version control system and its properties.

 A few commented examples follow to provide reference when writing or
 modifying paragraphs or option/command explanations that contain options
 or commands:

 Literal examples (e.g. use of command-line options, command names,
 branch names, configuration and environment variables) must be
 typeset in monospace (i.e. wrapped with backticks):
   `--pretty=oneline`
   `git rev-list`
   `remote.pushDefault`
   `GIT_DIR`
   `HEAD`

 An environment variable must be prefixed with "$" only when referring to its
 value and not when referring to the variable itself, in this case there is
 nothing to add except the backticks:
   `GIT_DIR` is specified
   `$GIT_DIR/hooks/pre-receive`

 Word phrases enclosed in `backtick characters` are rendered literally
 and will not be further expanded. The use of `backticks` to achieve the
 previous rule means that literal examples should not use AsciiDoc
 escapes.
   Correct:
      `--pretty=oneline`
   Incorrect:
      `\--pretty=oneline`

 If some place in the documentation needs to typeset a command usage
 example with inline substitutions, it is fine to use +monospaced and
 inline substituted text+ instead of `monospaced literal text`, and with
 the former, the part that should not get substituted must be
 quoted/escaped.