Merge branch 'es/osx-header-pollutes-mask-macro' into maint
[git/git.git] / Documentation / CodingGuidelines
1 Like other projects, we also have some guidelines to keep to the
2 code. For Git in general, a few rough rules are:
3
4 - Most importantly, we never say "It's in POSIX; we'll happily
5 ignore your needs should your system not conform to it."
6 We live in the real world.
7
8 - However, we often say "Let's stay away from that construct,
9 it's not even in POSIX".
10
11 - In spite of the above two rules, we sometimes say "Although
12 this is not in POSIX, it (is so convenient | makes the code
13 much more readable | has other good characteristics) and
14 practically all the platforms we care about support it, so
15 let's use it".
16
17 Again, we live in the real world, and it is sometimes a
18 judgement call, the decision based more on real world
19 constraints people face than what the paper standard says.
20
21 - Fixing style violations while working on a real change as a
22 preparatory clean-up step is good, but otherwise avoid useless code
23 churn for the sake of conforming to the style.
24
25 "Once it _is_ in the tree, it's not really worth the patch noise to
26 go and fix it up."
27 Cf. http://article.gmane.org/gmane.linux.kernel/943020
28
29 Make your code readable and sensible, and don't try to be clever.
30
31 As for more concrete guidelines, just imitate the existing code
32 (this is a good guideline, no matter which project you are
33 contributing to). It is always preferable to match the _local_
34 convention. New code added to Git suite is expected to match
35 the overall style of existing code. Modifications to existing
36 code is expected to match the style the surrounding code already
37 uses (even if it doesn't match the overall style of existing code).
38
39 But if you must have a list of rules, here they are.
40
41 For shell scripts specifically (not exhaustive):
42
43 - We use tabs for indentation.
44
45 - Case arms are indented at the same depth as case and esac lines,
46 like this:
47
48 case "$variable" in
49 pattern1)
50 do this
51 ;;
52 pattern2)
53 do that
54 ;;
55 esac
56
57 - Redirection operators should be written with space before, but no
58 space after them. In other words, write 'echo test >"$file"'
59 instead of 'echo test> $file' or 'echo test > $file'. Note that
60 even though it is not required by POSIX to double-quote the
61 redirection target in a variable (as shown above), our code does so
62 because some versions of bash issue a warning without the quotes.
63
64 (incorrect)
65 cat hello > world < universe
66 echo hello >$world
67
68 (correct)
69 cat hello >world <universe
70 echo hello >"$world"
71
72 - We prefer $( ... ) for command substitution; unlike ``, it
73 properly nests. It should have been the way Bourne spelled
74 it from day one, but unfortunately isn't.
75
76 - If you want to find out if a command is available on the user's
77 $PATH, you should use 'type <command>', instead of 'which <command>'.
78 The output of 'which' is not machine parseable and its exit code
79 is not reliable across platforms.
80
81 - We use POSIX compliant parameter substitutions and avoid bashisms;
82 namely:
83
84 - We use ${parameter-word} and its [-=?+] siblings, and their
85 colon'ed "unset or null" form.
86
87 - We use ${parameter#word} and its [#%] siblings, and their
88 doubled "longest matching" form.
89
90 - No "Substring Expansion" ${parameter:offset:length}.
91
92 - No shell arrays.
93
94 - No strlen ${#parameter}.
95
96 - No pattern replacement ${parameter/pattern/string}.
97
98 - We use Arithmetic Expansion $(( ... )).
99
100 - Inside Arithmetic Expansion, spell shell variables with $ in front
101 of them, as some shells do not grok $((x)) while accepting $(($x))
102 just fine (e.g. dash older than 0.5.4).
103
104 - We do not use Process Substitution <(list) or >(list).
105
106 - Do not write control structures on a single line with semicolon.
107 "then" should be on the next line for if statements, and "do"
108 should be on the next line for "while" and "for".
109
110 (incorrect)
111 if test -f hello; then
112 do this
113 fi
114
115 (correct)
116 if test -f hello
117 then
118 do this
119 fi
120
121 - We prefer "test" over "[ ... ]".
122
123 - We do not write the noiseword "function" in front of shell
124 functions.
125
126 - We prefer a space between the function name and the parentheses,
127 and no space inside the parentheses. The opening "{" should also
128 be on the same line.
129
130 (incorrect)
131 my_function(){
132 ...
133
134 (correct)
135 my_function () {
136 ...
137
138 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
139 [::], [==], or [..]) for portability.
140
141 - We do not use \{m,n\};
142
143 - We do not use -E;
144
145 - We do not use ? or + (which are \{0,1\} and \{1,\}
146 respectively in BRE) but that goes without saying as these
147 are ERE elements not BRE (note that \? and \+ are not even part
148 of BRE -- making them accessible from BRE is a GNU extension).
149
150 - Use Git's gettext wrappers in git-sh-i18n to make the user
151 interface translatable. See "Marking strings for translation" in
152 po/README.
153
154 - We do not write our "test" command with "-a" and "-o" and use "&&"
155 or "||" to concatenate multiple "test" commands instead, because
156 the use of "-a/-o" is often error-prone. E.g.
157
158 test -n "$x" -a "$a" = "$b"
159
160 is buggy and breaks when $x is "=", but
161
162 test -n "$x" && test "$a" = "$b"
163
164 does not have such a problem.
165
166
167 For C programs:
168
169 - We use tabs to indent, and interpret tabs as taking up to
170 8 spaces.
171
172 - We try to keep to at most 80 characters per line.
173
174 - We try to support a wide range of C compilers to compile Git with,
175 including old ones. That means that you should not use C99
176 initializers, even if a lot of compilers grok it.
177
178 - Variables have to be declared at the beginning of the block.
179
180 - NULL pointers shall be written as NULL, not as 0.
181
182 - When declaring pointers, the star sides with the variable
183 name, i.e. "char *string", not "char* string" or
184 "char * string". This makes it easier to understand code
185 like "char *string, c;".
186
187 - Use whitespace around operators and keywords, but not inside
188 parentheses and not around functions. So:
189
190 while (condition)
191 func(bar + 1);
192
193 and not:
194
195 while( condition )
196 func (bar+1);
197
198 - We avoid using braces unnecessarily. I.e.
199
200 if (bla) {
201 x = 1;
202 }
203
204 is frowned upon. A gray area is when the statement extends
205 over a few lines, and/or you have a lengthy comment atop of
206 it. Also, like in the Linux kernel, if there is a long list
207 of "else if" statements, it can make sense to add braces to
208 single line blocks.
209
210 - We try to avoid assignments in the condition of an "if" statement.
211
212 - Try to make your code understandable. You may put comments
213 in, but comments invariably tend to stale out when the code
214 they were describing changes. Often splitting a function
215 into two makes the intention of the code much clearer.
216
217 - Multi-line comments include their delimiters on separate lines from
218 the text. E.g.
219
220 /*
221 * A very long
222 * multi-line comment.
223 */
224
225 Note however that a comment that explains a translatable string to
226 translators uses a convention of starting with a magic token
227 "TRANSLATORS: " immediately after the opening delimiter, even when
228 it spans multiple lines. We do not add an asterisk at the beginning
229 of each line, either. E.g.
230
231 /* TRANSLATORS: here is a comment that explains the string
232 to be translated, that follows immediately after it */
233 _("Here is a translatable string explained by the above.");
234
235 - Double negation is often harder to understand than no negation
236 at all.
237
238 - There are two schools of thought when it comes to comparison,
239 especially inside a loop. Some people prefer to have the less stable
240 value on the left hand side and the more stable value on the right hand
241 side, e.g. if you have a loop that counts variable i down to the
242 lower bound,
243
244 while (i > lower_bound) {
245 do something;
246 i--;
247 }
248
249 Other people prefer to have the textual order of values match the
250 actual order of values in their comparison, so that they can
251 mentally draw a number line from left to right and place these
252 values in order, i.e.
253
254 while (lower_bound < i) {
255 do something;
256 i--;
257 }
258
259 Both are valid, and we use both. However, the more "stable" the
260 stable side becomes, the more we tend to prefer the former
261 (comparison with a constant, "i > 0", is an extreme example).
262 Just do not mix styles in the same part of the code and mimic
263 existing styles in the neighbourhood.
264
265 - There are two schools of thought when it comes to splitting a long
266 logical line into multiple lines. Some people push the second and
267 subsequent lines far enough to the right with tabs and align them:
268
269 if (the_beginning_of_a_very_long_expression_that_has_to ||
270 span_more_than_a_single_line_of ||
271 the_source_text) {
272 ...
273
274 while other people prefer to align the second and the subsequent
275 lines with the column immediately inside the opening parenthesis,
276 with tabs and spaces, following our "tabstop is always a multiple
277 of 8" convention:
278
279 if (the_beginning_of_a_very_long_expression_that_has_to ||
280 span_more_than_a_single_line_of ||
281 the_source_text) {
282 ...
283
284 Both are valid, and we use both. Again, just do not mix styles in
285 the same part of the code and mimic existing styles in the
286 neighbourhood.
287
288 - When splitting a long logical line, some people change line before
289 a binary operator, so that the result looks like a parse tree when
290 you turn your head 90-degrees counterclockwise:
291
292 if (the_beginning_of_a_very_long_expression_that_has_to
293 || span_more_than_a_single_line_of_the_source_text) {
294
295 while other people prefer to leave the operator at the end of the
296 line:
297
298 if (the_beginning_of_a_very_long_expression_that_has_to ||
299 span_more_than_a_single_line_of_the_source_text) {
300
301 Both are valid, but we tend to use the latter more, unless the
302 expression gets fairly complex, in which case the former tends to
303 be easier to read. Again, just do not mix styles in the same part
304 of the code and mimic existing styles in the neighbourhood.
305
306 - When splitting a long logical line, with everything else being
307 equal, it is preferable to split after the operator at higher
308 level in the parse tree. That is, this is more preferable:
309
310 if (a_very_long_variable * that_is_used_in +
311 a_very_long_expression) {
312 ...
313
314 than
315
316 if (a_very_long_variable *
317 that_is_used_in + a_very_long_expression) {
318 ...
319
320 - Some clever tricks, like using the !! operator with arithmetic
321 constructs, can be extremely confusing to others. Avoid them,
322 unless there is a compelling reason to use them.
323
324 - Use the API. No, really. We have a strbuf (variable length
325 string), several arrays with the ALLOC_GROW() macro, a
326 string_list for sorted string lists, a hash map (mapping struct
327 objects) named "struct decorate", amongst other things.
328
329 - When you come up with an API, document it.
330
331 - The first #include in C files, except in platform specific compat/
332 implementations, must be either "git-compat-util.h", "cache.h" or
333 "builtin.h". You do not have to include more than one of these.
334
335 - A C file must directly include the header files that declare the
336 functions and the types it uses, except for the functions and types
337 that are made available to it by including one of the header files
338 it must include by the previous rule.
339
340 - If you are planning a new command, consider writing it in shell
341 or perl first, so that changes in semantics can be easily
342 changed and discussed. Many Git commands started out like
343 that, and a few are still scripts.
344
345 - Avoid introducing a new dependency into Git. This means you
346 usually should stay away from scripting languages not already
347 used in the Git core command set (unless your command is clearly
348 separate from it, such as an importer to convert random-scm-X
349 repositories to Git).
350
351 - When we pass <string, length> pair to functions, we should try to
352 pass them in that order.
353
354 - Use Git's gettext wrappers to make the user interface
355 translatable. See "Marking strings for translation" in po/README.
356
357 For Perl programs:
358
359 - Most of the C guidelines above apply.
360
361 - We try to support Perl 5.8 and later ("use Perl 5.008").
362
363 - use strict and use warnings are strongly preferred.
364
365 - Don't overuse statement modifiers unless using them makes the
366 result easier to follow.
367
368 ... do something ...
369 do_this() unless (condition);
370 ... do something else ...
371
372 is more readable than:
373
374 ... do something ...
375 unless (condition) {
376 do_this();
377 }
378 ... do something else ...
379
380 *only* when the condition is so rare that do_this() will be almost
381 always called.
382
383 - We try to avoid assignments inside "if ()" conditions.
384
385 - Learn and use Git.pm if you need that functionality.
386
387 - For Emacs, it's useful to put the following in
388 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
389
390 ;; note the first part is useful for C editing, too
391 ((nil . ((indent-tabs-mode . t)
392 (tab-width . 8)
393 (fill-column . 80)))
394 (cperl-mode . ((cperl-indent-level . 8)
395 (cperl-extra-newline-before-brace . nil)
396 (cperl-merge-trailing-else . t))))
397
398 For Python scripts:
399
400 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
401
402 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
403
404 - Where required libraries do not restrict us to Python 2, we try to
405 also be compatible with Python 3.1 and later.
406
407 - When you must differentiate between Unicode literals and byte string
408 literals, it is OK to use the 'b' prefix. Even though the Python
409 documentation for version 2.6 does not mention this prefix, it has
410 been supported since version 2.6.0.
411
412 Error Messages
413
414 - Do not end error messages with a full stop.
415
416 - Do not capitalize ("unable to open %s", not "Unable to open %s")
417
418 - Say what the error is first ("cannot open %s", not "%s: cannot open")
419
420
421 Externally Visible Names
422
423 - For configuration variable names, follow the existing convention:
424
425 . The section name indicates the affected subsystem.
426
427 . The subsection name, if any, indicates which of an unbounded set
428 of things to set the value for.
429
430 . The variable name describes the effect of tweaking this knob.
431
432 The section and variable names that consist of multiple words are
433 formed by concatenating the words without punctuations (e.g. `-`),
434 and are broken using bumpyCaps in documentation as a hint to the
435 reader.
436
437 When choosing the variable namespace, do not use variable name for
438 specifying possibly unbounded set of things, most notably anything
439 an end user can freely come up with (e.g. branch names). Instead,
440 use subsection names or variable values, like the existing variable
441 branch.<name>.description does.
442
443
444 Writing Documentation:
445
446 Most (if not all) of the documentation pages are written in the
447 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
448 processed into HTML and manpages (e.g. git.html and git.1 in the
449 same directory).
450
451 The documentation liberally mixes US and UK English (en_US/UK)
452 norms for spelling and grammar, which is somewhat unfortunate.
453 In an ideal world, it would have been better if it consistently
454 used only one and not the other, and we would have picked en_US
455 (if you wish to correct the English of some of the existing
456 documentation, please see the documentation-related advice in the
457 Documentation/SubmittingPatches file).
458
459 Every user-visible change should be reflected in the documentation.
460 The same general rule as for code applies -- imitate the existing
461 conventions.
462
463 A few commented examples follow to provide reference when writing or
464 modifying command usage strings and synopsis sections in the manual
465 pages:
466
467 Placeholders are spelled in lowercase and enclosed in angle brackets:
468 <file>
469 --sort=<key>
470 --abbrev[=<n>]
471
472 If a placeholder has multiple words, they are separated by dashes:
473 <new-branch-name>
474 --template=<template-directory>
475
476 Possibility of multiple occurrences is indicated by three dots:
477 <file>...
478 (One or more of <file>.)
479
480 Optional parts are enclosed in square brackets:
481 [<extra>]
482 (Zero or one <extra>.)
483
484 --exec-path[=<path>]
485 (Option with an optional argument. Note that the "=" is inside the
486 brackets.)
487
488 [<patch>...]
489 (Zero or more of <patch>. Note that the dots are inside, not
490 outside the brackets.)
491
492 Multiple alternatives are indicated with vertical bars:
493 [-q | --quiet]
494 [--utf8 | --no-utf8]
495
496 Parentheses are used for grouping:
497 [(<rev> | <range>)...]
498 (Any number of either <rev> or <range>. Parens are needed to make
499 it clear that "..." pertains to both <rev> and <range>.)
500
501 [(-p <parent>)...]
502 (Any number of option -p, each with one <parent> argument.)
503
504 git remote set-head <name> (-a | -d | <branch>)
505 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
506 brackets) be provided.)
507
508 And a somewhat more contrived example:
509 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
510 Here "=" is outside the brackets, because "--diff-filter=" is a
511 valid usage. "*" has its own pair of brackets, because it can
512 (optionally) be specified only when one or more of the letters is
513 also provided.
514
515 A note on notation:
516 Use 'git' (all lowercase) when talking about commands i.e. something
517 the user would type into a shell and use 'Git' (uppercase first letter)
518 when talking about the version control system and its properties.
519
520 A few commented examples follow to provide reference when writing or
521 modifying paragraphs or option/command explanations that contain options
522 or commands:
523
524 Literal examples (e.g. use of command-line options, command names, and
525 configuration variables) are typeset in monospace, and if you can use
526 `backticks around word phrases`, do so.
527 `--pretty=oneline`
528 `git rev-list`
529 `remote.pushDefault`
530
531 Word phrases enclosed in `backtick characters` are rendered literally
532 and will not be further expanded. The use of `backticks` to achieve the
533 previous rule means that literal examples should not use AsciiDoc
534 escapes.
535 Correct:
536 `--pretty=oneline`
537 Incorrect:
538 `\--pretty=oneline`
539
540 If some place in the documentation needs to typeset a command usage
541 example with inline substitutions, it is fine to use +monospaced and
542 inline substituted text+ instead of `monospaced literal text`, and with
543 the former, the part that should not get substituted must be
544 quoted/escaped.