CodingGuidelines: spell out post-C89 rules
[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://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html
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 - If a command sequence joined with && or || or | spans multiple
122 lines, put each command on a separate line and put && and || and |
123 operators at the end of each line, rather than the start. This
124 means you don't need to use \ to join lines, since the above
125 operators imply the sequence isn't finished.
126
127 (incorrect)
128 grep blob verify_pack_result \
129 | awk -f print_1.awk \
130 | sort >actual &&
131 ...
132
133 (correct)
134 grep blob verify_pack_result |
135 awk -f print_1.awk |
136 sort >actual &&
137 ...
138
139 - We prefer "test" over "[ ... ]".
140
141 - We do not write the noiseword "function" in front of shell
142 functions.
143
144 - We prefer a space between the function name and the parentheses,
145 and no space inside the parentheses. The opening "{" should also
146 be on the same line.
147
148 (incorrect)
149 my_function(){
150 ...
151
152 (correct)
153 my_function () {
154 ...
155
156 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
157 [::], [==], or [..]) for portability.
158
159 - We do not use \{m,n\};
160
161 - We do not use -E;
162
163 - We do not use ? or + (which are \{0,1\} and \{1,\}
164 respectively in BRE) but that goes without saying as these
165 are ERE elements not BRE (note that \? and \+ are not even part
166 of BRE -- making them accessible from BRE is a GNU extension).
167
168 - Use Git's gettext wrappers in git-sh-i18n to make the user
169 interface translatable. See "Marking strings for translation" in
170 po/README.
171
172 - We do not write our "test" command with "-a" and "-o" and use "&&"
173 or "||" to concatenate multiple "test" commands instead, because
174 the use of "-a/-o" is often error-prone. E.g.
175
176 test -n "$x" -a "$a" = "$b"
177
178 is buggy and breaks when $x is "=", but
179
180 test -n "$x" && test "$a" = "$b"
181
182 does not have such a problem.
183
184
185 For C programs:
186
187 - We use tabs to indent, and interpret tabs as taking up to
188 8 spaces.
189
190 - We try to keep to at most 80 characters per line.
191
192 - As a Git developer we assume you have a reasonably modern compiler
193 and we recommend you to enable the DEVELOPER makefile knob to
194 ensure your patch is clear of all compiler warnings we care about,
195 by e.g. "echo DEVELOPER=1 >>config.mak".
196
197 - We try to support a wide range of C compilers to compile Git with,
198 including old ones. You should not use features from newer C
199 standard, even if your compiler groks them.
200
201 There are a few exceptions to this guideline:
202
203 . since early 2012 with e1327023ea, we have been using an enum
204 definition whose last element is followed by a comma. This, like
205 an array initializer that ends with a trailing comma, can be used
206 to reduce the patch noise when adding a new identifer at the end.
207
208 . since mid 2017 with cbc0f81d, we have been using designated
209 initializers for struct (e.g. "struct t v = { .val = 'a' };").
210
211 . since mid 2017 with 512f41cf, we have been using designated
212 initializers for array (e.g. "int array[10] = { [5] = 2 }").
213
214 These used to be forbidden, but we have not heard any breakage
215 report, and they are assumed to be safe.
216
217 - Variables have to be declared at the beginning of the block, before
218 the first statement (i.e. -Wdeclaration-after-statement).
219
220 - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
221 is still not allowed in this codebase.
222
223 - NULL pointers shall be written as NULL, not as 0.
224
225 - When declaring pointers, the star sides with the variable
226 name, i.e. "char *string", not "char* string" or
227 "char * string". This makes it easier to understand code
228 like "char *string, c;".
229
230 - Use whitespace around operators and keywords, but not inside
231 parentheses and not around functions. So:
232
233 while (condition)
234 func(bar + 1);
235
236 and not:
237
238 while( condition )
239 func (bar+1);
240
241 - We avoid using braces unnecessarily. I.e.
242
243 if (bla) {
244 x = 1;
245 }
246
247 is frowned upon. But there are a few exceptions:
248
249 - When the statement extends over a few lines (e.g., a while loop
250 with an embedded conditional, or a comment). E.g.:
251
252 while (foo) {
253 if (x)
254 one();
255 else
256 two();
257 }
258
259 if (foo) {
260 /*
261 * This one requires some explanation,
262 * so we're better off with braces to make
263 * it obvious that the indentation is correct.
264 */
265 doit();
266 }
267
268 - When there are multiple arms to a conditional and some of them
269 require braces, enclose even a single line block in braces for
270 consistency. E.g.:
271
272 if (foo) {
273 doit();
274 } else {
275 one();
276 two();
277 three();
278 }
279
280 - We try to avoid assignments in the condition of an "if" statement.
281
282 - Try to make your code understandable. You may put comments
283 in, but comments invariably tend to stale out when the code
284 they were describing changes. Often splitting a function
285 into two makes the intention of the code much clearer.
286
287 - Multi-line comments include their delimiters on separate lines from
288 the text. E.g.
289
290 /*
291 * A very long
292 * multi-line comment.
293 */
294
295 Note however that a comment that explains a translatable string to
296 translators uses a convention of starting with a magic token
297 "TRANSLATORS: ", e.g.
298
299 /*
300 * TRANSLATORS: here is a comment that explains the string to
301 * be translated, that follows immediately after it.
302 */
303 _("Here is a translatable string explained by the above.");
304
305 - Double negation is often harder to understand than no negation
306 at all.
307
308 - There are two schools of thought when it comes to comparison,
309 especially inside a loop. Some people prefer to have the less stable
310 value on the left hand side and the more stable value on the right hand
311 side, e.g. if you have a loop that counts variable i down to the
312 lower bound,
313
314 while (i > lower_bound) {
315 do something;
316 i--;
317 }
318
319 Other people prefer to have the textual order of values match the
320 actual order of values in their comparison, so that they can
321 mentally draw a number line from left to right and place these
322 values in order, i.e.
323
324 while (lower_bound < i) {
325 do something;
326 i--;
327 }
328
329 Both are valid, and we use both. However, the more "stable" the
330 stable side becomes, the more we tend to prefer the former
331 (comparison with a constant, "i > 0", is an extreme example).
332 Just do not mix styles in the same part of the code and mimic
333 existing styles in the neighbourhood.
334
335 - There are two schools of thought when it comes to splitting a long
336 logical line into multiple lines. Some people push the second and
337 subsequent lines far enough to the right with tabs and align them:
338
339 if (the_beginning_of_a_very_long_expression_that_has_to ||
340 span_more_than_a_single_line_of ||
341 the_source_text) {
342 ...
343
344 while other people prefer to align the second and the subsequent
345 lines with the column immediately inside the opening parenthesis,
346 with tabs and spaces, following our "tabstop is always a multiple
347 of 8" convention:
348
349 if (the_beginning_of_a_very_long_expression_that_has_to ||
350 span_more_than_a_single_line_of ||
351 the_source_text) {
352 ...
353
354 Both are valid, and we use both. Again, just do not mix styles in
355 the same part of the code and mimic existing styles in the
356 neighbourhood.
357
358 - When splitting a long logical line, some people change line before
359 a binary operator, so that the result looks like a parse tree when
360 you turn your head 90-degrees counterclockwise:
361
362 if (the_beginning_of_a_very_long_expression_that_has_to
363 || span_more_than_a_single_line_of_the_source_text) {
364
365 while other people prefer to leave the operator at the end of the
366 line:
367
368 if (the_beginning_of_a_very_long_expression_that_has_to ||
369 span_more_than_a_single_line_of_the_source_text) {
370
371 Both are valid, but we tend to use the latter more, unless the
372 expression gets fairly complex, in which case the former tends to
373 be easier to read. Again, just do not mix styles in the same part
374 of the code and mimic existing styles in the neighbourhood.
375
376 - When splitting a long logical line, with everything else being
377 equal, it is preferable to split after the operator at higher
378 level in the parse tree. That is, this is more preferable:
379
380 if (a_very_long_variable * that_is_used_in +
381 a_very_long_expression) {
382 ...
383
384 than
385
386 if (a_very_long_variable *
387 that_is_used_in + a_very_long_expression) {
388 ...
389
390 - Some clever tricks, like using the !! operator with arithmetic
391 constructs, can be extremely confusing to others. Avoid them,
392 unless there is a compelling reason to use them.
393
394 - Use the API. No, really. We have a strbuf (variable length
395 string), several arrays with the ALLOC_GROW() macro, a
396 string_list for sorted string lists, a hash map (mapping struct
397 objects) named "struct decorate", amongst other things.
398
399 - When you come up with an API, document its functions and structures
400 in the header file that exposes the API to its callers. Use what is
401 in "strbuf.h" as a model for the appropriate tone and level of
402 detail.
403
404 - The first #include in C files, except in platform specific compat/
405 implementations, must be either "git-compat-util.h", "cache.h" or
406 "builtin.h". You do not have to include more than one of these.
407
408 - A C file must directly include the header files that declare the
409 functions and the types it uses, except for the functions and types
410 that are made available to it by including one of the header files
411 it must include by the previous rule.
412
413 - If you are planning a new command, consider writing it in shell
414 or perl first, so that changes in semantics can be easily
415 changed and discussed. Many Git commands started out like
416 that, and a few are still scripts.
417
418 - Avoid introducing a new dependency into Git. This means you
419 usually should stay away from scripting languages not already
420 used in the Git core command set (unless your command is clearly
421 separate from it, such as an importer to convert random-scm-X
422 repositories to Git).
423
424 - When we pass <string, length> pair to functions, we should try to
425 pass them in that order.
426
427 - Use Git's gettext wrappers to make the user interface
428 translatable. See "Marking strings for translation" in po/README.
429
430 - Variables and functions local to a given source file should be marked
431 with "static". Variables that are visible to other source files
432 must be declared with "extern" in header files. However, function
433 declarations should not use "extern", as that is already the default.
434
435 For Perl programs:
436
437 - Most of the C guidelines above apply.
438
439 - We try to support Perl 5.8 and later ("use Perl 5.008").
440
441 - use strict and use warnings are strongly preferred.
442
443 - Don't overuse statement modifiers unless using them makes the
444 result easier to follow.
445
446 ... do something ...
447 do_this() unless (condition);
448 ... do something else ...
449
450 is more readable than:
451
452 ... do something ...
453 unless (condition) {
454 do_this();
455 }
456 ... do something else ...
457
458 *only* when the condition is so rare that do_this() will be almost
459 always called.
460
461 - We try to avoid assignments inside "if ()" conditions.
462
463 - Learn and use Git.pm if you need that functionality.
464
465 - For Emacs, it's useful to put the following in
466 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
467
468 ;; note the first part is useful for C editing, too
469 ((nil . ((indent-tabs-mode . t)
470 (tab-width . 8)
471 (fill-column . 80)))
472 (cperl-mode . ((cperl-indent-level . 8)
473 (cperl-extra-newline-before-brace . nil)
474 (cperl-merge-trailing-else . t))))
475
476 For Python scripts:
477
478 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
479
480 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
481
482 - Where required libraries do not restrict us to Python 2, we try to
483 also be compatible with Python 3.1 and later.
484
485 - When you must differentiate between Unicode literals and byte string
486 literals, it is OK to use the 'b' prefix. Even though the Python
487 documentation for version 2.6 does not mention this prefix, it has
488 been supported since version 2.6.0.
489
490 Error Messages
491
492 - Do not end error messages with a full stop.
493
494 - Do not capitalize ("unable to open %s", not "Unable to open %s")
495
496 - Say what the error is first ("cannot open %s", not "%s: cannot open")
497
498
499 Externally Visible Names
500
501 - For configuration variable names, follow the existing convention:
502
503 . The section name indicates the affected subsystem.
504
505 . The subsection name, if any, indicates which of an unbounded set
506 of things to set the value for.
507
508 . The variable name describes the effect of tweaking this knob.
509
510 The section and variable names that consist of multiple words are
511 formed by concatenating the words without punctuations (e.g. `-`),
512 and are broken using bumpyCaps in documentation as a hint to the
513 reader.
514
515 When choosing the variable namespace, do not use variable name for
516 specifying possibly unbounded set of things, most notably anything
517 an end user can freely come up with (e.g. branch names). Instead,
518 use subsection names or variable values, like the existing variable
519 branch.<name>.description does.
520
521
522 Writing Documentation:
523
524 Most (if not all) of the documentation pages are written in the
525 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
526 processed into HTML and manpages (e.g. git.html and git.1 in the
527 same directory).
528
529 The documentation liberally mixes US and UK English (en_US/UK)
530 norms for spelling and grammar, which is somewhat unfortunate.
531 In an ideal world, it would have been better if it consistently
532 used only one and not the other, and we would have picked en_US
533 (if you wish to correct the English of some of the existing
534 documentation, please see the documentation-related advice in the
535 Documentation/SubmittingPatches file).
536
537 Every user-visible change should be reflected in the documentation.
538 The same general rule as for code applies -- imitate the existing
539 conventions.
540
541 A few commented examples follow to provide reference when writing or
542 modifying command usage strings and synopsis sections in the manual
543 pages:
544
545 Placeholders are spelled in lowercase and enclosed in angle brackets:
546 <file>
547 --sort=<key>
548 --abbrev[=<n>]
549
550 If a placeholder has multiple words, they are separated by dashes:
551 <new-branch-name>
552 --template=<template-directory>
553
554 Possibility of multiple occurrences is indicated by three dots:
555 <file>...
556 (One or more of <file>.)
557
558 Optional parts are enclosed in square brackets:
559 [<extra>]
560 (Zero or one <extra>.)
561
562 --exec-path[=<path>]
563 (Option with an optional argument. Note that the "=" is inside the
564 brackets.)
565
566 [<patch>...]
567 (Zero or more of <patch>. Note that the dots are inside, not
568 outside the brackets.)
569
570 Multiple alternatives are indicated with vertical bars:
571 [-q | --quiet]
572 [--utf8 | --no-utf8]
573
574 Parentheses are used for grouping:
575 [(<rev> | <range>)...]
576 (Any number of either <rev> or <range>. Parens are needed to make
577 it clear that "..." pertains to both <rev> and <range>.)
578
579 [(-p <parent>)...]
580 (Any number of option -p, each with one <parent> argument.)
581
582 git remote set-head <name> (-a | -d | <branch>)
583 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
584 brackets) be provided.)
585
586 And a somewhat more contrived example:
587 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
588 Here "=" is outside the brackets, because "--diff-filter=" is a
589 valid usage. "*" has its own pair of brackets, because it can
590 (optionally) be specified only when one or more of the letters is
591 also provided.
592
593 A note on notation:
594 Use 'git' (all lowercase) when talking about commands i.e. something
595 the user would type into a shell and use 'Git' (uppercase first letter)
596 when talking about the version control system and its properties.
597
598 A few commented examples follow to provide reference when writing or
599 modifying paragraphs or option/command explanations that contain options
600 or commands:
601
602 Literal examples (e.g. use of command-line options, command names,
603 branch names, URLs, pathnames (files and directories), configuration and
604 environment variables) must be typeset in monospace (i.e. wrapped with
605 backticks):
606 `--pretty=oneline`
607 `git rev-list`
608 `remote.pushDefault`
609 `http://git.example.com`
610 `.git/config`
611 `GIT_DIR`
612 `HEAD`
613
614 An environment variable must be prefixed with "$" only when referring to its
615 value and not when referring to the variable itself, in this case there is
616 nothing to add except the backticks:
617 `GIT_DIR` is specified
618 `$GIT_DIR/hooks/pre-receive`
619
620 Word phrases enclosed in `backtick characters` are rendered literally
621 and will not be further expanded. The use of `backticks` to achieve the
622 previous rule means that literal examples should not use AsciiDoc
623 escapes.
624 Correct:
625 `--pretty=oneline`
626 Incorrect:
627 `\--pretty=oneline`
628
629 If some place in the documentation needs to typeset a command usage
630 example with inline substitutions, it is fine to use +monospaced and
631 inline substituted text+ instead of `monospaced literal text`, and with
632 the former, the part that should not get substituted must be
633 quoted/escaped.