compat/bswap.h: fix endianness detection
[git/git.git] / Documentation / CodingGuidelines
1 Like other projects, we also have some guidelines to keep to the
2 code. For Git in general, three 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 Make your code readable and sensible, and don't try to be clever.
22
23 As for more concrete guidelines, just imitate the existing code
24 (this is a good guideline, no matter which project you are
25 contributing to). It is always preferable to match the _local_
26 convention. New code added to Git suite is expected to match
27 the overall style of existing code. Modifications to existing
28 code is expected to match the style the surrounding code already
29 uses (even if it doesn't match the overall style of existing code).
30
31 But if you must have a list of rules, here they are.
32
33 For shell scripts specifically (not exhaustive):
34
35 - We use tabs for indentation.
36
37 - Case arms are indented at the same depth as case and esac lines.
38
39 - Redirection operators should be written with space before, but no
40 space after them. In other words, write 'echo test >"$file"'
41 instead of 'echo test> $file' or 'echo test > $file'. Note that
42 even though it is not required by POSIX to double-quote the
43 redirection target in a variable (as shown above), our code does so
44 because some versions of bash issue a warning without the quotes.
45
46 - We prefer $( ... ) for command substitution; unlike ``, it
47 properly nests. It should have been the way Bourne spelled
48 it from day one, but unfortunately isn't.
49
50 - If you want to find out if a command is available on the user's
51 $PATH, you should use 'type <command>', instead of 'which <command>'.
52 The output of 'which' is not machine parseable and its exit code
53 is not reliable across platforms.
54
55 - We use POSIX compliant parameter substitutions and avoid bashisms;
56 namely:
57
58 - We use ${parameter-word} and its [-=?+] siblings, and their
59 colon'ed "unset or null" form.
60
61 - We use ${parameter#word} and its [#%] siblings, and their
62 doubled "longest matching" form.
63
64 - No "Substring Expansion" ${parameter:offset:length}.
65
66 - No shell arrays.
67
68 - No strlen ${#parameter}.
69
70 - No pattern replacement ${parameter/pattern/string}.
71
72 - We use Arithmetic Expansion $(( ... )).
73
74 - Inside Arithmetic Expansion, spell shell variables with $ in front
75 of them, as some shells do not grok $((x)) while accepting $(($x))
76 just fine (e.g. dash older than 0.5.4).
77
78 - We do not use Process Substitution <(list) or >(list).
79
80 - Do not write control structures on a single line with semicolon.
81 "then" should be on the next line for if statements, and "do"
82 should be on the next line for "while" and "for".
83
84 - We prefer "test" over "[ ... ]".
85
86 - We do not write the noiseword "function" in front of shell
87 functions.
88
89 - We prefer a space between the function name and the parentheses. The
90 opening "{" should also be on the same line.
91 E.g.: my_function () {
92
93 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
94 [::], [==], or [..]) for portability.
95
96 - We do not use \{m,n\};
97
98 - We do not use -E;
99
100 - We do not use ? or + (which are \{0,1\} and \{1,\}
101 respectively in BRE) but that goes without saying as these
102 are ERE elements not BRE (note that \? and \+ are not even part
103 of BRE -- making them accessible from BRE is a GNU extension).
104
105 - Use Git's gettext wrappers in git-sh-i18n to make the user
106 interface translatable. See "Marking strings for translation" in
107 po/README.
108
109 For C programs:
110
111 - We use tabs to indent, and interpret tabs as taking up to
112 8 spaces.
113
114 - We try to keep to at most 80 characters per line.
115
116 - We try to support a wide range of C compilers to compile Git with,
117 including old ones. That means that you should not use C99
118 initializers, even if a lot of compilers grok it.
119
120 - Variables have to be declared at the beginning of the block.
121
122 - NULL pointers shall be written as NULL, not as 0.
123
124 - When declaring pointers, the star sides with the variable
125 name, i.e. "char *string", not "char* string" or
126 "char * string". This makes it easier to understand code
127 like "char *string, c;".
128
129 - Use whitespace around operators and keywords, but not inside
130 parentheses and not around functions. So:
131
132 while (condition)
133 func(bar + 1);
134
135 and not:
136
137 while( condition )
138 func (bar+1);
139
140 - We avoid using braces unnecessarily. I.e.
141
142 if (bla) {
143 x = 1;
144 }
145
146 is frowned upon. A gray area is when the statement extends
147 over a few lines, and/or you have a lengthy comment atop of
148 it. Also, like in the Linux kernel, if there is a long list
149 of "else if" statements, it can make sense to add braces to
150 single line blocks.
151
152 - We try to avoid assignments inside if().
153
154 - Try to make your code understandable. You may put comments
155 in, but comments invariably tend to stale out when the code
156 they were describing changes. Often splitting a function
157 into two makes the intention of the code much clearer.
158
159 - Multi-line comments include their delimiters on separate lines from
160 the text. E.g.
161
162 /*
163 * A very long
164 * multi-line comment.
165 */
166
167 Note however that a comment that explains a translatable string to
168 translators uses a convention of starting with a magic token
169 "TRANSLATORS: " immediately after the opening delimiter, even when
170 it spans multiple lines. We do not add an asterisk at the beginning
171 of each line, either. E.g.
172
173 /* TRANSLATORS: here is a comment that explains the string
174 to be translated, that follows immediately after it */
175 _("Here is a translatable string explained by the above.");
176
177 - Double negation is often harder to understand than no negation
178 at all.
179
180 - Some clever tricks, like using the !! operator with arithmetic
181 constructs, can be extremely confusing to others. Avoid them,
182 unless there is a compelling reason to use them.
183
184 - Use the API. No, really. We have a strbuf (variable length
185 string), several arrays with the ALLOC_GROW() macro, a
186 string_list for sorted string lists, a hash map (mapping struct
187 objects) named "struct decorate", amongst other things.
188
189 - When you come up with an API, document it.
190
191 - The first #include in C files, except in platform specific
192 compat/ implementations, should be git-compat-util.h or another
193 header file that includes it, such as cache.h or builtin.h.
194
195 - If you are planning a new command, consider writing it in shell
196 or perl first, so that changes in semantics can be easily
197 changed and discussed. Many Git commands started out like
198 that, and a few are still scripts.
199
200 - Avoid introducing a new dependency into Git. This means you
201 usually should stay away from scripting languages not already
202 used in the Git core command set (unless your command is clearly
203 separate from it, such as an importer to convert random-scm-X
204 repositories to Git).
205
206 - When we pass <string, length> pair to functions, we should try to
207 pass them in that order.
208
209 - Use Git's gettext wrappers to make the user interface
210 translatable. See "Marking strings for translation" in po/README.
211
212 For Perl programs:
213
214 - Most of the C guidelines above apply.
215
216 - We try to support Perl 5.8 and later ("use Perl 5.008").
217
218 - use strict and use warnings are strongly preferred.
219
220 - Don't overuse statement modifiers unless using them makes the
221 result easier to follow.
222
223 ... do something ...
224 do_this() unless (condition);
225 ... do something else ...
226
227 is more readable than:
228
229 ... do something ...
230 unless (condition) {
231 do_this();
232 }
233 ... do something else ...
234
235 *only* when the condition is so rare that do_this() will be almost
236 always called.
237
238 - We try to avoid assignments inside "if ()" conditions.
239
240 - Learn and use Git.pm if you need that functionality.
241
242 - For Emacs, it's useful to put the following in
243 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
244
245 ;; note the first part is useful for C editing, too
246 ((nil . ((indent-tabs-mode . t)
247 (tab-width . 8)
248 (fill-column . 80)))
249 (cperl-mode . ((cperl-indent-level . 8)
250 (cperl-extra-newline-before-brace . nil)
251 (cperl-merge-trailing-else . t))))
252
253 For Python scripts:
254
255 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
256
257 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
258
259 - Where required libraries do not restrict us to Python 2, we try to
260 also be compatible with Python 3.1 and later.
261
262 - When you must differentiate between Unicode literals and byte string
263 literals, it is OK to use the 'b' prefix. Even though the Python
264 documentation for version 2.6 does not mention this prefix, it has
265 been supported since version 2.6.0.
266
267 Writing Documentation:
268
269 Most (if not all) of the documentation pages are written in the
270 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
271 processed into HTML and manpages (e.g. git.html and git.1 in the
272 same directory).
273
274 The documentation liberally mixes US and UK English (en_US/UK)
275 norms for spelling and grammar, which is somewhat unfortunate.
276 In an ideal world, it would have been better if it consistently
277 used only one and not the other, and we would have picked en_US
278 (if you wish to correct the English of some of the existing
279 documentation, please see the documentation-related advice in the
280 Documentation/SubmittingPatches file).
281
282 Every user-visible change should be reflected in the documentation.
283 The same general rule as for code applies -- imitate the existing
284 conventions.
285
286 A few commented examples follow to provide reference when writing or
287 modifying command usage strings and synopsis sections in the manual
288 pages:
289
290 Placeholders are spelled in lowercase and enclosed in angle brackets:
291 <file>
292 --sort=<key>
293 --abbrev[=<n>]
294
295 Possibility of multiple occurrences is indicated by three dots:
296 <file>...
297 (One or more of <file>.)
298
299 Optional parts are enclosed in square brackets:
300 [<extra>]
301 (Zero or one <extra>.)
302
303 --exec-path[=<path>]
304 (Option with an optional argument. Note that the "=" is inside the
305 brackets.)
306
307 [<patch>...]
308 (Zero or more of <patch>. Note that the dots are inside, not
309 outside the brackets.)
310
311 Multiple alternatives are indicated with vertical bar:
312 [-q | --quiet]
313 [--utf8 | --no-utf8]
314
315 Parentheses are used for grouping:
316 [(<rev>|<range>)...]
317 (Any number of either <rev> or <range>. Parens are needed to make
318 it clear that "..." pertains to both <rev> and <range>.)
319
320 [(-p <parent>)...]
321 (Any number of option -p, each with one <parent> argument.)
322
323 git remote set-head <name> (-a | -d | <branch>)
324 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
325 brackets) be provided.)
326
327 And a somewhat more contrived example:
328 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
329 Here "=" is outside the brackets, because "--diff-filter=" is a
330 valid usage. "*" has its own pair of brackets, because it can
331 (optionally) be specified only when one or more of the letters is
332 also provided.
333
334 A note on notation:
335 Use 'git' (all lowercase) when talking about commands i.e. something
336 the user would type into a shell and use 'Git' (uppercase first letter)
337 when talking about the version control system and its properties.
338
339 A few commented examples follow to provide reference when writing or
340 modifying paragraphs or option/command explanations that contain options
341 or commands:
342
343 Literal examples (e.g. use of command-line options, command names, and
344 configuration variables) are typeset in monospace, and if you can use
345 `backticks around word phrases`, do so.
346 `--pretty=oneline`
347 `git rev-list`
348 `remote.pushdefault`
349
350 Word phrases enclosed in `backtick characters` are rendered literally
351 and will not be further expanded. The use of `backticks` to achieve the
352 previous rule means that literal examples should not use AsciiDoc
353 escapes.
354 Correct:
355 `--pretty=oneline`
356 Incorrect:
357 `\--pretty=oneline`
358
359 If some place in the documentation needs to typeset a command usage
360 example with inline substitutions, it is fine to use +monospaced and
361 inline substituted text+ instead of `monospaced literal text`, and with
362 the former, the part that should not get substituted must be
363 quoted/escaped.