Add Documentation/CodingGuidelines
[git/git.git] / Documentation / CodingGuidelines
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1Like other projects, we also have some guidelines to keep to the
2code. 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
22As for more concrete guidelines, just imitate the existing code
23(this is a good guideline, no matter which project you are
24contributing to). But if you must have a list of rules,
25here they are.
26
27For shell scripts specifically (not exhaustive):
28
29 - We prefer $( ... ) for command substitution; unlike ``, it
30 properly nests. It should have been the way Bourne spelled
31 it from day one, but unfortunately isn't.
32
33 - We use ${parameter-word} and its [-=?+] siblings, and their
34 colon'ed "unset or null" form.
35
36 - We use ${parameter#word} and its [#%] siblings, and their
37 doubled "longest matching" form.
38
39 - We use Arithmetic Expansion $(( ... )).
40
41 - No "Substring Expansion" ${parameter:offset:length}.
42
43 - No shell arrays.
44
45 - No strlen ${#parameter}.
46
47 - No regexp ${parameter/pattern/string}.
48
49 - We do not use Process Substitution <(list) or >(list).
50
51 - We prefer "test" over "[ ... ]".
52
53 - We do not write the noiseword "function" in front of shell
54 functions.
55
56For C programs:
57
58 - We use tabs to indent, and interpret tabs as taking up to
59 8 spaces.
60
61 - We try to keep to at most 80 characters per line.
62
63 - When declaring pointers, the star sides with the variable
64 name, i.e. "char *string", not "char* string" or
65 "char * string". This makes it easier to understand code
66 like "char *string, c;".
67
68 - We avoid using braces unnecessarily. I.e.
69
70 if (bla) {
71 x = 1;
72 }
73
74 is frowned upon. A gray area is when the statement extends
75 over a few lines, and/or you have a lengthy comment atop of
76 it. Also, like in the Linux kernel, if there is a long list
77 of "else if" statements, it can make sense to add braces to
78 single line blocks.
79
80 - Try to make your code understandable. You may put comments
81 in, but comments invariably tend to stale out when the code
82 they were describing changes. Often splitting a function
83 into two makes the intention of the code much clearer.
84
85 - Double negation is often harder to understand than no negation
86 at all.
87
88 - Some clever tricks, like using the !! operator with arithmetic
89 constructs, can be extremely confusing to others. Avoid them,
90 unless there is a compelling reason to use them.
91
92 - Use the API. No, really. We have a strbuf (variable length
93 string), several arrays with the ALLOC_GROW() macro, a
94 path_list for sorted string lists, a hash map (mapping struct
95 objects) named "struct decorate", amongst other things.
96
97 - When you come up with an API, document it.
98
99 - The first #include in C files, except in platform specific
100 compat/ implementations, should be git-compat-util.h or another
101 header file that includes it, such as cache.h or builtin.h.
102
103 - If you are planning a new command, consider writing it in shell
104 or perl first, so that changes in semantics can be easily
105 changed and discussed. Many git commands started out like
106 that, and a few are still scripts.
107
108 - Avoid introducing a new dependency into git. This means you
109 usually should stay away from scripting languages not already
110 used in the git core command set (unless your command is clearly
111 separate from it, such as an importer to convert random-scm-X
112 repositories to git).