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[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
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56 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
57 [::], [==], nor [..]) for portability.
58
59 - We do not use \{m,n\};
60
61 - We do not use -E;
62
63 - We do not use ? nor + (which are \{0,1\} and \{1,\}
64 respectively in BRE) but that goes without saying as these
65 are ERE elements not BRE (note that \? and \+ are not even part
66 of BRE -- making them accessible from BRE is a GNU extension).
67
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68For C programs:
69
70 - We use tabs to indent, and interpret tabs as taking up to
71 8 spaces.
72
73 - We try to keep to at most 80 characters per line.
74
75 - When declaring pointers, the star sides with the variable
76 name, i.e. "char *string", not "char* string" or
77 "char * string". This makes it easier to understand code
78 like "char *string, c;".
79
80 - We avoid using braces unnecessarily. I.e.
81
82 if (bla) {
83 x = 1;
84 }
85
86 is frowned upon. A gray area is when the statement extends
87 over a few lines, and/or you have a lengthy comment atop of
88 it. Also, like in the Linux kernel, if there is a long list
89 of "else if" statements, it can make sense to add braces to
90 single line blocks.
91
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92 - We try to avoid assignments inside if().
93
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94 - Try to make your code understandable. You may put comments
95 in, but comments invariably tend to stale out when the code
96 they were describing changes. Often splitting a function
97 into two makes the intention of the code much clearer.
98
99 - Double negation is often harder to understand than no negation
100 at all.
101
102 - Some clever tricks, like using the !! operator with arithmetic
103 constructs, can be extremely confusing to others. Avoid them,
104 unless there is a compelling reason to use them.
105
106 - Use the API. No, really. We have a strbuf (variable length
107 string), several arrays with the ALLOC_GROW() macro, a
108 path_list for sorted string lists, a hash map (mapping struct
109 objects) named "struct decorate", amongst other things.
110
111 - When you come up with an API, document it.
112
113 - The first #include in C files, except in platform specific
114 compat/ implementations, should be git-compat-util.h or another
115 header file that includes it, such as cache.h or builtin.h.
116
117 - If you are planning a new command, consider writing it in shell
118 or perl first, so that changes in semantics can be easily
119 changed and discussed. Many git commands started out like
120 that, and a few are still scripts.
121
122 - Avoid introducing a new dependency into git. This means you
123 usually should stay away from scripting languages not already
124 used in the git core command set (unless your command is clearly
125 separate from it, such as an importer to convert random-scm-X
126 repositories to git).