Remove redundant code, eliminate one static variable
[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
92 - Try to make your code understandable. You may put comments
93 in, but comments invariably tend to stale out when the code
94 they were describing changes. Often splitting a function
95 into two makes the intention of the code much clearer.
96
97 - Double negation is often harder to understand than no negation
98 at all.
99
100 - Some clever tricks, like using the !! operator with arithmetic
101 constructs, can be extremely confusing to others. Avoid them,
102 unless there is a compelling reason to use them.
103
104 - Use the API. No, really. We have a strbuf (variable length
105 string), several arrays with the ALLOC_GROW() macro, a
106 path_list for sorted string lists, a hash map (mapping struct
107 objects) named "struct decorate", amongst other things.
108
109 - When you come up with an API, document it.
110
111 - The first #include in C files, except in platform specific
112 compat/ implementations, should be git-compat-util.h or another
113 header file that includes it, such as cache.h or builtin.h.
114
115 - If you are planning a new command, consider writing it in shell
116 or perl first, so that changes in semantics can be easily
117 changed and discussed. Many git commands started out like
118 that, and a few are still scripts.
119
120 - Avoid introducing a new dependency into git. This means you
121 usually should stay away from scripting languages not already
122 used in the git core command set (unless your command is clearly
123 separate from it, such as an importer to convert random-scm-X
124 repositories to git).