Documentation: update-index: -z applies also to --index-info
[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
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24contributing to). It is always preferable to match the _local_
25convention. New code added to git suite is expected to match
26the overall style of existing code. Modifications to existing
27code is expected to match the style the surrounding code already
28uses (even if it doesn't match the overall style of existing code).
29
30But if you must have a list of rules, here they are.
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31
32For shell scripts specifically (not exhaustive):
33
34 - We prefer $( ... ) for command substitution; unlike ``, it
35 properly nests. It should have been the way Bourne spelled
36 it from day one, but unfortunately isn't.
37
38 - We use ${parameter-word} and its [-=?+] siblings, and their
39 colon'ed "unset or null" form.
40
41 - We use ${parameter#word} and its [#%] siblings, and their
42 doubled "longest matching" form.
43
44 - We use Arithmetic Expansion $(( ... )).
45
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46 - Inside Arithmetic Expansion, spell shell variables with $ in front
47 of them, as some shells do not grok $((x)) while accepting $(($x))
48 just fine (e.g. dash older than 0.5.4).
49
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50 - No "Substring Expansion" ${parameter:offset:length}.
51
52 - No shell arrays.
53
54 - No strlen ${#parameter}.
55
56 - No regexp ${parameter/pattern/string}.
57
58 - We do not use Process Substitution <(list) or >(list).
59
60 - We prefer "test" over "[ ... ]".
61
62 - We do not write the noiseword "function" in front of shell
63 functions.
64
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65 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
66 [::], [==], nor [..]) for portability.
67
68 - We do not use \{m,n\};
69
70 - We do not use -E;
71
72 - We do not use ? nor + (which are \{0,1\} and \{1,\}
73 respectively in BRE) but that goes without saying as these
74 are ERE elements not BRE (note that \? and \+ are not even part
75 of BRE -- making them accessible from BRE is a GNU extension).
76
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77For C programs:
78
79 - We use tabs to indent, and interpret tabs as taking up to
80 8 spaces.
81
82 - We try to keep to at most 80 characters per line.
83
84 - When declaring pointers, the star sides with the variable
85 name, i.e. "char *string", not "char* string" or
86 "char * string". This makes it easier to understand code
87 like "char *string, c;".
88
89 - We avoid using braces unnecessarily. I.e.
90
91 if (bla) {
92 x = 1;
93 }
94
95 is frowned upon. A gray area is when the statement extends
96 over a few lines, and/or you have a lengthy comment atop of
97 it. Also, like in the Linux kernel, if there is a long list
98 of "else if" statements, it can make sense to add braces to
99 single line blocks.
100
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101 - We try to avoid assignments inside if().
102
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103 - Try to make your code understandable. You may put comments
104 in, but comments invariably tend to stale out when the code
105 they were describing changes. Often splitting a function
106 into two makes the intention of the code much clearer.
107
108 - Double negation is often harder to understand than no negation
109 at all.
110
111 - Some clever tricks, like using the !! operator with arithmetic
112 constructs, can be extremely confusing to others. Avoid them,
113 unless there is a compelling reason to use them.
114
115 - Use the API. No, really. We have a strbuf (variable length
116 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 117 string_list for sorted string lists, a hash map (mapping struct
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118 objects) named "struct decorate", amongst other things.
119
120 - When you come up with an API, document it.
121
122 - The first #include in C files, except in platform specific
123 compat/ implementations, should be git-compat-util.h or another
124 header file that includes it, such as cache.h or builtin.h.
125
126 - If you are planning a new command, consider writing it in shell
127 or perl first, so that changes in semantics can be easily
128 changed and discussed. Many git commands started out like
129 that, and a few are still scripts.
130
131 - Avoid introducing a new dependency into git. This means you
132 usually should stay away from scripting languages not already
133 used in the git core command set (unless your command is clearly
134 separate from it, such as an importer to convert random-scm-X
135 repositories to git).
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136
137 - When we pass <string, length> pair to functions, we should try to
138 pass them in that order.