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
  22 As for more concrete guidelines, just imitate the existing code
  23 (this is a good guideline, no matter which project you are
  24 contributing to). It is always preferable to match the _local_
  25 convention. New code added to git suite is expected to match
  26 the overall style of existing code. Modifications to existing
  27 code is expected to match the style the surrounding code already
  28 uses (even if it doesn't match the overall style of existing code).
  29
  30 But if you must have a list of rules, here they are.
  31
  32 For shell scripts specifically (not exhaustive):
  33
  34  - We use tabs for indentation.
  35
  36  - Case arms are indented at the same depth as case and esac lines.
  37
  38  - We prefer $( ... ) for command substitution; unlike ``, it
  39    properly nests.  It should have been the way Bourne spelled
  40    it from day one, but unfortunately isn't.
  41
  42  - We use POSIX compliant parameter substitutions and avoid bashisms;
  43    namely:
  44
  45    - We use ${parameter-word} and its [-=?+] siblings, and their
  46      colon'ed "unset or null" form.
  47
  48    - We use ${parameter#word} and its [#%] siblings, and their
  49      doubled "longest matching" form.
  50
  51    - No "Substring Expansion" ${parameter:offset:length}.
  52
  53    - No shell arrays.
  54
  55    - No strlen ${#parameter}.
  56
  57    - No pattern replacement ${parameter/pattern/string}.
  58
  59  - We use Arithmetic Expansion $(( ... )).
  60
  61  - Inside Arithmetic Expansion, spell shell variables with $ in front
  62    of them, as some shells do not grok $((x)) while accepting $(($x))
  63    just fine (e.g. dash older than 0.5.4).
  64
  65  - We do not use Process Substitution <(list) or >(list).
  66
  67  - We prefer "test" over "[ ... ]".
  68
  69  - We do not write the noiseword "function" in front of shell
  70    functions.
  71
  72  - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
  73    [::], [==], nor [..]) for portability.
  74
  75    - We do not use \{m,n\};
  76
  77    - We do not use -E;
  78
  79    - We do not use ? nor + (which are \{0,1\} and \{1,\}
  80      respectively in BRE) but that goes without saying as these
  81      are ERE elements not BRE (note that \? and \+ are not even part
  82      of BRE -- making them accessible from BRE is a GNU extension).
  83
  84 For C programs:
  85
  86  - We use tabs to indent, and interpret tabs as taking up to
  87    8 spaces.
  88
  89  - We try to keep to at most 80 characters per line.
  90
  91  - When declaring pointers, the star sides with the variable
  92    name, i.e. "char *string", not "char* string" or
  93    "char * string".  This makes it easier to understand code
  94    like "char *string, c;".
  95
  96  - We avoid using braces unnecessarily.  I.e.
  97
  98         if (bla) {
  99                 x = 1;
 100         }
 101
 102    is frowned upon.  A gray area is when the statement extends
 103    over a few lines, and/or you have a lengthy comment atop of
 104    it.  Also, like in the Linux kernel, if there is a long list
 105    of "else if" statements, it can make sense to add braces to
 106    single line blocks.
 107
 108  - We try to avoid assignments inside if().
 109
 110  - Try to make your code understandable.  You may put comments
 111    in, but comments invariably tend to stale out when the code
 112    they were describing changes.  Often splitting a function
 113    into two makes the intention of the code much clearer.
 114
 115  - Double negation is often harder to understand than no negation
 116    at all.
 117
 118  - Some clever tricks, like using the !! operator with arithmetic
 119    constructs, can be extremely confusing to others.  Avoid them,
 120    unless there is a compelling reason to use them.
 121
 122  - Use the API.  No, really.  We have a strbuf (variable length
 123    string), several arrays with the ALLOC_GROW() macro, a
 124    string_list for sorted string lists, a hash map (mapping struct
 125    objects) named "struct decorate", amongst other things.
 126
 127  - When you come up with an API, document it.
 128
 129  - The first #include in C files, except in platform specific
 130    compat/ implementations, should be git-compat-util.h or another
 131    header file that includes it, such as cache.h or builtin.h.
 132
 133  - If you are planning a new command, consider writing it in shell
 134    or perl first, so that changes in semantics can be easily
 135    changed and discussed.  Many git commands started out like
 136    that, and a few are still scripts.
 137
 138  - Avoid introducing a new dependency into git. This means you
 139    usually should stay away from scripting languages not already
 140    used in the git core command set (unless your command is clearly
 141    separate from it, such as an importer to convert random-scm-X
 142    repositories to git).
 143
 144  - When we pass <string, length> pair to functions, we should try to
 145    pass them in that order.
 146
 147 Writing Documentation:
 148
 149  Every user-visible change should be reflected in the documentation.
 150  The same general rule as for code applies -- imitate the existing
 151  conventions.  A few commented examples follow to provide reference
 152  when writing or modifying command usage strings and synopsis sections
 153  in the manual pages:
 154
 155  Placeholders are spelled in lowercase and enclosed in angle brackets:
 156    <file>
 157    --sort=<key>
 158    --abbrev[=<n>]
 159
 160  Possibility of multiple occurrences is indicated by three dots:
 161    <file>...
 162    (One or more of <file>.)
 163
 164  Optional parts are enclosed in square brackets:
 165    [<extra>]
 166    (Zero or one <extra>.)
 167
 168    --exec-path[=<path>]
 169    (Option with an optional argument.  Note that the "=" is inside the
 170    brackets.)
 171
 172    [<patch>...]
 173    (Zero or more of <patch>.  Note that the dots are inside, not
 174    outside the brackets.)
 175
 176  Multiple alternatives are indicated with vertical bar:
 177    [-q | --quiet]
 178    [--utf8 | --no-utf8]
 179
 180  Parentheses are used for grouping:
 181    [(<rev>|<range>)...]
 182    (Any number of either <rev> or <range>.  Parens are needed to make
 183    it clear that "..." pertains to both <rev> and <range>.)
 184
 185    [(-p <parent>)...]
 186    (Any number of option -p, each with one <parent> argument.)
 187
 188    git remote set-head <name> (-a | -d | <branch>)
 189    (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 190    brackets) be provided.)
 191
 192  And a somewhat more contrived example:
 193    --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 194    Here "=" is outside the brackets, because "--diff-filter=" is a
 195    valid usage.  "*" has its own pair of brackets, because it can
 196    (optionally) be specified only when one or more of the letters is
 197    also provided.