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 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 POSIX compliant parameter substitutions and avoid bashisms;
  39    namely:
  40
  41    - We use ${parameter-word} and its [-=?+] siblings, and their
  42      colon'ed "unset or null" form.
  43
  44    - We use ${parameter#word} and its [#%] siblings, and their
  45      doubled "longest matching" form.
  46
  47    - No "Substring Expansion" ${parameter:offset:length}.
  48
  49    - No shell arrays.
  50
  51    - No strlen ${#parameter}.
  52
  53    - No pattern replacement ${parameter/pattern/string}.
  54
  55  - We use Arithmetic Expansion $(( ... )).
  56
  57  - Inside Arithmetic Expansion, spell shell variables with $ in front
  58    of them, as some shells do not grok $((x)) while accepting $(($x))
  59    just fine (e.g. dash older than 0.5.4).
  60
  61  - We do not use Process Substitution <(list) or >(list).
  62
  63  - We prefer "test" over "[ ... ]".
  64
  65  - We do not write the noiseword "function" in front of shell
  66    functions.
  67
  68  - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
  69    [::], [==], nor [..]) for portability.
  70
  71    - We do not use \{m,n\};
  72
  73    - We do not use -E;
  74
  75    - We do not use ? nor + (which are \{0,1\} and \{1,\}
  76      respectively in BRE) but that goes without saying as these
  77      are ERE elements not BRE (note that \? and \+ are not even part
  78      of BRE -- making them accessible from BRE is a GNU extension).
  79
  80 For C programs:
  81
  82  - We use tabs to indent, and interpret tabs as taking up to
  83    8 spaces.
  84
  85  - We try to keep to at most 80 characters per line.
  86
  87  - When declaring pointers, the star sides with the variable
  88    name, i.e. "char *string", not "char* string" or
  89    "char * string".  This makes it easier to understand code
  90    like "char *string, c;".
  91
  92  - We avoid using braces unnecessarily.  I.e.
  93
  94         if (bla) {
  95                 x = 1;
  96         }
  97
  98    is frowned upon.  A gray area is when the statement extends
  99    over a few lines, and/or you have a lengthy comment atop of
 100    it.  Also, like in the Linux kernel, if there is a long list
 101    of "else if" statements, it can make sense to add braces to
 102    single line blocks.
 103
 104  - We try to avoid assignments inside if().
 105
 106  - Try to make your code understandable.  You may put comments
 107    in, but comments invariably tend to stale out when the code
 108    they were describing changes.  Often splitting a function
 109    into two makes the intention of the code much clearer.
 110
 111  - Double negation is often harder to understand than no negation
 112    at all.
 113
 114  - Some clever tricks, like using the !! operator with arithmetic
 115    constructs, can be extremely confusing to others.  Avoid them,
 116    unless there is a compelling reason to use them.
 117
 118  - Use the API.  No, really.  We have a strbuf (variable length
 119    string), several arrays with the ALLOC_GROW() macro, a
 120    string_list for sorted string lists, a hash map (mapping struct
 121    objects) named "struct decorate", amongst other things.
 122
 123  - When you come up with an API, document it.
 124
 125  - The first #include in C files, except in platform specific
 126    compat/ implementations, should be git-compat-util.h or another
 127    header file that includes it, such as cache.h or builtin.h.
 128
 129  - If you are planning a new command, consider writing it in shell
 130    or perl first, so that changes in semantics can be easily
 131    changed and discussed.  Many git commands started out like
 132    that, and a few are still scripts.
 133
 134  - Avoid introducing a new dependency into git. This means you
 135    usually should stay away from scripting languages not already
 136    used in the git core command set (unless your command is clearly
 137    separate from it, such as an importer to convert random-scm-X
 138    repositories to git).
 139
 140  - When we pass <string, length> pair to functions, we should try to
 141    pass them in that order.