Documentation / CodingGuidelineson commit Merge branch 'cm/notes-comment-fix' (521d3ef)
   1Like other projects, we also have some guidelines to keep to the
   2code.  For Git in general, a few 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 - Fixing style violations while working on a real change as a
  22   preparatory clean-up step is good, but otherwise avoid useless code
  23   churn for the sake of conforming to the style.
  24
  25   "Once it _is_ in the tree, it's not really worth the patch noise to
  26   go and fix it up."
  27   Cf. http://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html
  28
  29Make your code readable and sensible, and don't try to be clever.
  30
  31As for more concrete guidelines, just imitate the existing code
  32(this is a good guideline, no matter which project you are
  33contributing to). It is always preferable to match the _local_
  34convention. New code added to Git suite is expected to match
  35the overall style of existing code. Modifications to existing
  36code is expected to match the style the surrounding code already
  37uses (even if it doesn't match the overall style of existing code).
  38
  39But if you must have a list of rules, here they are.
  40
  41For shell scripts specifically (not exhaustive):
  42
  43 - We use tabs for indentation.
  44
  45 - Case arms are indented at the same depth as case and esac lines,
  46   like this:
  47
  48        case "$variable" in
  49        pattern1)
  50                do this
  51                ;;
  52        pattern2)
  53                do that
  54                ;;
  55        esac
  56
  57 - Redirection operators should be written with space before, but no
  58   space after them.  In other words, write 'echo test >"$file"'
  59   instead of 'echo test> $file' or 'echo test > $file'.  Note that
  60   even though it is not required by POSIX to double-quote the
  61   redirection target in a variable (as shown above), our code does so
  62   because some versions of bash issue a warning without the quotes.
  63
  64        (incorrect)
  65        cat hello > world < universe
  66        echo hello >$world
  67
  68        (correct)
  69        cat hello >world <universe
  70        echo hello >"$world"
  71
  72 - We prefer $( ... ) for command substitution; unlike ``, it
  73   properly nests.  It should have been the way Bourne spelled
  74   it from day one, but unfortunately isn't.
  75
  76 - If you want to find out if a command is available on the user's
  77   $PATH, you should use 'type <command>', instead of 'which <command>'.
  78   The output of 'which' is not machine parseable and its exit code
  79   is not reliable across platforms.
  80
  81 - We use POSIX compliant parameter substitutions and avoid bashisms;
  82   namely:
  83
  84   - We use ${parameter-word} and its [-=?+] siblings, and their
  85     colon'ed "unset or null" form.
  86
  87   - We use ${parameter#word} and its [#%] siblings, and their
  88     doubled "longest matching" form.
  89
  90   - No "Substring Expansion" ${parameter:offset:length}.
  91
  92   - No shell arrays.
  93
  94   - No strlen ${#parameter}.
  95
  96   - No pattern replacement ${parameter/pattern/string}.
  97
  98 - We use Arithmetic Expansion $(( ... )).
  99
 100 - Inside Arithmetic Expansion, spell shell variables with $ in front
 101   of them, as some shells do not grok $((x)) while accepting $(($x))
 102   just fine (e.g. dash older than 0.5.4).
 103
 104 - We do not use Process Substitution <(list) or >(list).
 105
 106 - Do not write control structures on a single line with semicolon.
 107   "then" should be on the next line for if statements, and "do"
 108   should be on the next line for "while" and "for".
 109
 110        (incorrect)
 111        if test -f hello; then
 112                do this
 113        fi
 114
 115        (correct)
 116        if test -f hello
 117        then
 118                do this
 119        fi
 120
 121 - If a command sequence joined with && or || or | spans multiple
 122   lines, put each command on a separate line and put && and || and |
 123   operators at the end of each line, rather than the start. This
 124   means you don't need to use \ to join lines, since the above
 125   operators imply the sequence isn't finished.
 126
 127        (incorrect)
 128        grep blob verify_pack_result \
 129        | awk -f print_1.awk \
 130        | sort >actual &&
 131        ...
 132
 133        (correct)
 134        grep blob verify_pack_result |
 135        awk -f print_1.awk |
 136        sort >actual &&
 137        ...
 138
 139 - We prefer "test" over "[ ... ]".
 140
 141 - We do not write the noiseword "function" in front of shell
 142   functions.
 143
 144 - We prefer a space between the function name and the parentheses,
 145   and no space inside the parentheses. The opening "{" should also
 146   be on the same line.
 147
 148        (incorrect)
 149        my_function(){
 150                ...
 151
 152        (correct)
 153        my_function () {
 154                ...
 155
 156 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
 157   [::], [==], or [..]) for portability.
 158
 159   - We do not use \{m,n\};
 160
 161   - We do not use -E;
 162
 163   - We do not use ? or + (which are \{0,1\} and \{1,\}
 164     respectively in BRE) but that goes without saying as these
 165     are ERE elements not BRE (note that \? and \+ are not even part
 166     of BRE -- making them accessible from BRE is a GNU extension).
 167
 168 - Use Git's gettext wrappers in git-sh-i18n to make the user
 169   interface translatable. See "Marking strings for translation" in
 170   po/README.
 171
 172 - We do not write our "test" command with "-a" and "-o" and use "&&"
 173   or "||" to concatenate multiple "test" commands instead, because
 174   the use of "-a/-o" is often error-prone.  E.g.
 175
 176     test -n "$x" -a "$a" = "$b"
 177
 178   is buggy and breaks when $x is "=", but
 179
 180     test -n "$x" && test "$a" = "$b"
 181
 182   does not have such a problem.
 183
 184
 185For C programs:
 186
 187 - We use tabs to indent, and interpret tabs as taking up to
 188   8 spaces.
 189
 190 - We try to keep to at most 80 characters per line.
 191
 192 - As a Git developer we assume you have a reasonably modern compiler
 193   and we recommend you to enable the DEVELOPER makefile knob to
 194   ensure your patch is clear of all compiler warnings we care about,
 195   by e.g. "echo DEVELOPER=1 >>config.mak".
 196
 197 - We try to support a wide range of C compilers to compile Git with,
 198   including old ones. That means that you should not use C99
 199   initializers, even if a lot of compilers grok it.
 200
 201 - Variables have to be declared at the beginning of the block.
 202
 203 - NULL pointers shall be written as NULL, not as 0.
 204
 205 - When declaring pointers, the star sides with the variable
 206   name, i.e. "char *string", not "char* string" or
 207   "char * string".  This makes it easier to understand code
 208   like "char *string, c;".
 209
 210 - Use whitespace around operators and keywords, but not inside
 211   parentheses and not around functions. So:
 212
 213        while (condition)
 214                func(bar + 1);
 215
 216   and not:
 217
 218        while( condition )
 219                func (bar+1);
 220
 221 - We avoid using braces unnecessarily.  I.e.
 222
 223        if (bla) {
 224                x = 1;
 225        }
 226
 227   is frowned upon. But there are a few exceptions:
 228
 229        - When the statement extends over a few lines (e.g., a while loop
 230          with an embedded conditional, or a comment). E.g.:
 231
 232                while (foo) {
 233                        if (x)
 234                                one();
 235                        else
 236                                two();
 237                }
 238
 239                if (foo) {
 240                        /*
 241                         * This one requires some explanation,
 242                         * so we're better off with braces to make
 243                         * it obvious that the indentation is correct.
 244                         */
 245                        doit();
 246                }
 247
 248        - When there are multiple arms to a conditional and some of them
 249          require braces, enclose even a single line block in braces for
 250          consistency. E.g.:
 251
 252                if (foo) {
 253                        doit();
 254                } else {
 255                        one();
 256                        two();
 257                        three();
 258                }
 259
 260 - We try to avoid assignments in the condition of an "if" statement.
 261
 262 - Try to make your code understandable.  You may put comments
 263   in, but comments invariably tend to stale out when the code
 264   they were describing changes.  Often splitting a function
 265   into two makes the intention of the code much clearer.
 266
 267 - Multi-line comments include their delimiters on separate lines from
 268   the text.  E.g.
 269
 270        /*
 271         * A very long
 272         * multi-line comment.
 273         */
 274
 275   Note however that a comment that explains a translatable string to
 276   translators uses a convention of starting with a magic token
 277   "TRANSLATORS: ", e.g.
 278
 279        /*
 280         * TRANSLATORS: here is a comment that explains the string to
 281         * be translated, that follows immediately after it.
 282         */
 283        _("Here is a translatable string explained by the above.");
 284
 285 - Double negation is often harder to understand than no negation
 286   at all.
 287
 288 - There are two schools of thought when it comes to comparison,
 289   especially inside a loop. Some people prefer to have the less stable
 290   value on the left hand side and the more stable value on the right hand
 291   side, e.g. if you have a loop that counts variable i down to the
 292   lower bound,
 293
 294        while (i > lower_bound) {
 295                do something;
 296                i--;
 297        }
 298
 299   Other people prefer to have the textual order of values match the
 300   actual order of values in their comparison, so that they can
 301   mentally draw a number line from left to right and place these
 302   values in order, i.e.
 303
 304        while (lower_bound < i) {
 305                do something;
 306                i--;
 307        }
 308
 309   Both are valid, and we use both.  However, the more "stable" the
 310   stable side becomes, the more we tend to prefer the former
 311   (comparison with a constant, "i > 0", is an extreme example).
 312   Just do not mix styles in the same part of the code and mimic
 313   existing styles in the neighbourhood.
 314
 315 - There are two schools of thought when it comes to splitting a long
 316   logical line into multiple lines.  Some people push the second and
 317   subsequent lines far enough to the right with tabs and align them:
 318
 319        if (the_beginning_of_a_very_long_expression_that_has_to ||
 320                span_more_than_a_single_line_of ||
 321                the_source_text) {
 322                ...
 323
 324   while other people prefer to align the second and the subsequent
 325   lines with the column immediately inside the opening parenthesis,
 326   with tabs and spaces, following our "tabstop is always a multiple
 327   of 8" convention:
 328
 329        if (the_beginning_of_a_very_long_expression_that_has_to ||
 330            span_more_than_a_single_line_of ||
 331            the_source_text) {
 332                ...
 333
 334   Both are valid, and we use both.  Again, just do not mix styles in
 335   the same part of the code and mimic existing styles in the
 336   neighbourhood.
 337
 338 - When splitting a long logical line, some people change line before
 339   a binary operator, so that the result looks like a parse tree when
 340   you turn your head 90-degrees counterclockwise:
 341
 342        if (the_beginning_of_a_very_long_expression_that_has_to
 343            || span_more_than_a_single_line_of_the_source_text) {
 344
 345   while other people prefer to leave the operator at the end of the
 346   line:
 347
 348        if (the_beginning_of_a_very_long_expression_that_has_to ||
 349            span_more_than_a_single_line_of_the_source_text) {
 350
 351   Both are valid, but we tend to use the latter more, unless the
 352   expression gets fairly complex, in which case the former tends to
 353   be easier to read.  Again, just do not mix styles in the same part
 354   of the code and mimic existing styles in the neighbourhood.
 355
 356 - When splitting a long logical line, with everything else being
 357   equal, it is preferable to split after the operator at higher
 358   level in the parse tree.  That is, this is more preferable:
 359
 360        if (a_very_long_variable * that_is_used_in +
 361            a_very_long_expression) {
 362                ...
 363
 364   than
 365
 366        if (a_very_long_variable *
 367            that_is_used_in + a_very_long_expression) {
 368                ...
 369
 370 - Some clever tricks, like using the !! operator with arithmetic
 371   constructs, can be extremely confusing to others.  Avoid them,
 372   unless there is a compelling reason to use them.
 373
 374 - Use the API.  No, really.  We have a strbuf (variable length
 375   string), several arrays with the ALLOC_GROW() macro, a
 376   string_list for sorted string lists, a hash map (mapping struct
 377   objects) named "struct decorate", amongst other things.
 378
 379 - When you come up with an API, document its functions and structures
 380   in the header file that exposes the API to its callers. Use what is
 381   in "strbuf.h" as a model for the appropriate tone and level of
 382   detail.
 383
 384 - The first #include in C files, except in platform specific compat/
 385   implementations, must be either "git-compat-util.h", "cache.h" or
 386   "builtin.h".  You do not have to include more than one of these.
 387
 388 - A C file must directly include the header files that declare the
 389   functions and the types it uses, except for the functions and types
 390   that are made available to it by including one of the header files
 391   it must include by the previous rule.
 392
 393 - If you are planning a new command, consider writing it in shell
 394   or perl first, so that changes in semantics can be easily
 395   changed and discussed.  Many Git commands started out like
 396   that, and a few are still scripts.
 397
 398 - Avoid introducing a new dependency into Git. This means you
 399   usually should stay away from scripting languages not already
 400   used in the Git core command set (unless your command is clearly
 401   separate from it, such as an importer to convert random-scm-X
 402   repositories to Git).
 403
 404 - When we pass <string, length> pair to functions, we should try to
 405   pass them in that order.
 406
 407 - Use Git's gettext wrappers to make the user interface
 408   translatable. See "Marking strings for translation" in po/README.
 409
 410 - Variables and functions local to a given source file should be marked
 411   with "static". Variables that are visible to other source files
 412   must be declared with "extern" in header files. However, function
 413   declarations should not use "extern", as that is already the default.
 414
 415For Perl programs:
 416
 417 - Most of the C guidelines above apply.
 418
 419 - We try to support Perl 5.8 and later ("use Perl 5.008").
 420
 421 - use strict and use warnings are strongly preferred.
 422
 423 - Don't overuse statement modifiers unless using them makes the
 424   result easier to follow.
 425
 426        ... do something ...
 427        do_this() unless (condition);
 428        ... do something else ...
 429
 430   is more readable than:
 431
 432        ... do something ...
 433        unless (condition) {
 434                do_this();
 435        }
 436        ... do something else ...
 437
 438   *only* when the condition is so rare that do_this() will be almost
 439   always called.
 440
 441 - We try to avoid assignments inside "if ()" conditions.
 442
 443 - Learn and use Git.pm if you need that functionality.
 444
 445 - For Emacs, it's useful to put the following in
 446   GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
 447
 448    ;; note the first part is useful for C editing, too
 449    ((nil . ((indent-tabs-mode . t)
 450                  (tab-width . 8)
 451                  (fill-column . 80)))
 452     (cperl-mode . ((cperl-indent-level . 8)
 453                    (cperl-extra-newline-before-brace . nil)
 454                    (cperl-merge-trailing-else . t))))
 455
 456For Python scripts:
 457
 458 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
 459
 460 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
 461
 462 - Where required libraries do not restrict us to Python 2, we try to
 463   also be compatible with Python 3.1 and later.
 464
 465 - When you must differentiate between Unicode literals and byte string
 466   literals, it is OK to use the 'b' prefix.  Even though the Python
 467   documentation for version 2.6 does not mention this prefix, it has
 468   been supported since version 2.6.0.
 469
 470Error Messages
 471
 472 - Do not end error messages with a full stop.
 473
 474 - Do not capitalize ("unable to open %s", not "Unable to open %s")
 475
 476 - Say what the error is first ("cannot open %s", not "%s: cannot open")
 477
 478
 479Externally Visible Names
 480
 481 - For configuration variable names, follow the existing convention:
 482
 483   . The section name indicates the affected subsystem.
 484
 485   . The subsection name, if any, indicates which of an unbounded set
 486     of things to set the value for.
 487
 488   . The variable name describes the effect of tweaking this knob.
 489
 490   The section and variable names that consist of multiple words are
 491   formed by concatenating the words without punctuations (e.g. `-`),
 492   and are broken using bumpyCaps in documentation as a hint to the
 493   reader.
 494
 495   When choosing the variable namespace, do not use variable name for
 496   specifying possibly unbounded set of things, most notably anything
 497   an end user can freely come up with (e.g. branch names).  Instead,
 498   use subsection names or variable values, like the existing variable
 499   branch.<name>.description does.
 500
 501
 502Writing Documentation:
 503
 504 Most (if not all) of the documentation pages are written in the
 505 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
 506 processed into HTML and manpages (e.g. git.html and git.1 in the
 507 same directory).
 508
 509 The documentation liberally mixes US and UK English (en_US/UK)
 510 norms for spelling and grammar, which is somewhat unfortunate.
 511 In an ideal world, it would have been better if it consistently
 512 used only one and not the other, and we would have picked en_US
 513 (if you wish to correct the English of some of the existing
 514 documentation, please see the documentation-related advice in the
 515 Documentation/SubmittingPatches file).
 516
 517 Every user-visible change should be reflected in the documentation.
 518 The same general rule as for code applies -- imitate the existing
 519 conventions.
 520
 521 A few commented examples follow to provide reference when writing or
 522 modifying command usage strings and synopsis sections in the manual
 523 pages:
 524
 525 Placeholders are spelled in lowercase and enclosed in angle brackets:
 526   <file>
 527   --sort=<key>
 528   --abbrev[=<n>]
 529
 530 If a placeholder has multiple words, they are separated by dashes:
 531   <new-branch-name>
 532   --template=<template-directory>
 533
 534 Possibility of multiple occurrences is indicated by three dots:
 535   <file>...
 536   (One or more of <file>.)
 537
 538 Optional parts are enclosed in square brackets:
 539   [<extra>]
 540   (Zero or one <extra>.)
 541
 542   --exec-path[=<path>]
 543   (Option with an optional argument.  Note that the "=" is inside the
 544   brackets.)
 545
 546   [<patch>...]
 547   (Zero or more of <patch>.  Note that the dots are inside, not
 548   outside the brackets.)
 549
 550 Multiple alternatives are indicated with vertical bars:
 551   [-q | --quiet]
 552   [--utf8 | --no-utf8]
 553
 554 Parentheses are used for grouping:
 555   [(<rev> | <range>)...]
 556   (Any number of either <rev> or <range>.  Parens are needed to make
 557   it clear that "..." pertains to both <rev> and <range>.)
 558
 559   [(-p <parent>)...]
 560   (Any number of option -p, each with one <parent> argument.)
 561
 562   git remote set-head <name> (-a | -d | <branch>)
 563   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 564   brackets) be provided.)
 565
 566 And a somewhat more contrived example:
 567   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 568   Here "=" is outside the brackets, because "--diff-filter=" is a
 569   valid usage.  "*" has its own pair of brackets, because it can
 570   (optionally) be specified only when one or more of the letters is
 571   also provided.
 572
 573  A note on notation:
 574   Use 'git' (all lowercase) when talking about commands i.e. something
 575   the user would type into a shell and use 'Git' (uppercase first letter)
 576   when talking about the version control system and its properties.
 577
 578 A few commented examples follow to provide reference when writing or
 579 modifying paragraphs or option/command explanations that contain options
 580 or commands:
 581
 582 Literal examples (e.g. use of command-line options, command names,
 583 branch names, URLs, pathnames (files and directories), configuration and
 584 environment variables) must be typeset in monospace (i.e. wrapped with
 585 backticks):
 586   `--pretty=oneline`
 587   `git rev-list`
 588   `remote.pushDefault`
 589   `http://git.example.com`
 590   `.git/config`
 591   `GIT_DIR`
 592   `HEAD`
 593
 594 An environment variable must be prefixed with "$" only when referring to its
 595 value and not when referring to the variable itself, in this case there is
 596 nothing to add except the backticks:
 597   `GIT_DIR` is specified
 598   `$GIT_DIR/hooks/pre-receive`
 599
 600 Word phrases enclosed in `backtick characters` are rendered literally
 601 and will not be further expanded. The use of `backticks` to achieve the
 602 previous rule means that literal examples should not use AsciiDoc
 603 escapes.
 604   Correct:
 605      `--pretty=oneline`
 606   Incorrect:
 607      `\--pretty=oneline`
 608
 609 If some place in the documentation needs to typeset a command usage
 610 example with inline substitutions, it is fine to use +monospaced and
 611 inline substituted text+ instead of `monospaced literal text`, and with
 612 the former, the part that should not get substituted must be
 613 quoted/escaped.