Documentation / CodingGuidelineson commit Merge branch 'sg/travis-gcc-4.8' (995ec8a)
   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.  You should not use features from newer C
 199   standard, even if your compiler groks them.
 200
 201   There are a few exceptions to this guideline:
 202
 203   . since early 2012 with e1327023ea, we have been using an enum
 204     definition whose last element is followed by a comma.  This, like
 205     an array initializer that ends with a trailing comma, can be used
 206     to reduce the patch noise when adding a new identifer at the end.
 207
 208   . since mid 2017 with cbc0f81d, we have been using designated
 209     initializers for struct (e.g. "struct t v = { .val = 'a' };").
 210
 211   . since mid 2017 with 512f41cf, we have been using designated
 212     initializers for array (e.g. "int array[10] = { [5] = 2 }").
 213
 214   These used to be forbidden, but we have not heard any breakage
 215   report, and they are assumed to be safe.
 216
 217 - Variables have to be declared at the beginning of the block, before
 218   the first statement (i.e. -Wdeclaration-after-statement).
 219
 220 - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
 221   is still not allowed in this codebase.
 222
 223 - NULL pointers shall be written as NULL, not as 0.
 224
 225 - When declaring pointers, the star sides with the variable
 226   name, i.e. "char *string", not "char* string" or
 227   "char * string".  This makes it easier to understand code
 228   like "char *string, c;".
 229
 230 - Use whitespace around operators and keywords, but not inside
 231   parentheses and not around functions. So:
 232
 233        while (condition)
 234                func(bar + 1);
 235
 236   and not:
 237
 238        while( condition )
 239                func (bar+1);
 240
 241 - We avoid using braces unnecessarily.  I.e.
 242
 243        if (bla) {
 244                x = 1;
 245        }
 246
 247   is frowned upon. But there are a few exceptions:
 248
 249        - When the statement extends over a few lines (e.g., a while loop
 250          with an embedded conditional, or a comment). E.g.:
 251
 252                while (foo) {
 253                        if (x)
 254                                one();
 255                        else
 256                                two();
 257                }
 258
 259                if (foo) {
 260                        /*
 261                         * This one requires some explanation,
 262                         * so we're better off with braces to make
 263                         * it obvious that the indentation is correct.
 264                         */
 265                        doit();
 266                }
 267
 268        - When there are multiple arms to a conditional and some of them
 269          require braces, enclose even a single line block in braces for
 270          consistency. E.g.:
 271
 272                if (foo) {
 273                        doit();
 274                } else {
 275                        one();
 276                        two();
 277                        three();
 278                }
 279
 280 - We try to avoid assignments in the condition of an "if" statement.
 281
 282 - Try to make your code understandable.  You may put comments
 283   in, but comments invariably tend to stale out when the code
 284   they were describing changes.  Often splitting a function
 285   into two makes the intention of the code much clearer.
 286
 287 - Multi-line comments include their delimiters on separate lines from
 288   the text.  E.g.
 289
 290        /*
 291         * A very long
 292         * multi-line comment.
 293         */
 294
 295   Note however that a comment that explains a translatable string to
 296   translators uses a convention of starting with a magic token
 297   "TRANSLATORS: ", e.g.
 298
 299        /*
 300         * TRANSLATORS: here is a comment that explains the string to
 301         * be translated, that follows immediately after it.
 302         */
 303        _("Here is a translatable string explained by the above.");
 304
 305 - Double negation is often harder to understand than no negation
 306   at all.
 307
 308 - There are two schools of thought when it comes to comparison,
 309   especially inside a loop. Some people prefer to have the less stable
 310   value on the left hand side and the more stable value on the right hand
 311   side, e.g. if you have a loop that counts variable i down to the
 312   lower bound,
 313
 314        while (i > lower_bound) {
 315                do something;
 316                i--;
 317        }
 318
 319   Other people prefer to have the textual order of values match the
 320   actual order of values in their comparison, so that they can
 321   mentally draw a number line from left to right and place these
 322   values in order, i.e.
 323
 324        while (lower_bound < i) {
 325                do something;
 326                i--;
 327        }
 328
 329   Both are valid, and we use both.  However, the more "stable" the
 330   stable side becomes, the more we tend to prefer the former
 331   (comparison with a constant, "i > 0", is an extreme example).
 332   Just do not mix styles in the same part of the code and mimic
 333   existing styles in the neighbourhood.
 334
 335 - There are two schools of thought when it comes to splitting a long
 336   logical line into multiple lines.  Some people push the second and
 337   subsequent lines far enough to the right with tabs and align them:
 338
 339        if (the_beginning_of_a_very_long_expression_that_has_to ||
 340                span_more_than_a_single_line_of ||
 341                the_source_text) {
 342                ...
 343
 344   while other people prefer to align the second and the subsequent
 345   lines with the column immediately inside the opening parenthesis,
 346   with tabs and spaces, following our "tabstop is always a multiple
 347   of 8" convention:
 348
 349        if (the_beginning_of_a_very_long_expression_that_has_to ||
 350            span_more_than_a_single_line_of ||
 351            the_source_text) {
 352                ...
 353
 354   Both are valid, and we use both.  Again, just do not mix styles in
 355   the same part of the code and mimic existing styles in the
 356   neighbourhood.
 357
 358 - When splitting a long logical line, some people change line before
 359   a binary operator, so that the result looks like a parse tree when
 360   you turn your head 90-degrees counterclockwise:
 361
 362        if (the_beginning_of_a_very_long_expression_that_has_to
 363            || span_more_than_a_single_line_of_the_source_text) {
 364
 365   while other people prefer to leave the operator at the end of the
 366   line:
 367
 368        if (the_beginning_of_a_very_long_expression_that_has_to ||
 369            span_more_than_a_single_line_of_the_source_text) {
 370
 371   Both are valid, but we tend to use the latter more, unless the
 372   expression gets fairly complex, in which case the former tends to
 373   be easier to read.  Again, just do not mix styles in the same part
 374   of the code and mimic existing styles in the neighbourhood.
 375
 376 - When splitting a long logical line, with everything else being
 377   equal, it is preferable to split after the operator at higher
 378   level in the parse tree.  That is, this is more preferable:
 379
 380        if (a_very_long_variable * that_is_used_in +
 381            a_very_long_expression) {
 382                ...
 383
 384   than
 385
 386        if (a_very_long_variable *
 387            that_is_used_in + a_very_long_expression) {
 388                ...
 389
 390 - Some clever tricks, like using the !! operator with arithmetic
 391   constructs, can be extremely confusing to others.  Avoid them,
 392   unless there is a compelling reason to use them.
 393
 394 - Use the API.  No, really.  We have a strbuf (variable length
 395   string), several arrays with the ALLOC_GROW() macro, a
 396   string_list for sorted string lists, a hash map (mapping struct
 397   objects) named "struct decorate", amongst other things.
 398
 399 - When you come up with an API, document its functions and structures
 400   in the header file that exposes the API to its callers. Use what is
 401   in "strbuf.h" as a model for the appropriate tone and level of
 402   detail.
 403
 404 - The first #include in C files, except in platform specific compat/
 405   implementations, must be either "git-compat-util.h", "cache.h" or
 406   "builtin.h".  You do not have to include more than one of these.
 407
 408 - A C file must directly include the header files that declare the
 409   functions and the types it uses, except for the functions and types
 410   that are made available to it by including one of the header files
 411   it must include by the previous rule.
 412
 413 - If you are planning a new command, consider writing it in shell
 414   or perl first, so that changes in semantics can be easily
 415   changed and discussed.  Many Git commands started out like
 416   that, and a few are still scripts.
 417
 418 - Avoid introducing a new dependency into Git. This means you
 419   usually should stay away from scripting languages not already
 420   used in the Git core command set (unless your command is clearly
 421   separate from it, such as an importer to convert random-scm-X
 422   repositories to Git).
 423
 424 - When we pass <string, length> pair to functions, we should try to
 425   pass them in that order.
 426
 427 - Use Git's gettext wrappers to make the user interface
 428   translatable. See "Marking strings for translation" in po/README.
 429
 430 - Variables and functions local to a given source file should be marked
 431   with "static". Variables that are visible to other source files
 432   must be declared with "extern" in header files. However, function
 433   declarations should not use "extern", as that is already the default.
 434
 435 - You can launch gdb around your program using the shorthand GIT_DEBUGGER.
 436   Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or
 437   run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to
 438   use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
 439   ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
 440
 441For Perl programs:
 442
 443 - Most of the C guidelines above apply.
 444
 445 - We try to support Perl 5.8 and later ("use Perl 5.008").
 446
 447 - use strict and use warnings are strongly preferred.
 448
 449 - Don't overuse statement modifiers unless using them makes the
 450   result easier to follow.
 451
 452        ... do something ...
 453        do_this() unless (condition);
 454        ... do something else ...
 455
 456   is more readable than:
 457
 458        ... do something ...
 459        unless (condition) {
 460                do_this();
 461        }
 462        ... do something else ...
 463
 464   *only* when the condition is so rare that do_this() will be almost
 465   always called.
 466
 467 - We try to avoid assignments inside "if ()" conditions.
 468
 469 - Learn and use Git.pm if you need that functionality.
 470
 471 - For Emacs, it's useful to put the following in
 472   GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
 473
 474    ;; note the first part is useful for C editing, too
 475    ((nil . ((indent-tabs-mode . t)
 476                  (tab-width . 8)
 477                  (fill-column . 80)))
 478     (cperl-mode . ((cperl-indent-level . 8)
 479                    (cperl-extra-newline-before-brace . nil)
 480                    (cperl-merge-trailing-else . t))))
 481
 482For Python scripts:
 483
 484 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
 485
 486 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
 487
 488 - Where required libraries do not restrict us to Python 2, we try to
 489   also be compatible with Python 3.1 and later.
 490
 491 - When you must differentiate between Unicode literals and byte string
 492   literals, it is OK to use the 'b' prefix.  Even though the Python
 493   documentation for version 2.6 does not mention this prefix, it has
 494   been supported since version 2.6.0.
 495
 496Error Messages
 497
 498 - Do not end error messages with a full stop.
 499
 500 - Do not capitalize ("unable to open %s", not "Unable to open %s")
 501
 502 - Say what the error is first ("cannot open %s", not "%s: cannot open")
 503
 504
 505Externally Visible Names
 506
 507 - For configuration variable names, follow the existing convention:
 508
 509   . The section name indicates the affected subsystem.
 510
 511   . The subsection name, if any, indicates which of an unbounded set
 512     of things to set the value for.
 513
 514   . The variable name describes the effect of tweaking this knob.
 515
 516   The section and variable names that consist of multiple words are
 517   formed by concatenating the words without punctuations (e.g. `-`),
 518   and are broken using bumpyCaps in documentation as a hint to the
 519   reader.
 520
 521   When choosing the variable namespace, do not use variable name for
 522   specifying possibly unbounded set of things, most notably anything
 523   an end user can freely come up with (e.g. branch names).  Instead,
 524   use subsection names or variable values, like the existing variable
 525   branch.<name>.description does.
 526
 527
 528Writing Documentation:
 529
 530 Most (if not all) of the documentation pages are written in the
 531 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
 532 processed into HTML and manpages (e.g. git.html and git.1 in the
 533 same directory).
 534
 535 The documentation liberally mixes US and UK English (en_US/UK)
 536 norms for spelling and grammar, which is somewhat unfortunate.
 537 In an ideal world, it would have been better if it consistently
 538 used only one and not the other, and we would have picked en_US
 539 (if you wish to correct the English of some of the existing
 540 documentation, please see the documentation-related advice in the
 541 Documentation/SubmittingPatches file).
 542
 543 Every user-visible change should be reflected in the documentation.
 544 The same general rule as for code applies -- imitate the existing
 545 conventions.
 546
 547 A few commented examples follow to provide reference when writing or
 548 modifying command usage strings and synopsis sections in the manual
 549 pages:
 550
 551 Placeholders are spelled in lowercase and enclosed in angle brackets:
 552   <file>
 553   --sort=<key>
 554   --abbrev[=<n>]
 555
 556 If a placeholder has multiple words, they are separated by dashes:
 557   <new-branch-name>
 558   --template=<template-directory>
 559
 560 Possibility of multiple occurrences is indicated by three dots:
 561   <file>...
 562   (One or more of <file>.)
 563
 564 Optional parts are enclosed in square brackets:
 565   [<extra>]
 566   (Zero or one <extra>.)
 567
 568   --exec-path[=<path>]
 569   (Option with an optional argument.  Note that the "=" is inside the
 570   brackets.)
 571
 572   [<patch>...]
 573   (Zero or more of <patch>.  Note that the dots are inside, not
 574   outside the brackets.)
 575
 576 Multiple alternatives are indicated with vertical bars:
 577   [-q | --quiet]
 578   [--utf8 | --no-utf8]
 579
 580 Parentheses are used for grouping:
 581   [(<rev> | <range>)...]
 582   (Any number of either <rev> or <range>.  Parens are needed to make
 583   it clear that "..." pertains to both <rev> and <range>.)
 584
 585   [(-p <parent>)...]
 586   (Any number of option -p, each with one <parent> argument.)
 587
 588   git remote set-head <name> (-a | -d | <branch>)
 589   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 590   brackets) be provided.)
 591
 592 And a somewhat more contrived example:
 593   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 594   Here "=" is outside the brackets, because "--diff-filter=" is a
 595   valid usage.  "*" has its own pair of brackets, because it can
 596   (optionally) be specified only when one or more of the letters is
 597   also provided.
 598
 599  A note on notation:
 600   Use 'git' (all lowercase) when talking about commands i.e. something
 601   the user would type into a shell and use 'Git' (uppercase first letter)
 602   when talking about the version control system and its properties.
 603
 604 A few commented examples follow to provide reference when writing or
 605 modifying paragraphs or option/command explanations that contain options
 606 or commands:
 607
 608 Literal examples (e.g. use of command-line options, command names,
 609 branch names, URLs, pathnames (files and directories), configuration and
 610 environment variables) must be typeset in monospace (i.e. wrapped with
 611 backticks):
 612   `--pretty=oneline`
 613   `git rev-list`
 614   `remote.pushDefault`
 615   `http://git.example.com`
 616   `.git/config`
 617   `GIT_DIR`
 618   `HEAD`
 619
 620 An environment variable must be prefixed with "$" only when referring to its
 621 value and not when referring to the variable itself, in this case there is
 622 nothing to add except the backticks:
 623   `GIT_DIR` is specified
 624   `$GIT_DIR/hooks/pre-receive`
 625
 626 Word phrases enclosed in `backtick characters` are rendered literally
 627 and will not be further expanded. The use of `backticks` to achieve the
 628 previous rule means that literal examples should not use AsciiDoc
 629 escapes.
 630   Correct:
 631      `--pretty=oneline`
 632   Incorrect:
 633      `\--pretty=oneline`
 634
 635 If some place in the documentation needs to typeset a command usage
 636 example with inline substitutions, it is fine to use +monospaced and
 637 inline substituted text+ instead of `monospaced literal text`, and with
 638 the former, the part that should not get substituted must be
 639 quoted/escaped.