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Diffstat (limited to 'Documentation/CodingGuidelines')
| -rw-r--r-- | Documentation/CodingGuidelines | 452 |
1 files changed, 431 insertions, 21 deletions
diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines index b8bf618..c6e536f 100644 --- a/Documentation/CodingGuidelines +++ b/Documentation/CodingGuidelines @@ -1,5 +1,5 @@ Like other projects, we also have some guidelines to keep to the -code. For git in general, three rough rules are: +code. For Git in general, a few rough rules are: - Most importantly, we never say "It's in POSIX; we'll happily ignore your needs should your system not conform to it." @@ -18,11 +18,20 @@ code. For git in general, three rough rules are: judgement call, the decision based more on real world constraints people face than what the paper standard says. + - Fixing style violations while working on a real change as a + preparatory clean-up step is good, but otherwise avoid useless code + churn for the sake of conforming to the style. + + "Once it _is_ in the tree, it's not really worth the patch noise to + go and fix it up." + Cf. http://article.gmane.org/gmane.linux.kernel/943020 + +Make your code readable and sensible, and don't try to be clever. As for more concrete guidelines, just imitate the existing code (this is a good guideline, no matter which project you are contributing to). It is always preferable to match the _local_ -convention. New code added to git suite is expected to match +convention. New code added to Git suite is expected to match the overall style of existing code. Modifications to existing code is expected to match the style the surrounding code already uses (even if it doesn't match the overall style of existing code). @@ -31,45 +40,130 @@ But if you must have a list of rules, here they are. For shell scripts specifically (not exhaustive): + - We use tabs for indentation. + + - Case arms are indented at the same depth as case and esac lines, + like this: + + case "$variable" in + pattern1) + do this + ;; + pattern2) + do that + ;; + esac + + - Redirection operators should be written with space before, but no + space after them. In other words, write 'echo test >"$file"' + instead of 'echo test> $file' or 'echo test > $file'. Note that + even though it is not required by POSIX to double-quote the + redirection target in a variable (as shown above), our code does so + because some versions of bash issue a warning without the quotes. + + (incorrect) + cat hello > world < universe + echo hello >$world + + (correct) + cat hello >world <universe + echo hello >"$world" + - We prefer $( ... ) for command substitution; unlike ``, it properly nests. It should have been the way Bourne spelled it from day one, but unfortunately isn't. - - We use ${parameter-word} and its [-=?+] siblings, and their - colon'ed "unset or null" form. + - If you want to find out if a command is available on the user's + $PATH, you should use 'type <command>', instead of 'which <command>'. + The output of 'which' is not machine parseable and its exit code + is not reliable across platforms. - - We use ${parameter#word} and its [#%] siblings, and their - doubled "longest matching" form. + - We use POSIX compliant parameter substitutions and avoid bashisms; + namely: - - We use Arithmetic Expansion $(( ... )). + - We use ${parameter-word} and its [-=?+] siblings, and their + colon'ed "unset or null" form. - - No "Substring Expansion" ${parameter:offset:length}. + - We use ${parameter#word} and its [#%] siblings, and their + doubled "longest matching" form. - - No shell arrays. + - No "Substring Expansion" ${parameter:offset:length}. - - No strlen ${#parameter}. + - No shell arrays. - - No regexp ${parameter/pattern/string}. + - No strlen ${#parameter}. + + - No pattern replacement ${parameter/pattern/string}. + + - We use Arithmetic Expansion $(( ... )). + + - Inside Arithmetic Expansion, spell shell variables with $ in front + of them, as some shells do not grok $((x)) while accepting $(($x)) + just fine (e.g. dash older than 0.5.4). - We do not use Process Substitution <(list) or >(list). + - Do not write control structures on a single line with semicolon. + "then" should be on the next line for if statements, and "do" + should be on the next line for "while" and "for". + + (incorrect) + if test -f hello; then + do this + fi + + (correct) + if test -f hello + then + do this + fi + - We prefer "test" over "[ ... ]". - We do not write the noiseword "function" in front of shell functions. + - We prefer a space between the function name and the parentheses, + and no space inside the parentheses. The opening "{" should also + be on the same line. + + (incorrect) + my_function(){ + ... + + (correct) + my_function () { + ... + - As to use of grep, stick to a subset of BRE (namely, no \{m,n\}, - [::], [==], nor [..]) for portability. + [::], [==], or [..]) for portability. - We do not use \{m,n\}; - We do not use -E; - - We do not use ? nor + (which are \{0,1\} and \{1,\} + - We do not use ? or + (which are \{0,1\} and \{1,\} respectively in BRE) but that goes without saying as these are ERE elements not BRE (note that \? and \+ are not even part of BRE -- making them accessible from BRE is a GNU extension). + - Use Git's gettext wrappers in git-sh-i18n to make the user + interface translatable. See "Marking strings for translation" in + po/README. + + - We do not write our "test" command with "-a" and "-o" and use "&&" + or "||" to concatenate multiple "test" commands instead, because + the use of "-a/-o" is often error-prone. E.g. + + test -n "$x" -a "$a" = "$b" + + is buggy and breaks when $x is "=", but + + test -n "$x" && test "$a" = "$b" + + does not have such a problem. + + For C programs: - We use tabs to indent, and interpret tabs as taking up to @@ -77,11 +171,30 @@ For C programs: - We try to keep to at most 80 characters per line. + - We try to support a wide range of C compilers to compile Git with, + including old ones. That means that you should not use C99 + initializers, even if a lot of compilers grok it. + + - Variables have to be declared at the beginning of the block. + + - NULL pointers shall be written as NULL, not as 0. + - When declaring pointers, the star sides with the variable name, i.e. "char *string", not "char* string" or "char * string". This makes it easier to understand code like "char *string, c;". + - Use whitespace around operators and keywords, but not inside + parentheses and not around functions. So: + + while (condition) + func(bar + 1); + + and not: + + while( condition ) + func (bar+1); + - We avoid using braces unnecessarily. I.e. if (bla) { @@ -94,16 +207,116 @@ For C programs: of "else if" statements, it can make sense to add braces to single line blocks. - - We try to avoid assignments inside if(). + - We try to avoid assignments in the condition of an "if" statement. - Try to make your code understandable. You may put comments in, but comments invariably tend to stale out when the code they were describing changes. Often splitting a function into two makes the intention of the code much clearer. + - Multi-line comments include their delimiters on separate lines from + the text. E.g. + + /* + * A very long + * multi-line comment. + */ + + Note however that a comment that explains a translatable string to + translators uses a convention of starting with a magic token + "TRANSLATORS: " immediately after the opening delimiter, even when + it spans multiple lines. We do not add an asterisk at the beginning + of each line, either. E.g. + + /* TRANSLATORS: here is a comment that explains the string + to be translated, that follows immediately after it */ + _("Here is a translatable string explained by the above."); + - Double negation is often harder to understand than no negation at all. + - There are two schools of thought when it comes to comparison, + especially inside a loop. Some people prefer to have the less stable + value on the left hand side and the more stable value on the right hand + side, e.g. if you have a loop that counts variable i down to the + lower bound, + + while (i > lower_bound) { + do something; + i--; + } + + Other people prefer to have the textual order of values match the + actual order of values in their comparison, so that they can + mentally draw a number line from left to right and place these + values in order, i.e. + + while (lower_bound < i) { + do something; + i--; + } + + Both are valid, and we use both. However, the more "stable" the + stable side becomes, the more we tend to prefer the former + (comparison with a constant, "i > 0", is an extreme example). + Just do not mix styles in the same part of the code and mimic + existing styles in the neighbourhood. + + - There are two schools of thought when it comes to splitting a long + logical line into multiple lines. Some people push the second and + subsequent lines far enough to the right with tabs and align them: + + if (the_beginning_of_a_very_long_expression_that_has_to || + span_more_than_a_single_line_of || + the_source_text) { + ... + + while other people prefer to align the second and the subsequent + lines with the column immediately inside the opening parenthesis, + with tabs and spaces, following our "tabstop is always a multiple + of 8" convention: + + if (the_beginning_of_a_very_long_expression_that_has_to || + span_more_than_a_single_line_of || + the_source_text) { + ... + + Both are valid, and we use both. Again, just do not mix styles in + the same part of the code and mimic existing styles in the + neighbourhood. + + - When splitting a long logical line, some people change line before + a binary operator, so that the result looks like a parse tree when + you turn your head 90-degrees counterclockwise: + + if (the_beginning_of_a_very_long_expression_that_has_to + || span_more_than_a_single_line_of_the_source_text) { + + while other people prefer to leave the operator at the end of the + line: + + if (the_beginning_of_a_very_long_expression_that_has_to || + span_more_than_a_single_line_of_the_source_text) { + + Both are valid, but we tend to use the latter more, unless the + expression gets fairly complex, in which case the former tends to + be easier to read. Again, just do not mix styles in the same part + of the code and mimic existing styles in the neighbourhood. + + - When splitting a long logical line, with everything else being + equal, it is preferable to split after the operator at higher + level in the parse tree. That is, this is more preferable: + + if (a_very_long_variable * that_is_used_in + + a_very_long_expression) { + ... + + than + + if (a_very_long_variable * + that_is_used_in + a_very_long_expression) { + ... + - Some clever tricks, like using the !! operator with arithmetic constructs, can be extremely confusing to others. Avoid them, unless there is a compelling reason to use them. @@ -115,20 +328,217 @@ For C programs: - When you come up with an API, document it. - - The first #include in C files, except in platform specific - compat/ implementations, should be git-compat-util.h or another - header file that includes it, such as cache.h or builtin.h. + - The first #include in C files, except in platform specific compat/ + implementations, must be either "git-compat-util.h", "cache.h" or + "builtin.h". You do not have to include more than one of these. + + - A C file must directly include the header files that declare the + functions and the types it uses, except for the functions and types + that are made available to it by including one of the header files + it must include by the previous rule. - If you are planning a new command, consider writing it in shell or perl first, so that changes in semantics can be easily - changed and discussed. Many git commands started out like + changed and discussed. Many Git commands started out like that, and a few are still scripts. - - Avoid introducing a new dependency into git. This means you + - Avoid introducing a new dependency into Git. This means you usually should stay away from scripting languages not already - used in the git core command set (unless your command is clearly + used in the Git core command set (unless your command is clearly separate from it, such as an importer to convert random-scm-X - repositories to git). + repositories to Git). - When we pass <string, length> pair to functions, we should try to pass them in that order. + + - Use Git's gettext wrappers to make the user interface + translatable. See "Marking strings for translation" in po/README. + +For Perl programs: + + - Most of the C guidelines above apply. + + - We try to support Perl 5.8 and later ("use Perl 5.008"). + + - use strict and use warnings are strongly preferred. + + - Don't overuse statement modifiers unless using them makes the + result easier to follow. + + ... do something ... + do_this() unless (condition); + ... do something else ... + + is more readable than: + + ... do something ... + unless (condition) { + do_this(); + } + ... do something else ... + + *only* when the condition is so rare that do_this() will be almost + always called. + + - We try to avoid assignments inside "if ()" conditions. + + - Learn and use Git.pm if you need that functionality. + + - For Emacs, it's useful to put the following in + GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode: + + ;; note the first part is useful for C editing, too + ((nil . ((indent-tabs-mode . t) + (tab-width . 8) + (fill-column . 80))) + (cperl-mode . ((cperl-indent-level . 8) + (cperl-extra-newline-before-brace . nil) + (cperl-merge-trailing-else . t)))) + +For Python scripts: + + - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/). + + - As a minimum, we aim to be compatible with Python 2.6 and 2.7. + + - Where required libraries do not restrict us to Python 2, we try to + also be compatible with Python 3.1 and later. + + - When you must differentiate between Unicode literals and byte string + literals, it is OK to use the 'b' prefix. Even though the Python + documentation for version 2.6 does not mention this prefix, it has + been supported since version 2.6.0. + +Error Messages + + - Do not end error messages with a full stop. + + - Do not capitalize ("unable to open %s", not "Unable to open %s") + + - Say what the error is first ("cannot open %s", not "%s: cannot open") + + +Externally Visible Names + + - For configuration variable names, follow the existing convention: + + . The section name indicates the affected subsystem. + + . The subsection name, if any, indicates which of an unbounded set + of things to set the value for. + + . The variable name describes the effect of tweaking this knob. + + The section and variable names that consist of multiple words are + formed by concatenating the words without punctuations (e.g. `-`), + and are broken using bumpyCaps in documentation as a hint to the + reader. + + When choosing the variable namespace, do not use variable name for + specifying possibly unbounded set of things, most notably anything + an end user can freely come up with (e.g. branch names). Instead, + use subsection names or variable values, like the existing variable + branch.<name>.description does. + + +Writing Documentation: + + Most (if not all) of the documentation pages are written in the + AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and + processed into HTML and manpages (e.g. git.html and git.1 in the + same directory). + + The documentation liberally mixes US and UK English (en_US/UK) + norms for spelling and grammar, which is somewhat unfortunate. + In an ideal world, it would have been better if it consistently + used only one and not the other, and we would have picked en_US + (if you wish to correct the English of some of the existing + documentation, please see the documentation-related advice in the + Documentation/SubmittingPatches file). + + Every user-visible change should be reflected in the documentation. + The same general rule as for code applies -- imitate the existing + conventions. + + A few commented examples follow to provide reference when writing or + modifying command usage strings and synopsis sections in the manual + pages: + + Placeholders are spelled in lowercase and enclosed in angle brackets: + <file> + --sort=<key> + --abbrev[=<n>] + + If a placeholder has multiple words, they are separated by dashes: + <new-branch-name> + --template=<template-directory> + + Possibility of multiple occurrences is indicated by three dots: + <file>... + (One or more of <file>.) + + Optional parts are enclosed in square brackets: + [<extra>] + (Zero or one <extra>.) + + --exec-path[=<path>] + (Option with an optional argument. Note that the "=" is inside the + brackets.) + + [<patch>...] + (Zero or more of <patch>. Note that the dots are inside, not + outside the brackets.) + + Multiple alternatives are indicated with vertical bars: + [-q | --quiet] + [--utf8 | --no-utf8] + + Parentheses are used for grouping: + [(<rev> | <range>)...] + (Any number of either <rev> or <range>. Parens are needed to make + it clear that "..." pertains to both <rev> and <range>.) + + [(-p <parent>)...] + (Any number of option -p, each with one <parent> argument.) + + git remote set-head <name> (-a | -d | <branch>) + (One and only one of "-a", "-d" or "<branch>" _must_ (no square + brackets) be provided.) + + And a somewhat more contrived example: + --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]] + Here "=" is outside the brackets, because "--diff-filter=" is a + valid usage. "*" has its own pair of brackets, because it can + (optionally) be specified only when one or more of the letters is + also provided. + + A note on notation: + Use 'git' (all lowercase) when talking about commands i.e. something + the user would type into a shell and use 'Git' (uppercase first letter) + when talking about the version control system and its properties. + + A few commented examples follow to provide reference when writing or + modifying paragraphs or option/command explanations that contain options + or commands: + + Literal examples (e.g. use of command-line options, command names, and + configuration variables) are typeset in monospace, and if you can use + `backticks around word phrases`, do so. + `--pretty=oneline` + `git rev-list` + `remote.pushDefault` + + Word phrases enclosed in `backtick characters` are rendered literally + and will not be further expanded. The use of `backticks` to achieve the + previous rule means that literal examples should not use AsciiDoc + escapes. + Correct: + `--pretty=oneline` + Incorrect: + `\--pretty=oneline` + + If some place in the documentation needs to typeset a command usage + example with inline substitutions, it is fine to use +monospaced and + inline substituted text+ instead of `monospaced literal text`, and with + the former, the part that should not get substituted must be + quoted/escaped. |