path: root/Documentation/CodingGuidelines

Like other projects, we also have some guidelines to keep to the
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."
   We live in the real world.

 - However, we often say "Let's stay away from that construct,
   it's not even in POSIX".

 - In spite of the above two rules, we sometimes say "Although
   this is not in POSIX, it (is so convenient | makes the code
   much more readable | has other good characteristics) and
   practically all the platforms we care about support it, so
   let's use it".

   Again, we live in the real world, and it is sometimes a
   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://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html

 - Log messages to explain your changes are as important as the
   changes themselves.  Clearly written code and in-code comments
   explain how the code works and what is assumed from the surrounding
   context.  The log messages explain what the changes wanted to
   achieve and why the changes were necessary (more on this in the
   accompanying SubmittingPatches document).

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
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).

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.

 - 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 parsable and its exit code
   is not reliable across platforms.

 - We use POSIX compliant parameter substitutions and avoid bashisms;
   namely:

   - We use ${parameter-word} and its [-=?+] siblings, and their
     colon'ed "unset or null" form.

   - We use ${parameter#word} and its [#%] siblings, and their
     doubled "longest matching" form.

   - No "Substring Expansion" ${parameter:offset:length}.

   - No shell arrays.

   - No pattern replacement ${parameter/pattern/string}.

 - We use Arithmetic Expansion $(( ... )).

 - 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

 - If a command sequence joined with && or || or | spans multiple
   lines, put each command on a separate line and put && and || and |
   operators at the end of each line, rather than the start. This
   means you don't need to use \ to join lines, since the above
   operators imply the sequence isn't finished.

	(incorrect)
	grep blob verify_pack_result \
	| awk -f print_1.awk \
	| sort >actual &&
	...

	(correct)
	grep blob verify_pack_result |
	awk -f print_1.awk |
	sort >actual &&
	...

 - 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\},
   [::], [==], or [..]) for portability.

   - We do not use \{m,n\};

   - We do not use -E;

   - 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.

 - Even though "local" is not part of POSIX, we make heavy use of it
   in our test suite.  We do not use it in scripted Porcelains, and
   hopefully nobody starts using "local" before they are reimplemented
   in C ;-)


For C programs:

 - We use tabs to indent, and interpret tabs as taking up to
   8 spaces.

 - We try to keep to at most 80 characters per line.

 - As a Git developer we assume you have a reasonably modern compiler
   and we recommend you to enable the DEVELOPER makefile knob to
   ensure your patch is clear of all compiler warnings we care about,
   by e.g. "echo DEVELOPER=1 >>config.mak".

 - We try to support a wide range of C compilers to compile Git with,
   including old ones.  You should not use features from newer C
   standard, even if your compiler groks them.

   There are a few exceptions to this guideline:

   . since early 2012 with e1327023ea, we have been using an enum
     definition whose last element is followed by a comma.  This, like
     an array initializer that ends with a trailing comma, can be used
     to reduce the patch noise when adding a new identifier at the end.

   . since mid 2017 with cbc0f81d, we have been using designated
     initializers for struct (e.g. "struct t v = { .val = 'a' };").

   . since mid 2017 with 512f41cf, we have been using designated
     initializers for array (e.g. "int array[10] = { [5] = 2 }").

   . since early 2021 with 765dc168882, we have been using variadic
     macros, mostly for printf-like trace and debug macros.

   These used to be forbidden, but we have not heard any breakage
   report, and they are assumed to be safe.

 - Variables have to be declared at the beginning of the block, before
   the first statement (i.e. -Wdeclaration-after-statement).

 - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
   is still not allowed in this codebase.

 - 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);

 - Do not explicitly compare an integral value with constant 0 or '\0',
   or a pointer value with constant NULL.  For instance, to validate that
   counted array <ptr, cnt> is initialized but has no elements, write:

	if (!ptr || cnt)
		BUG("empty array expected");

   and not:

	if (ptr == NULL || cnt != 0);
		BUG("empty array expected");

 - We avoid using braces unnecessarily.  I.e.

	if (bla) {
		x = 1;
	}

   is frowned upon. But there are a few exceptions:

	- When the statement extends over a few lines (e.g., a while loop
	  with an embedded conditional, or a comment). E.g.:

		while (foo) {
			if (x)
				one();
			else
				two();
		}

		if (foo) {
			/*
			 * This one requires some explanation,
			 * so we're better off with braces to make
			 * it obvious that the indentation is correct.
			 */
			doit();
		}

	- When there are multiple arms to a conditional and some of them
	  require braces, enclose even a single line block in braces for
	  consistency. E.g.:

		if (foo) {
			doit();
		} else {
			one();
			two();
			three();
		}

 - 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: ", 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