[thirdparty/git.git] / Documentation / CodingGuidelines

Like other projects, we also have some guidelines to keep to the
code.  For git in general, three 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.


As for more concrete guidelines, just imitate the existing code
(this is a good guideline, no matter which project you are
contributing to).  But if you must have a list of rules,
here they are.

For shell scripts specifically (not exhaustive):

 - 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.

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

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

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

 - No shell arrays.

 - No strlen ${#parameter}.

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

 - We do not use Process Substitution <(list) or >(list).

 - We prefer "test" over "[ ... ]".

 - We do not write the noiseword "function" in front of shell
   functions.

 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
   [::], [==], nor [..]) for portability.

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

   - We do not use -E;

   - We do not use ? nor + (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).

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.

 - 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;".

 - We avoid using braces unnecessarily.  I.e.

	if (bla) {
		x = 1;
	}

   is frowned upon.  A gray area is when the statement extends
   over a few lines, and/or you have a lengthy comment atop of
   it.  Also, like in the Linux kernel, if there is a long list
   of "else if" statements, it can make sense to add braces to
   single line blocks.

 - 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.

 - Double negation is often harder to understand than no negation
   at all.

 - 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.

 - Use the API.  No, really.  We have a strbuf (variable length
   string), several arrays with the ALLOC_GROW() macro, a
   path_list for sorted string lists, a hash map (mapping struct
   objects) named "struct decorate", amongst other things.

 - 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.

 - 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
   that, and a few are still scripts.

 - 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
   separate from it, such as an importer to convert random-scm-X
   repositories to git).
Commit	Line	Data
6d0618a8 JS	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). But if you must have a list of rules,
	25	here they are.
	26
	27	For shell scripts specifically (not exhaustive):
	28
	29	- We prefer $( ... ) for command substitution; unlike ``, it
	30	properly nests. It should have been the way Bourne spelled
	31	it from day one, but unfortunately isn't.
	32
	33	- We use ${parameter-word} and its [-=?+] siblings, and their
	34	colon'ed "unset or null" form.
	35
	36	- We use ${parameter#word} and its [#%] siblings, and their
	37	doubled "longest matching" form.
	38
	39	- We use Arithmetic Expansion $(( ... )).
	40
	41	- No "Substring Expansion" ${parameter:offset:length}.
	42
	43	- No shell arrays.
	44
	45	- No strlen ${#parameter}.
	46
	47	- No regexp ${parameter/pattern/string}.
	48
	49	- We do not use Process Substitution <(list) or >(list).
	50
	51	- We prefer "test" over "[ ... ]".
	52
	53	- We do not write the noiseword "function" in front of shell
	54	functions.
	55
009c98ee JH	56	- As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
	57	[::], [==], nor [..]) for portability.
	58
	59	- We do not use \{m,n\};
	60
	61	- We do not use -E;
	62
	63	- We do not use ? nor + (which are \{0,1\} and \{1,\}
	64	respectively in BRE) but that goes without saying as these
	65	are ERE elements not BRE (note that \? and \+ are not even part
	66	of BRE -- making them accessible from BRE is a GNU extension).
	67
6d0618a8 JS	68	For C programs:
	69
	70	- We use tabs to indent, and interpret tabs as taking up to
	71	8 spaces.
	72
	73	- We try to keep to at most 80 characters per line.
	74
	75	- When declaring pointers, the star sides with the variable
	76	name, i.e. "char string", not "char string" or
	77	"char * string". This makes it easier to understand code
	78	like "char *string, c;".
	79
	80	- We avoid using braces unnecessarily. I.e.
	81
	82	if (bla) {
	83	x = 1;
	84	}
	85
	86	is frowned upon. A gray area is when the statement extends
	87	over a few lines, and/or you have a lengthy comment atop of
	88	it. Also, like in the Linux kernel, if there is a long list
	89	of "else if" statements, it can make sense to add braces to
	90	single line blocks.
	91
	92	- Try to make your code understandable. You may put comments
	93	in, but comments invariably tend to stale out when the code
	94	they were describing changes. Often splitting a function
	95	into two makes the intention of the code much clearer.
	96
	97	- Double negation is often harder to understand than no negation
	98	at all.
	99
	100	- Some clever tricks, like using the !! operator with arithmetic
	101	constructs, can be extremely confusing to others. Avoid them,
	102	unless there is a compelling reason to use them.
	103
	104	- Use the API. No, really. We have a strbuf (variable length
	105	string), several arrays with the ALLOC_GROW() macro, a
	106	path_list for sorted string lists, a hash map (mapping struct
	107	objects) named "struct decorate", amongst other things.
	108
	109	- When you come up with an API, document it.
	110
	111	- The first #include in C files, except in platform specific
	112	compat/ implementations, should be git-compat-util.h or another
	113	header file that includes it, such as cache.h or builtin.h.
	114
	115	- If you are planning a new command, consider writing it in shell
	116	or perl first, so that changes in semantics can be easily
	117	changed and discussed. Many git commands started out like
	118	that, and a few are still scripts.
	119
	120	- Avoid introducing a new dependency into git. This means you
	121	usually should stay away from scripting languages not already
	122	used in the git core command set (unless your command is clearly
	123	separate from it, such as an importer to convert random-scm-X
	124	repositories to git).