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-rw-r--r--Documentation/CodingGuidelines150
1 files changed, 145 insertions, 5 deletions
diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index f424dbd..4d90c77 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -18,6 +18,14 @@ 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
@@ -34,7 +42,17 @@ For shell scripts specifically (not exhaustive):
- We use tabs for indentation.
- - Case arms are indented at the same depth as case and esac lines.
+ - 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"'
@@ -43,6 +61,14 @@ For shell scripts specifically (not exhaustive):
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 $( ... ) for command substitution; unlike ``, it
properly nests. It should have been the way Bourne spelled
it from day one, but unfortunately isn't.
@@ -81,14 +107,33 @@ For shell scripts specifically (not exhaustive):
"then" should be on the next line for if statements, and "do"
should be on the next line for "while" and "for".
+ (incorrect)
+ if test -f hello; then
+ do this
+ fi
+
+ (correct)
+ if test -f hello
+ then
+ do this
+ fi
+
- We prefer "test" over "[ ... ]".
- We do not write the noiseword "function" in front of shell
functions.
- - We prefer a space between the function name and the parentheses. The
- opening "{" should also be on the same line.
- E.g.: my_function () {
+ - We prefer a space between the function name and the parentheses,
+ and no space inside the parentheses. The opening "{" should also
+ be on the same line.
+
+ (incorrect)
+ my_function(){
+ ...
+
+ (correct)
+ my_function () {
+ ...
- As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
[::], [==], or [..]) for portability.
@@ -106,6 +151,19 @@ For shell scripts specifically (not exhaustive):
interface translatable. See "Marking strings for translation" in
po/README.
+ - We do not write our "test" command with "-a" and "-o" and use "&&"
+ or "||" to concatenate multiple "test" commands instead, because
+ the use of "-a/-o" is often error-prone. E.g.
+
+ test -n "$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
@@ -149,7 +207,7 @@ 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
@@ -177,6 +235,88 @@ For C programs:
- 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.