[Xen-devel] Libxenlight Coding Style

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

We posted a draft coding style document in September but failed to
actually commit it :-/.  I have now done so.

The most relevant thing is the line length.  Lines are supposed to be
no more than 75-80 characters long.  We've been seeing lots of new
patches coming in with long lines.

Longer lines make patches harder to review in an 80-column MUA as well
as making the code harder to read in an 80-column source editor.

Lots of libxl is unfortunately already full of long lines; we will
deal with this at some later point, or if it bothers anyone now, _pure
formatting_ patches to fix it will be accepted right away.

In the meantime patches which introduce new long lines will be
bounced, or perhaps - if I'm feeling generous :-) - fixed up.

Thanks for your attention,
Ian.

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1301925286 -3600
# Node ID 9fcfa0ca31419e7988804863694a13977906f65f
# Parent  777aaa2172c8cccffb31e59fcf43aa35e1727cf1
libxl: add CODING_STYLE

libxenlight and xl grew enough to need a CODING_STYLE, that I blatantly
copied from qemu and linux, just adding few specific modifications.
The result should be as less controversial as possible, mostly
documenting what we are already doing.

[ Message and document originally posted to xen-devel on 2010-09-01 ]

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

diff -r 777aaa2172c8 -r 9fcfa0ca3141 tools/libxl/CODING_STYLE
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/tools/libxl/CODING_STYLE  Mon Apr 04 14:54:46 2011 +0100
@@ -0,0 +1,135 @@
+Libxenlight Coding Style
+========================
+
+Blatantly copied from qemu and linux with few modifications.
+
+
+1. Whitespace
+
+Of course, the most important aspect in any coding style is whitespace.
+Crusty old coders who have trouble spotting the glasses on their noses
+can tell the difference between a tab and eight spaces from a distance
+of approximately fifteen parsecs.  Many a flamewar have been fought and
+lost on this issue.
+
+Libxenlight indents are four spaces.  Tabs are never used, except in
+Makefiles where they have been irreversibly coded into the syntax.
+Spaces of course are superior to tabs because:
+
+ - You have just one way to specify whitespace, not two.  Ambiguity breeds
+   mistakes.
+ - The confusion surrounding 'use tabs to indent, spaces to justify' is gone.
+ - Tab indents push your code to the right, making your screen seriously
+   unbalanced.
+ - Tabs will be rendered incorrectly on editors who are misconfigured not
+   to use tab stops of eight positions.
+ - Tabs are rendered badly in patches, causing off-by-one errors in almost
+   every line.
+ - It is the libxenlight coding style.
+
+Do not leave whitespace dangling off the ends of lines.
+
+
+2. Line width
+
+Lines are limited to 75-80 characters.
+
+Rationale:
+ - Some people like to tile their 24" screens with a 6x4 matrix of 80x24
+   xterms and use vi in all of them.  The best way to punish them is to
+   let them keep doing it.
+ - Code and especially patches is much more readable if limited to a sane
+   line length.  Eighty is traditional.
+ - It is the libxenlight coding style.
+
+
+3. Naming
+
+C is a Spartan language, and so should your naming be.  Unlike Modula-2
+and Pascal programmers, C programmers do not use cute names like
+ThisVariableIsATemporaryCounter.  A C programmer would call that
+variable "tmp", which is much easier to write, and not the least more
+difficult to understand.
+
+HOWEVER, while mixed-case names are frowned upon, descriptive names for
+global variables are a must.  To call a global function "foo" is a
+shooting offense.
+
+GLOBAL variables (to be used only if you _really_ need them) need to
+have descriptive names, as do global functions.  If you have a function
+that counts the number of active users, you should call that
+"count_active_users()" or similar, you should _not_ call it "cntusr()".
+
+Encoding the type of a function into the name (so-called Hungarian
+notation) is brain damaged - the compiler knows the types anyway and can
+check those, and it only confuses the programmer.
+
+LOCAL variable names should be short, and to the point.  If you have
+some random integer loop counter, it should probably be called "i".
+Calling it "loop_counter" is non-productive, if there is no chance of it
+being mis-understood.  Similarly, "tmp" can be just about any type of
+variable that is used to hold a temporary value.
+
+Local variables used to store return values should have descriptive name
+like "rc" or "ret". Following the same reasoning the label used as exit
+path should be called "out" or "error".
+
+Variables, type names and function names are
+lower_case_with_underscores.
+Type names and function names use the prefix libxl__ when internal to
+libxenlight and libxl_ when exported in libxl.h.
+Xl should avoid using libxl_ and libxl__ as prefix for its own function
+names.
+
+When wrapping standard library functions, use the prefix libxl_ to alert
+readers that they are seeing a wrapped version; otherwise avoid this prefix.
+
+Typedefs are used to eliminate the redundant 'struct' keyword.
+It is the libxenlight coding style.
+
+
+4. Statements
+
+Don't put multiple statements on a single line.
+Don't put multiple assignments on a single line either.
+Error code paths with an if statement and a goto or a return on the same
+line are allowed. Examples:
+
+    if (rc) goto out;
+    if (rc < 0) return;
+
+Libxenlight coding style is super simple.  Avoid tricky expressions.
+
+
+5. Block structure
+
+Every indented statement is braced apart from blocks that contain just
+one statement.
+The opening brace is on the line that contains the control flow
+statement that introduces the new block; the closing brace is on the
+same line as the else keyword, or on a line by itself if there is no
+else keyword.  Examples:
+
+    if (a == 5) {
+        printf("a was 5.\n");
+    } else if (a == 6) {
+        printf("a was 6.\n");
+    } else {
+        printf("a was something else entirely.\n");
+    }
+
+    if (a == 5)
+        printf("a was 5.\n");
+
+An exception is the opening brace for a function; for reasons of tradition
+and clarity it comes on a line by itself:
+
+    void a_function(void)
+    {
+        do_something();
+    }
+
+Rationale: a consistent (except for functions...) bracing style reduces
+ambiguity and avoids needless churn when lines are added or removed.
+Furthermore, it is the libxenlight coding style.
+

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