xl: add CODING_STYLE

Copy the one in libxl, remove the irrelevant bits about libxl. Replace
libxl with xl where appropriate.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/xl/CODING_STYLE

@@ -0,0 +1,202 @@
+XL CODING STYLE
+========================
+
+
+AN APOLOGY AND WARNING
+----------------------
+
+Much of the code in xl does not yet follow this coding style
+document in every respect.  However, new code is expected to conform.
+
+Patches to improve the style of existing code are welcome.  Please
+separate these out from functional changes.
+
+If it is not feasible to conform fully to the style while patching old
+code, without doing substantial style reengineering first, we may
+accept patches which contain nonconformant elements, provided that
+they don't make the coding style problem worse overall.
+
+In this case, the new code should conform to the prevailing style in
+the area being touched.
+
+ERROR HANDLING
+--------------
+
+Unless, there are good reasons to do otherwise, the following error
+handling and cleanup paradigm should be used:
+
+  * All local variables referring to resources which might need
+    cleaning up are declared at the top of the function, and
+    initialised to a sentinel value indicating "nothing allocated".
+    For example,
+            libxl_evgen_disk_eject *evg = NULL;
+            int nullfd = -1;
+
+  * If the function is to return a libxl error value, `rc' is
+    used to contain the error code, but it is NOT initialised:
+            int rc;
+
+  * There is only one error cleanup path out of the function.  It
+    starts with a label `out:'.  That error cleanup path checks for
+    each allocated resource and frees it iff necessary.  It then
+    returns rc.  For example,
+         out:
+             if (evg) libxl__evdisable_disk_eject(gc, evg);
+             if (nullfd >= 0) close(nullfd);
+             return rc;
+
+  * Function calls which might fail (ie most function calls) are
+    handled by putting the return/status value into a variable, and
+    then checking it in a separate statement:
+            char *dompath = libxl__xs_get_dompath(gc, bl->domid);
+            if (!dompath) { rc = ERROR_FAIL; goto out; }
+
+  * If a resource is freed in the main body of the function (for
+    example, in a loop), the corresponding variable has to be reset to
+    the sentinel at the point where it's freed.
+
+Whether to use the `out' path for successful returns as well as error
+returns is a matter of taste and convenience for the specific
+function.
+
+If you reuse the `out' path for successful returns, there may be
+resources which are to be returned to the caller rather than freed.
+In that case you have to reset the local variable to `nothing here',
+to avoid the resource being freed on the out path.  That resetting
+should be done immediately after the resource value is stored at the
+applicable _r function parameter (or equivalent).  Do not test `rc' in
+the out section, to discover whether to free things.
+
+The uses of the single-line formatting in the examples above are
+permitted exceptions to the usual xl code formatting rules.
+
+FORMATTING AND NAMING
+---------------------
+
+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.
+
+Xl 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 xl 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 xl 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".
+
+Function arguments which are used to return values to the caller
+should be suffixed `_r' or `_out'.
+
+Variables, type names and function names are
+lower_case_with_underscores.
+Xl should avoid using libxl_ and libxl__ as prefix for its own function
+names.
+
+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;
+
+Xl coding style is super simple.  Avoid tricky expressions.
+
+
+5. Block structure
+
+Every indented statement is braced, but blocks that contain just one
+statement may have the braces omitted.  To avoid confusion, either all
+the blocks in an if...else chain have braces, or none of them do.
+
+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 xl coding style.