xref: /xen/tools/libxl/CODING_STYLE

LIBXENLIGHT CODING STYLE
========================


AN APOLOGY AND WARNING
----------------------

Much of the code in libxl 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.


MEMORY ALLOCATION
-----------------

Memory allocation for libxl-internal purposes should normally be done
with the provided gc mechanisms; there is then no need to free.  See
"libxl memory management" in libxl.h.


CONVENTIONAL VARIABLE NAMES
---------------------------

The following local variable names should be used where applicable:

  int rc;    /* a libxl error code - and not anything else */
  int r;     /* the return value from a system call (or libxc call) */
  bool ok;   /* the success return value from a boolean function */

  uint32_t domid;
  libxl__gc *gc;
  libxl__egc *egc;
  libxl__ao *ao;

  libxl_foo_bar_state *fbs;    /* local variable */
  libxl_foo_bar_state foo_bar; /* inside another state struct */


CONVENIENCE MACROS
------------------

There are a number of convenience macros which shorten the program and
avoid opportunity for mistakes.  In some cases non-use of the macros
produces functional bugs or incorrect error handling.  Use the macros
whenever they are applicable.  For example:

 Usually, don't use:     Instead, use (see libxl_internal.h):
  libxl__log[v]           LOG, LOGE, LOGEV
  libxl__sprintf          GCSPRINTF
  libxl__*alloc et al.    GCNEW, GCNEW_ARRAY, GCREALLOC_ARRAY
  isalnum etc. directly   CTYPE
  libxl__ctx_[un]lock     CTX_LOCK, CTX_UNLOCK
  gc=...; ao=...;         EGC_GC, AO_GC, STATE_AO_GC
  explicit gc creation    GC_INIT, GC_FREE
  memset(..,0,sizeof..)   FILLZERO

Instead of malloc et al one should (as an exception to the above) use
libxl__{zalloc,calloc,realloc} etc but passing NOGC.

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.  Not reusing the out path is fine if the duplicated function
exit code is only `CTX_UNLOCK; GC_FREE;' (or similar).

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 libxl code formatting rules.



IDEMPOTENT DATA STRUCTURE CONSTRUCTION/DESTRUCTION
--------------------------------------------------

Nontrivial data structures (in structs) should come with an idempotent
_dispose function, which must free all resources associated with the
data structure (but not free the struct itself).

Such a struct should also come with an _init function which
initialises the struct so that _dispose is a no-op.


ASYNCHRONOUS/LONG-RUNNING OPERATIONS
------------------------------------

All long-running operations in libxl need to use the asynchronous
operation machinery.  Consult the programmer documentation in
libxl_internal.h for details - search for "Machinery for asynchronous
operations".

The code for asynchronous operations should be laid out in
chronological order.  That is, where there is a chain of callback
functions, each subsequent function should be, textually, the next
function in the file.  This will normally involve predeclaring the
callback functions.  Synchronous helper functions should be separated
out into a section preceding the main callback chain.

Control flow arrangements in asynchronous operations should be made as
simple as possible, because it can otherwise be very hard to see
through the tangle.


When inventing a new sub-operation in asynchronous code, consider
whether to structure it formally as a sub-operation with its own state
structure.  (See, for example, libxl__datacopier_*.)

An ao-suboperation state structure should contain, in this order:
  * fields that the caller must fill in, and which are,
    effectively, the parameters to the operation, including:
      - libxl__ao *ao
      - the callback function pointer(s), which
        should be named callback or callback_*.
  * shared information fields or ones used for returning information
    to the calling operation
  * private fields
These sections should be clearly demarcated by comments.

An asynchronous operation should normally have an idempotent stop or
cancel function.  It should normally also have an _init function for
its state struct, which arranges that the stop is a no-op.

The permitted order of calls into your ao operation's methods must be
documented in comments, if it is nontrivial.


When using an ao sub-operation, you should normally:
 * Physically include the sub-operation state struct in your
   own state struct;
 * Use CONTAINER_OF to find your own state struct at the start of
   your implementations of the sub-operation callback functions;
 * Unconditionally initialise the sub-operation's struct (with its
   _init method) in your own _init method.
 * Unconditionally cancel or destroy the sub-operation in your own
   cancel or destroy method.


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.

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 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.
 - In an 80 column terminal, some room needs to be left for > quoting
   characters, +/- diff characters, and so on, in emails.
 - 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".

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.
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, 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 libxenlight coding style.