libvirt/HACKING

Libvirt contributor guidelines
==============================


General tips for contributing patches
=====================================

(1) Discuss any large changes on the mailing list first.  Post patches
early and listen to feedback.

(2) Post patches in unified diff format.  A command similar to this
should work:

  diff -urp libvirt.orig/ libvirt.modified/ > libvirt-myfeature.patch

or:

  cvs diff -up > libvirt-myfeature.patch

(3) Split large changes into a series of smaller patches, self-contained
if possible, with an explanation of each patch and an explanation of how
the sequence of patches fits together.

(4) Make sure your patches apply against libvirt CVS.  Developers
only follow CVS and don't care much about released versions.

(5) Run the automated tests on your code before submitting any changes.
In particular, configure with compile warnings set to -Werror:

  ./configure --enable-compile-warnings=error

and run the tests:

  make check
  make syntax-check
  make -C tests valgrind

The latter test checks for memory leaks.

(6) Update tests and/or documentation, particularly if you are adding
a new feature or changing the output of a program.


There is more on this subject, including lots of links to background
reading on the subject, on this page:

  http://et.redhat.com/~rjones/how-to-supply-code-to-open-source-projects/



Code indentation
================
Libvirt's C source code generally adheres to some basic code-formatting
conventions.  The existing code base is not totally consistent on this
front, but we do prefer that contributed code be formatted similarly.
In short, use spaces-not-TABs for indentation, use 4 spaces for each
indentation level, and other than that, follow the K&R style.

If you use Emacs, add the following to one of one of your start-up files
(e.g., ~/.emacs), to help ensure that you get indentation right:

  ;;; When editing C sources in libvirt, use this style.
  (defun libvirt-c-mode ()
    "C mode with adjusted defaults for use with libvirt."
    (interactive)
    (c-set-style "K&R")
    (setq indent-tabs-mode nil) ; indent using spaces, not TABs
    (setq c-indent-level 4)
    (setq c-basic-offset 4))
  (add-hook 'c-mode-hook
	    '(lambda () (if (string-match "/libvirt" (buffer-file-name))
			    (libvirt-c-mode))))

Code formatting (especially for new code)
=========================================
With new code, we can be even more strict.
Please apply the following function (using GNU indent) to any new code.
Note that this also gives you an idea of the type of spacing we prefer
around operators and keywords:

  indent-libvirt()
  {
    indent -bad -bap -bbb -bli4 -br -ce -brs -cs -i4 -l75 -lc75 \
      -sbi4 -psl -saf -sai -saw -sbi4 -ss -sc -cdw -cli4 -npcs -nbc \
      --no-tabs "$@"
  }

Note that sometimes you'll have to postprocess that output further, by
piping it through "expand -i", since some leading TABs can get through.
Usually they're in macro definitions or strings, and should be converted
anyhow.


Low level memory management
===========================

Use of the malloc/free/realloc/calloc APIs is deprecated in the libvirt
codebase, because they encourage a number of serious coding bugs and do
not enable compile time verification of checks for NULL. Instead of these
routines, use the macros from memory.h

  - eg to allocate  a single object:

      virDomainPtr domain;

      if (VIR_ALLOC(domain) < 0) {
         __virRaiseError(VIR_ERROR_NO_MEMORY)
         return NULL;
      }


  - eg to allocate an array of objects

       virDomainPtr domains;
       int ndomains = 10;

       if (VIR_ALLOC_N(domains, ndomains) < 0) {
         __virRaiseError(VIR_ERROR_NO_MEMORY)
         return NULL;
       }

  - eg to allocate an array of object pointers

       virDomainPtr *domains;
       int ndomains = 10;

       if (VIR_ALLOC_N(domains, ndomains) < 0) {
         __virRaiseError(VIR_ERROR_NO_MEMORY)
         return NULL;
       }

   - eg to re-allocate the array of domains to be longer

       ndomains = 20

       if (VIR_REALLOC_N(domains, ndomains) < 0) {
         __virRaiseError(VIR_ERROR_NO_MEMORY)
         return NULL;
       }

   - eg to free the domain

       VIR_FREE(domain);



String comparisons
==================

Do not use the strcmp, strncmp, etc functions directly. Instead use
one of the following semantically named macros

  - For strict equality:

     STREQ(a,b)
     STRNEQ(a,b)

  - For case sensitive equality:
     STRCASEEQ(a,b)
     STRCASENEQ(a,b)

  - For strict equality of a substring:

     STREQLEN(a,b,n)
     STRNEQLEN(a,b,n)

  - For case sensitive equality of a substring:

     STRCASEEQLEN(a,b,n)
     STRCASENEQLEN(a,b,n)

  - For strict equality of a prefix:

     STRPREFIX(a,b)



Variable length string buffer
=============================

If there is a need for complex string concatenations, avoid using
the usual sequence of malloc/strcpy/strcat/snprintf functions and
make use of the virBuffer API described in buf.h

eg typical usage is as follows:

  char *
  somefunction(...) {
     virBuffer buf = VIR_BUFFER_INITIALIZER;

     ...

     virBufferAddLit(&buf, "<domain>\n");
     virBufferVSprint(&buf, "  <memory>%d</memory>\n", memory);
     ...
     virBufferAddLit(&buf, "</domain>\n");

     ....

     if (virBufferError(&buf)) {
         __virRaiseError(...);
         return NULL;
     }

     return virBufferContentAndReset(&buf);
  }


Include files
=============

There are now quite a large number of include files, both libvirt
internal and external, and system includes.  To manage all this
complexity it's best to stick to the following general plan for all
*.c source files:

  /*
   * Copyright notice
   * ....
   * ....
   * ....
   *
   */

  #include <config.h>             Must come first in every file.

  #include <stdio.h>              Any system includes you need.
  #include <string.h>
  #include <limits.h>

  #if HAVE_NUMACTL                Some system includes aren't supported
  #include <numa.h>               everywhere so need these #if defences.
  #endif

  #include "internal.h"           Include this first, after system includes.

  #include "util.h"               Any libvirt internal header files.
  #include "buf.h"

  static myInternalFunc ()        The actual code.
  {
    ...

Of particular note: *DO NOT* include libvirt/libvirt.h or
libvirt/virterror.h.  It is included by "internal.h" already and there
are some special reasons why you cannot include these files
explicitly.