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 Richard Jones' guide to working with 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.

C types

Use the right type.

Scalars

Of course, take all of the above with a grain of salt. If you're about to use some system interface that requires a type like size_t, pid_t or off_t, use matching types for any corresponding variables.

Also, if you try to use e.g., "unsigned int" as a type, and that conflicts with the signedness of a related variable, sometimes it's best just to use the *wrong* type, if "pulling the thread" and fixing all related variables would be too invasive.

Finally, while using descriptive types is important, be careful not to go overboard. If whatever you're doing causes warnings, or requires casts, then reconsider or ask for help.

Pointers

Ensure that all of your pointers are "const-correct". Unless a pointer is used to modify the pointed-to storage, give it the "const" attribute. That way, the reader knows up-front that this is a read-only pointer. Perhaps more importantly, if we're diligent about this, when you see a non-const pointer, you're guaranteed that it is used to modify the storage it points to, or it is aliased to another pointer that is.

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

String comparisons

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

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.

Printf-style functions

Whenever you add a new printf-style function, i.e., one with a format string argument and following "..." in its prototype, be sure to use gcc's printf attribute directive in the prototype. For example, here's the one for virAsprintf, in util.h: 

    int virAsprintf(char **strp, const char *fmt, ...)
	ATTRIBUTE_FORMAT(printf, 2, 3);

This makes it so gcc's -Wformat and -Wformat-security options can do their jobs and cross-check format strings with the number and types of arguments.

Libvirt commiters guidelines

The AUTHORS files indicates the list of people with commit acces right who can actually merge the patches.

The general rule for commiting patches is to make sure it has been reviewed properly in the mailing-list first, usually if a couple of persons gave an ACK or +1 to a patch and nobody raised an objection on the list it should be good to go. If the patch touches a part of the code where you're not the main maintainer or not have a very clear idea of how things work, it's better to wait for a more authoritative feedback though. Before commiting please also rebuild locally and run 'make check syntax-check' and make sure they don't raise error. Try to look for warnings too for example configure with --enable-compile-warnings=error which adds -Werror to compile flags, so no warnings get missed

Exceptions to that 'review and approval on the list first' is fixing failures to build: