libvirt/HACKING

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                    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:

  git diff > 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 GIT.  Developers
only follow GIT 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.

If you encounter any failing tests, the VIR_TEST_DEBUG environment variable
may provide extra information to debug the failures. Larger values of
VIR_TEST_DEBUG may provide larger amounts of information:

  VIR_TEST_DEBUG=1 make check    (or)
  VIR_TEST_DEBUG=2 make check

Also, individual tests can be run from inside the 'tests/' directory, like:

  ./qemuxml2xmltest

(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 post-process 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.


Curly braces
============
Omit the curly braces around an "if", "while", "for" etc. body only when that
body occupies a single line. In every other case we require the braces. This
ensures that it is trivially easy to identify a single-*statement* loop: each
has only one *line* in its body.

Omitting braces with a single-line body is fine:

   while (expr) // one-line body -> omitting curly braces is ok
       single_line_stmt ();

However, the moment your loop/if/else body extends onto a second line, for
whatever reason (even if it's just an added comment), then you should add
braces. Otherwise, it would be too easy to insert a statement just before that
comment (without adding braces), thinking it is already a multi-statement
loop:

   while (true) // BAD! multi-line body with no braces
       /* comment... */
       single_line_stmt ();

Do this instead:

   while (true) { // Always put braces around a multi-line body.
       /* comment... */
       single_line_stmt ();
   }

There is one exception: when the second body line is not at the same
indentation level as the first body line:

   if (expr)
       die ("a diagnostic that would make this line"
            " extend past the 80-column limit"));

It is safe to omit the braces in the code above, since the further-indented
second body line makes it obvious that this is still a single-statement body.


To reiterate, don't do this:

   if (expr)            // BAD: no braces around...
       while (expr_2) { // ... a multi-line body
           ...
       }

Do this, instead:

   if (expr) {
       while (expr_2) {
           ...
       }
   }

However, there is one exception in the other direction, when even a one-line
block should have braces. That occurs when that one-line, brace-less block is
an "else" block, and the corresponding "then" block *does* use braces. In that
case, either put braces around the "else" block, or negate the "if"-condition
and swap the bodies, putting the one-line block first and making the longer,
multi-line block be the "else" block.

   if (expr) {
       ...
       ...
   }
   else
       x = y;    // BAD: braceless "else" with braced "then"

This is preferred, especially when the multi-line body is more than a few
lines long, because it is easier to read and grasp the semantics of an if-
then-else block when the simpler block occurs first, rather than after the
more involved block:

   if (!expr)
     x = y; // putting the smaller block first is more readable
   else {
       ...
       ...
   }

If you'd rather not negate the condition, then at least add braces:

   if (expr) {
       ...
       ...
   } else {
       x = y;
   }


Preprocessor
============
For variadic macros, stick with C99 syntax:

#define vshPrint(_ctl, ...)   fprintf(stdout, __VA_ARGS__)

Use parenthesis when checking if a macro is defined, and use
indentation to track nesting:

#if defined(HAVE_POSIX_FALLOCATE) && !defined(HAVE_FALLOCATE)
# define fallocate(a,ignored,b,c) posix_fallocate(a,b,c)
#endif


C types
=======
Use the right type.

Scalars
-------
If you're using "int" or "long", odds are good that there's a better type.
If a variable is counting something, be sure to declare it with an
unsigned type.
If it's memory-size-related, use size_t (use ssize_t only if required).
If it's file-size related, use uintmax_t, or maybe off_t.
If it's file-offset related (i.e., signed), use off_t.
If it's just counting small numbers use "unsigned int";
(on all but oddball embedded systems, you can assume that that
type is at least four bytes wide).
If a variable has boolean semantics, give it the "bool" type
and use the corresponding "true" and "false" macros.  It's ok
to include <stdbool.h>, since libvirt's use of gnulib ensures
that it exists and is usable.
In the unusual event that you require a specific width, use a
standard type like int32_t, uint32_t, uint64_t, etc.

While using "bool" is good for readability, it comes with minor caveats:
 - Don't use "bool" in places where the type size must be constant across
   all systems, like public interfaces and on-the-wire protocols.  Note
   that it would be possible (albeit wasteful) to use "bool" in libvirt's
   logical wire protocol, since XDR maps that to its lower-level bool_t
   type, which *is* fixed-size.
 - Don't compare a bool variable against the literal, "true",
   since a value with a logical non-false value need not be "1".
   I.e., don't write "if (seen == true) ...".  Rather, write "if (seen)...".

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

  - eg to allocate a single object:

      virDomainPtr domain;

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


  - eg to allocate an array of objects

       virDomainPtr domains;
       int ndomains = 10;

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

  - eg to allocate an array of object pointers

       virDomainPtr *domains;
       int ndomains = 10;

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

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

       ndomains = 20

       if (VIR_REALLOC_N(domains, ndomains) < 0) {
           virReportOOMError();
           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 insensitive equality:
     STRCASEEQ(a,b)
     STRCASENEQ(a,b)

  - For strict equality of a substring:

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

  - For case insensitive equality of a substring:

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

  - For strict equality of a prefix:

     STRPREFIX(a,b)



String copying
==============

Do not use the strncpy function.  According to the man page, it does
*not* guarantee a NULL-terminated buffer, which makes it extremely dangerous
to use.  Instead, use one of the functionally equivalent functions:

  - virStrncpy(char *dest, const char *src, size_t n, size_t destbytes)
      The first three arguments have the same meaning as for strncpy; namely the
      destination, source, and number of bytes to copy, respectively.  The last
      argument is the number of bytes available in the destination string; if a
      copy of the source string (including a \0) will not fit into the
      destination, no bytes are copied and the routine returns NULL.
      Otherwise, n bytes from the source are copied into the destination and a
      trailing \0 is appended.

  - virStrcpy(char *dest, const char *src, size_t destbytes)
      Use this variant if you know you want to copy the entire src string
      into dest.  Note that this is a macro, so arguments could be
      evaluated more than once.  This is equivalent to
      virStrncpy(dest, src, strlen(src), destbytes)

  - virStrcpyStatic(char *dest, const char *src)
      Use this variant if you know you want to copy the entire src string
      into dest *and* you know that your destination string is a static string
      (i.e. that sizeof(dest) returns something meaningful).  Note that
      this is a macro, so arguments could be evaluated more than once.  This is
      equivalent to virStrncpy(dest, src, strlen(src), sizeof(dest)).



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)) {
         virBufferFreeAndReset(&buf);
         virReportOOMError();
         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 guards.
  #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_FMT_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 committer guidelines
                ============================

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

The general rule for committing a patch is to make sure it has been reviewed
properly in the mailing-list first, usually if a couple of people 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 committing 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:
  - if a recently committed patch breaks compilation on a platform
    or for a given driver then it's fine to commit a minimal fix
    directly without getting the review feedback first
  - similarly, if make check or make syntax-check breaks, if there is
    an obvious fix, it's fine to commit immediately
The patch should still be sent to the list (or tell what the fix was if
trivial) and 'make check syntax-check' should pass too before committing
anything
Similar fixes for documentation and code comments can be managed
in the same way, but still make sure they get reviewed if non-trivial.