libvirt/HACKING

-*- buffer-read-only: t -*- vi: set ro:
DO NOT EDIT THIS FILE!  IT IS GENERATED AUTOMATICALLY!



                         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

  Richard Jones' guide to working with open source projects
  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 "if" or "else" block, and the counterpart block *does* use braces. In that
case, put braces around both blocks. Also, if the "else" block is much shorter
than the "if" block, consider negating the "if"-condition and swapping the
bodies, putting the short block first and making the longer, multi-line block
be the "else" block.

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

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

Keeping braces consistent and putting the short block first 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 {
      ...
      ...
  }

But if negating a complex condition is too ugly, then at least add braces:

  if (complex expr not worth negating) {
      ...
      ...
  } 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.

- To allocate a single object:

  virDomainPtr domain;

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



- To allocate an array of objects:

  virDomainPtr domains;
  size_t ndomains = 10;

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



- To allocate an array of object pointers:

  virDomainPtr *domains;
  size_t ndomains = 10;

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



- To re-allocate the array of domains to be 1 element longer (however, note that
repeatedly expanding an array by 1 scales quadratically, so this is
recommended only for smaller arrays):

  virDomainPtr domains;
  size_t ndomains = 0;

  if (VIR_EXPAND_N(domains, ndomains, 1) < 0) {
      virReportOOMError();
      return NULL;
  }
  domains[ndomains - 1] = domain;



- To ensure an array has room to hold at least one more element (this approach
scales better, but requires tracking allocation separately from usage)

  virDomainPtr domains;
  size_t ndomains = 0;
  size_t ndomains_max = 0;

  if (VIR_RESIZE_N(domains, ndomains_max, ndomains, 1) < 0) {
      virReportOOMError();
      return NULL;
  }
  domains[ndomains++] = domain;



- To trim an array of domains from its allocated size down to the actual used
size:

  virDomainPtr domains;
  size_t ndomains = x;
  size_t ndomains_max = y;

  VIR_SHRINK_N(domains, ndomains_max, ndomains_max - ndomains);



- To free an array of domains:

  virDomainPtr domains;
  size_t ndomains = x;
  size_t ndomains_max = y;
  size_t i;

  for (i = 0; i < ndomains; i++)
      VIR_FREE(domains[i]);
  VIR_FREE(domains);
  ndomains_max = ndomains = 0;






File handling
=============
Usage of the "fdopen()", "close()", "fclose()" APIs is deprecated in libvirt
code base to help avoiding double-closing of files or file descriptors, which
is particulary dangerous in a multi-threaded applications. Instead of these
APIs, use the macros from files.h

- Open a file from a file descriptor:

  if ((file = VIR_FDOPEN(fd, "r")) == NULL) {
      virReportSystemError(errno, "%s",
                           _("failed to open file from file descriptor"));
      return -1;
  }
  /* fd is now invalid; only access the file using file variable */



- Close a file descriptor:

  if (VIR_CLOSE(fd) < 0) {
      virReportSystemError(errno, "%s", _("failed to close file"));
  }



- Close a file:

  if (VIR_FCLOSE(file) < 0) {
      virReportSystemError(errno, "%s", _("failed to close file"));
  }



- Close a file or file descriptor in an error path, without losing the previous
"errno" value:

  VIR_FORCE_CLOSE(fd);
  VIR_FORCE_FCLOSE(file);






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

Typical usage is as follows:

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

     ...

     virBufferAddLit(&buf, "<domain>\n");
     virBufferVSprintf(&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 int
  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.

When printing to a string, consider using virBuffer for incremental
allocations, virAsprintf for a one-shot allocation, and snprintf for
fixed-width buffers. Do not use sprintf, even if you can prove the buffer
won't overflow, since gnulib does not provide the same portability guarantees
for sprintf as it does for snprintf.


Use of goto
===========
The use of goto is not forbidden, and goto is widely used throughout libvirt.
While the uncontrolled use of goto will quickly lead to unmaintainable code,
there is a place for it in well structured code where its use increases
readability and maintainability. In general, if goto is used for error
recovery, it's likely to be ok, otherwise, be cautious or avoid it all
together.

The typical use of goto is to jump to cleanup code in the case of a long list
of actions, any of which may fail and cause the entire operation to fail. In
this case, a function will have a single label at the end of the function.
It's almost always ok to use this style. In particular, if the cleanup code
only involves free'ing memory, then having multiple labels is overkill.
VIR_FREE() and every function named XXXFree() in libvirt is required to handle
NULL as its arg. Thus you can safely call free on all the variables even if
they were not yet allocated (yes they have to have been initialized to NULL).
This is much simpler and clearer than having multiple labels.

There are a couple of signs that a particular use of goto is not ok:

- You're using multiple labels. If you find yourself using multiple labels,
you're strongly encouraged to rework your code to eliminate all but one of
them.

- The goto jumps back up to a point above the current line of code being
executed. Please use some combination of looping constructs to re-execute code
instead; it's almost certainly going to be more understandable by others. One
well-known exception to this rule is restarting an i/o operation following
EINTR.

- The goto jumps down to an arbitrary place in the middle of a function followed
by further potentially failing calls. You should almost certainly be using a
conditional and a block instead of a goto. Perhaps some of your function's
logic would be better pulled out into a helper function.



Although libvirt does not encourage the Linux kernel wind/unwind style of
multiple labels, there's a good general discussion of the issue archived at

  KernelTrap
  http://kerneltrap.org/node/553/2131

When using goto, please use one of these standard labels if it makes sense:

      error: A path only taken upon return with an error code
    cleanup: A path taken upon return with success code + optional error
  no_memory: A path only taken upon return with an OOM error code
      retry: If needing to jump upwards (e.g., retry on EINTR)


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 where you do 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, run 'make check syntax-check', and
make sure you don't raise errors. 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

An exception to '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

- 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

- fixes for documentation and code comments can be managed in the same way, but
still make sure they get reviewed if non-trivial.