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libvirt/docs/hacking.html.in
Ján Tomko cae955eba2 docs: hacking: document removal of VIR_STR(N)DUP 
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
2019-12-12 14:24:35 +01:00

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<body>
<h1>Contributor guidelines</h1>
<ul id="toc"></ul>
<h2><a id="patches">General tips for contributing patches</a></h2>
<ol>
<li>
<p>Discuss any large changes on the mailing list first. Post patches
early and listen to feedback.</p>
</li>
<li>
<p>Official upstream repository is kept in git
(<code>https://libvirt.org/git/libvirt.git</code>) and is browsable
along with other libvirt-related repositories
(e.g. libvirt-python) <a href="https://libvirt.org/git/">online</a>.</p>
</li>
<li>
<p>Patches to translations are maintained via
the <a href="https://fedora.zanata.org/">zanata project</a>.
If you want to fix a translation in a .po file, join the
appropriate language team. The libvirt release process
automatically pulls the latest version of each translation
file from zanata.</p>
</li>
<li><p>The simplest way to send patches is to use the
<a href="https://github.com/stefanha/git-publish"><code>git-publish</code></a>
tool. All libvirt-related repositories contain a config file that
tells git-publish to use the correct mailing list and subject prefix.</p>
<p>Alternatively, you may send patches using <code>git send-email</code>.</p>
<p>Also, for code motion patches, you may find that <code>git
diff --patience</code> provides an easier-to-read patch.
However, the usual workflow of libvirt developer is:</p>
<pre>
git checkout master
git pull
git checkout -t origin -b workbranch
Hack, committing any changes along the way
</pre>
<p>More hints on compiling can be
found <a href="compiling.html">here</a>. When you want to
post your patches:</p>
<pre>
git pull --rebase
(fix any conflicts)
git send-email --cover-letter --no-chain-reply-to --annotate \
--confirm=always --to=libvir-list@redhat.com master
</pre>
<p>For a single patch you can omit
<code>--cover-letter</code>, but a series of two or more
patches needs a cover letter.</p>
<p>Note that the <code>git send-email</code> subcommand may not
be in the main git package and using it may require installation
of a separate package, for example the "git-email" package in
Fedora and Debian. If this is your first time using
<code>git send-email</code>, you might need to configure it to
point it to your SMTP server with something like:</p>
<pre>
git config --global sendemail.smtpServer stmp.youremailprovider.net
</pre>
<p>If you get tired of typing
<code>--to=libvir-list@redhat.com</code> all the time, you can
configure that to be automatically handled as well:</p>
<pre>
git config sendemail.to libvir-list@redhat.com
</pre>
<p>As a rule, patches should be sent to the mailing list only: all
developers are subscribed to libvir-list and read it regularly, so
<strong>please don't CC individual developers</strong> unless
they've explicitly asked you to.</p>
<p>Avoid using mail clients for sending patches, as most of them
will mangle the messages in some way, making them unusable for our
purposes. Gmail and other Web-based mail clients are particularly
bad at this.</p>
<p>If everything went well, your patch should show up on the
<a href="https://www.redhat.com/archives/libvir-list/">libvir-list
archives</a> in a matter of minutes; if you still can't find it on
there after an hour or so, you should double-check your setup.
<strong>Note that, if you are not already a subscriber, your very
first post to the mailing list will be
subject to moderation</strong>, and it's not uncommon for that to
take around a day.</p>
<p>Please follow this as close as you can, especially the rebase and
<code>git send-email</code> part, as it makes life easier for other
developers to review your patch set.</p>
<p>One should avoid sending patches as attachments,
but rather send them in email body along with commit message. If a
developer is sending another version of the patch (e.g. to address
review comments), they are advised to note differences to previous
versions after the <code>---</code> line in the patch so that it helps
reviewers but doesn't become part of git history. Moreover, such patch
needs to be prefixed correctly with
<code>--subject-prefix=PATCHv2</code> appended to <code>git
send-email</code> (substitute <code>v2</code> with the correct
version if needed though).</p>
</li>
<li><p>In your commit message, make the summary line reasonably
short (60 characters is typical), followed by a blank line,
followed by any longer description of why your patch makes
sense. If the patch fixes a regression, and you know what
commit introduced the problem, mentioning that is useful.
If the patch resolves a bugzilla report, mentioning the URL
of the bug number is useful; but also summarize the issue
rather than making all readers follow the link. You can use
'git shortlog -30' to get an idea of typical summary lines.
</p>
</li>
<li><p>Contributors to libvirt projects <strong>must</strong>
assert that they are in compliance with the
<a href="https://developercertificate.org/">Developer
Certificate of Origin 1.1</a>. This is achieved by adding
a "Signed-off-by" line containing the contributor's name
and e-mail to every commit message. The presence
of this line attests that the contributor has read the
above lined DCO and agrees with its statements.
</p></li>
<li><p>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. Moreover, please keep in mind that it's required to
be able to compile cleanly (<b>including</b> <code>make
check</code> and <code>make syntax-check</code>) after each
patch. A feature does not have to work until the end of a
series, but intermediate patches must compile and not cause
test-suite failures (this is to preserve the usefulness
of <code>git bisect</code>, among other things).</p>
</li>
<li>
<p>Make sure your patches apply against libvirt GIT. Developers
only follow GIT and don't care much about released versions.</p>
</li>
<li><p>Run the automated tests on your code before submitting any changes.
That is:
</p>
<pre>
make check
make syntax-check
make -C tests valgrind
</pre>
<p><a href="http://valgrind.org/">Valgrind</a> is a test that checks
for memory management issues, such as leaks or use of uninitialized
variables.
</p>
<p>
Some tests are skipped by default in a development environment,
based on the time they take in comparison to the likelihood
that those tests will turn up problems during incremental builds.
These tests default to being run when building from a
tarball or with the configure option --enable-expensive-tests;
you can also force a one-time toggle of these tests by
setting VIR_TEST_EXPENSIVE to 0 or 1 at make time, as in:
</p>
<pre>
make check VIR_TEST_EXPENSIVE=1
</pre>
<p>
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:
</p>
<pre>
VIR_TEST_DEBUG=1 make check (or)
VIR_TEST_DEBUG=2 make check
</pre>
<p>
When debugging failures during development, it is possible
to focus in on just the failing subtests by using
VIR_TEST_RANGE. I.e. to run all tests from 3 to 20 with the
exception of tests 6 and 16, use:
</p>
<pre>
VIR_TEST_DEBUG=1 VIR_TEST_RANGE=3-5,7-20,^16 ./run tests/qemuxml2argvtest
</pre>
<p>
Also, individual tests can be run from inside the <code>tests/</code>
directory, like:
</p>
<pre>
./qemuxml2xmltest
</pre>
<p>
If you are adding new test cases, or making changes that alter
existing test output, you can use the environment variable
VIR_TEST_REGENERATE_OUTPUT to quickly update the saved test data.
Of course you still need to review the changes VERY CAREFULLY to
ensure they are correct.
</p>
<pre>
VIR_TEST_REGENERATE_OUTPUT=1 ./qemuxml2argvtest
</pre>
<p>There is also a <code>./run</code> script at the top level,
to make it easier to run programs that have not yet been
installed, as well as to wrap invocations of various tests
under gdb or Valgrind.
</p>
<p>When running our test suite it may happen that the test result is
nondeterministic because of the test suite relying on a particular file
in the system being accessible or having some specific value. To catch
this kind of errors, the test suite has a module for that prints any
path touched that fulfils constraints described above
into a file. To enable it just set
<code>VIR_TEST_FILE_ACCESS</code> environment variable.
Then <code>VIR_TEST_FILE_ACCESS_OUTPUT</code> environment
variable can alter location where the file is stored.</p>
<pre>
VIR_TEST_FILE_ACCESS=1 VIR_TEST_FILE_ACCESS_OUTPUT="/tmp/file_access.txt" ./qemuxml2argvtest
</pre>
</li>
<li><p>The Valgrind test should produce similar output to
<code>make check</code>. If the output has traces within libvirt
API's, then investigation is required in order to determine the
cause of the issue. Output such as the following indicates some
sort of leak:
</p>
<pre>
==5414== 4 bytes in 1 blocks are definitely lost in loss record 3 of 89
==5414== at 0x4A0881C: malloc (vg_replace_malloc.c:270)
==5414== by 0x34DE0AAB85: xmlStrndup (in /usr/lib64/libxml2.so.2.7.8)
==5414== by 0x4CC97A6: virDomainVideoDefParseXML (domain_conf.c:7410)
==5414== by 0x4CD581D: virDomainDefParseXML (domain_conf.c:10188)
==5414== by 0x4CD8C73: virDomainDefParseNode (domain_conf.c:10640)
==5414== by 0x4CD8DDB: virDomainDefParse (domain_conf.c:10590)
==5414== by 0x41CB1D: testCompareXMLToArgvHelper (qemuxml2argvtest.c:100)
==5414== by 0x41E20F: virtTestRun (testutils.c:161)
==5414== by 0x41C7CB: mymain (qemuxml2argvtest.c:866)
==5414== by 0x41E84A: virtTestMain (testutils.c:723)
==5414== by 0x34D9021734: (below main) (in /usr/lib64/libc-2.15.so)
</pre>
<p>In this example, the <code>virDomainDefParseXML()</code> had
an error path where the <code>virDomainVideoDefPtr video</code>
pointer was not properly disposed. By simply adding a
<code>virDomainVideoDefFree(video);</code> in the error path,
the issue was resolved.
</p>
<p>Another common mistake is calling a printing function, such as
<code>VIR_DEBUG()</code> without initializing a variable to be
printed. The following example involved a call which could return
an error, but not set variables passed by reference to the call.
The solution was to initialize the variables prior to the call.
</p>
<pre>
==4749== Use of uninitialised value of size 8
==4749== at 0x34D904650B: _itoa_word (in /usr/lib64/libc-2.15.so)
==4749== by 0x34D9049118: vfprintf (in /usr/lib64/libc-2.15.so)
==4749== by 0x34D9108F60: __vasprintf_chk (in /usr/lib64/libc-2.15.so)
==4749== by 0x4CAEEF7: virVasprintf (stdio2.h:199)
==4749== by 0x4C8A55E: virLogVMessage (virlog.c:814)
==4749== by 0x4C8AA96: virLogMessage (virlog.c:751)
==4749== by 0x4DA0056: virNetTLSContextCheckCertKeyUsage (virnettlscontext.c:225)
==4749== by 0x4DA06DB: virNetTLSContextCheckCert (virnettlscontext.c:439)
==4749== by 0x4DA1620: virNetTLSContextNew (virnettlscontext.c:562)
==4749== by 0x4DA26FC: virNetTLSContextNewServer (virnettlscontext.c:927)
==4749== by 0x409C39: testTLSContextInit (virnettlscontexttest.c:467)
==4749== by 0x40AB8F: virtTestRun (testutils.c:161)
</pre>
<p>Valgrind will also find some false positives or code paths
which cannot be resolved by making changes to the libvirt code.
For these paths, it is possible to add a filter to avoid the
errors. For example:
</p>
<pre>
==4643== 7 bytes in 1 blocks are possibly lost in loss record 4 of 20
==4643== at 0x4A0881C: malloc (vg_replace_malloc.c:270)
==4643== by 0x34D90853F1: strdup (in /usr/lib64/libc-2.15.so)
==4643== by 0x34EEC2C08A: ??? (in /usr/lib64/libnl.so.1.1)
==4643== by 0x34EEC15B81: ??? (in /usr/lib64/libnl.so.1.1)
==4643== by 0x34D8C0EE15: call_init.part.0 (in /usr/lib64/ld-2.15.so)
==4643== by 0x34D8C0EECF: _dl_init (in /usr/lib64/ld-2.15.so)
==4643== by 0x34D8C01569: ??? (in /usr/lib64/ld-2.15.so)
</pre>
<p>In this instance, it is acceptable to modify the
<code>tests/.valgrind.supp</code> file in order to add a
suppression filter. The filter should be unique enough to
not suppress real leaks, but it should be generic enough to
cover multiple code paths. The format of the entry can be
found in the documentation found at the
<a href="http://valgrind.org/">Valgrind home page</a>.
The following trace was added to <code>tests/.valgrind.supp</code>
in order to suppress the warning:
</p>
<pre>
{
dlInitMemoryLeak1
Memcheck:Leak
fun:?alloc
...
fun:call_init.part.0
fun:_dl_init
...
obj:*/lib*/ld-2.*so*
}
</pre>
</li>
<li>
<p>Update tests and/or documentation, particularly if you are adding
a new feature or changing the output of a program.</p>
</li>
<li>
<p>Don't forget to update the <a href="news.html">release notes</a>
by changing <code>docs/news.xml</code> if your changes are
significant. All user-visible changes, such as adding new XML elements
or fixing all but the most obscure bugs, must be (briefly) described
in a release notes entry; changes that are only relevant to other
libvirt developers, such as code refactoring, don't belong in the
release notes. Note that <code>docs/news.xml</code> should be updated
in its own commit not to get in the way of backports.</p>
</li>
</ol>
<p>
There is more on this subject, including lots of links to background
reading on the subject, on
<a href="http://people.redhat.com/rjones/how-to-supply-code-to-open-source-projects/">
Richard Jones' guide to working with open source projects</a>.
</p>
<h2><a id="lang">Language Usage</a></h2>
<p>
The libvirt repository makes use of a large number of programming
languages. It is anticipated that in the future libvirt will adopt
use of other new languages. To reduce the overall burden on developers,
there is thus a general desire to phase out usage of some of the
existing languages.
</p>
<p>
The preferred languages at this time are:
</p>
<ul>
<li>C - for the main libvirt codebase. Dialect supported by
GCC/CLang only.</li>
<li>Python - for supporting build scripts / tools. Code must
run with both version 2.7 and 3.x at this time.</li>
</ul>
<p>
Languages that should not be used for any new contributions:
</p>
<ul>
<li>Perl - build scripts must be written in Python instead.</li>
<li>Shell - build scripts must be written in Python instead.</li>
</ul>
<h2><a id="tooling">Tooling</a></h2>
<p>
libvirt includes support for some useful development tools right in its
source repository, meaning users will be able to take advantage of them
without little or no configuration. Examples include:
</p>
<ul>
<li>
<a href="https://github.com/jeaye/color_coded">color_coded</a>,
a vim plugin for libclang-powered semantic syntax highlighting;
</li>
<li>
<a href="http://valloric.github.io/YouCompleteMe/">YouCompleteMe</a>,
a vim plugin for libclang-powered semantic code completion.
</li>
</ul>
<h2><a id="naming">Naming conventions</a></h2>
<p>
When reading libvirt code, a number of different naming conventions will
be evident due to various changes in thinking over the course of the
project's lifetime. The conventions documented below should be followed
when creating any entirely new files in libvirt. When working on existing
files, while it is desirable to apply these conventions, keeping a
consistent style with existing code in that particular file is generally
more important. The overall guiding principal is that every file, enum,
struct, function, macro and typedef name must have a 'vir' or 'VIR' prefix.
All local scope variable names are exempt, and global variables are exempt,
unless exported in a header file.
</p>
<dl>
<dt>File names</dt>
<dd>
<p>
File naming varies depending on the subdirectory. The preferred
style is to have a 'vir' prefix, followed by a name which matches
the name of the functions / objects inside the file. For example,
a file containing an object 'virHashtable' is stored in files
'virhashtable.c' and 'virhashtable.h'. Sometimes, methods which
would otherwise be declared 'static' need to be exported for use
by a test suite. For this purpose a second header file should be
added with a suffix of 'priv', e.g. 'virhashtablepriv.h'. Use of
underscores in file names is discouraged when using the 'vir'
prefix style. The 'vir' prefix naming applies to src/util,
src/rpc and tests/ directories. Most other directories do not
follow this convention.
</p>
</dd>
<dt>Enum type &amp; field names</dt>
<dd>
<p>
All enums should have a 'vir' prefix in their typedef name,
and each following word should have its first letter in
uppercase. The enum name should match the typedef name with
a leading underscore. The enum member names should be in all
uppercase, and use an underscore to separate each word. The
enum member name prefix should match the enum typedef name.
</p>
<pre>
typedef enum _virSocketType virSocketType;
enum _virSocketType {
VIR_SOCKET_TYPE_IPV4,
VIR_SOCKET_TYPE_IPV6,
};</pre>
</dd>
<dt>Struct type names</dt>
<dd>
<p>
All structs should have a 'vir' prefix in their typedef name,
and each following word should have its first letter in
uppercase. The struct name should be the same as the typedef
name with a leading underscore. A second typedef should be
given for a pointer to the struct with a 'Ptr' suffix.
</p>
<pre>
typedef struct _virHashTable virHashTable;
typedef virHashTable *virHashTablePtr;
struct _virHashTable {
...
};</pre>
</dd>
<dt>Function names</dt>
<dd>
<p>
All functions should have a 'vir' prefix in their name,
followed by one or more words with first letter of each
word capitalized. Underscores should not be used in function
names. If the function is operating on an object, then the
function name prefix should match the object typedef name,
otherwise it should match the filename. Following this
comes the verb / action name, and finally an optional
subject name. For example, given an object 'virHashTable',
all functions should have a name 'virHashTable$VERB' or
'virHashTable$VERB$SUBJECT", e.g. 'virHashTableLookup'
or 'virHashTableGetValue'.
</p>
</dd>
<dt>Macro names</dt>
<dd>
<p>
All macros should have a "VIR" prefix in their name, followed
by one or more uppercase words separated by underscores. The
macro argument names should be in lowercase. Aside from having
a "VIR" prefix there are no common practices for the rest of
the macro name.
</p>
</dd>
</dl>
<h2><a id="indent">Code indentation</a></h2>
<p>
Libvirt's C source code generally adheres to some basic code-formatting
conventions. The existing code base is not totally consistent on this
front, but we do prefer that contributed code be formatted similarly.
In short, use spaces-not-TABs for indentation, use 4 spaces for each
indentation level, and other than that, follow the K&amp;R style.
</p>
<p>
If you use Emacs, the project includes a file .dir-locals.el
that sets up the preferred indentation. If you use vim,
append the following to your ~/.vimrc file:
</p>
<pre>
set nocompatible
filetype on
set autoindent
set smartindent
set cindent
set tabstop=8
set shiftwidth=4
set expandtab
set cinoptions=(0,:0,l1,t0,L3
filetype plugin indent on
au FileType make setlocal noexpandtab
au BufRead,BufNewFile *.am setlocal noexpandtab
match ErrorMsg /\s\+$\| \+\ze\t/
</pre>
<p>
Or if you don't want to mess your ~/.vimrc up, you can save the above
into a file called .lvimrc (not .vimrc) located at the root of libvirt
source, then install a vim script from
http://www.vim.org/scripts/script.php?script_id=1408,
which will load the .lvimrc only when you edit libvirt code.
</p>
<h2><a id="formatting">Code formatting (especially for new code)</a></h2>
<p>
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:
</p>
<pre>
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 "$@"
}
</pre>
<p>
Note that sometimes you'll have to post-process that output further, by
piping it through <code>expand -i</code>, since some leading TABs can get through.
Usually they're in macro definitions or strings, and should be converted
anyhow.
</p>
<p>
Libvirt requires a C99 compiler for various reasons. However,
most of the code base prefers to stick to C89 syntax unless
there is a compelling reason otherwise. For example, it is
preferable to use <code>/* */</code> comments rather
than <code>//</code>. Also, when declaring local variables, the
prevailing style has been to declare them at the beginning of a
scope, rather than immediately before use.
</p>
<h2><a id="bracket_spacing">Bracket spacing</a></h2>
<p>
The keywords <code>if</code>, <code>for</code>, <code>while</code>,
and <code>switch</code> must have a single space following them
before the opening bracket. E.g.
</p>
<pre>
if(foo) // Bad
if (foo) // Good
</pre>
<p>
Function implementations must <strong>not</strong> have any whitespace
between the function name and the opening bracket. E.g.
</p>
<pre>
int foo (int wizz) // Bad
int foo(int wizz) // Good
</pre>
<p>
Function calls must <strong>not</strong> have any whitespace
between the function name and the opening bracket. E.g.
</p>
<pre>
bar = foo (wizz); // Bad
bar = foo(wizz); // Good
</pre>
<p>
Function typedefs must <strong>not</strong> have any whitespace
between the closing bracket of the function name and opening
bracket of the arg list. E.g.
</p>
<pre>
typedef int (*foo) (int wizz); // Bad
typedef int (*foo)(int wizz); // Good
</pre>
<p>
There must not be any whitespace immediately following any
opening bracket, or immediately prior to any closing bracket. E.g.
</p>
<pre>
int foo( int wizz ); // Bad
int foo(int wizz); // Good
</pre>
<h2><a id="comma">Commas</a></h2>
<p>
Commas should always be followed by a space or end of line, and
never have leading space; this is enforced during 'make
syntax-check'.
</p>
<pre>
call(a,b ,c);// Bad
call(a, b, c); // Good
</pre>
<p>
When declaring an enum or using a struct initializer that
occupies more than one line, use a trailing comma. That way,
future edits to extend the list only have to add a line, rather
than modify an existing line to add the intermediate comma. Any
sentinel enumerator value with a name ending in _LAST is exempt,
since you would extend such an enum before the _LAST element.
Another reason to favor trailing commas is that it requires less
effort to produce via code generators. Note that the syntax
checker is unable to enforce a style of trailing commas, so
there are counterexamples in existing code which do not use it;
also, while C99 allows trailing commas, remember that JSON and
XDR do not.
</p>
<pre>
enum {
VALUE_ONE,
VALUE_TWO // Bad
};
enum {
VALUE_THREE,
VALUE_FOUR, // Good
};
</pre>
<h2><a id="semicolon">Semicolons</a></h2>
<p>
Semicolons should never have a space beforehand. Inside the
condition of a <code>for</code> loop, there should always be a
space or line break after each semicolon, except for the special
case of an infinite loop (although more infinite loops
use <code>while</code>). While not enforced, loop counters
generally use post-increment.
</p>
<pre>
for (i = 0 ;i &lt; limit ; ++i) { // Bad
for (i = 0; i &lt; limit; i++) { // Good
for (;;) { // ok
while (1) { // Better
</pre>
<p>
Empty loop bodies are better represented with curly braces and a
comment, although use of a semicolon is not currently rejected.
</p>
<pre>
while ((rc = waitpid(pid, &amp;st, 0) == -1) &amp;&amp;
errno == EINTR); // ok
while ((rc = waitpid(pid, &amp;st, 0) == -1) &amp;&amp;
errno == EINTR) { // Better
/* nothing */
}
</pre>
<h2><a id="curly_braces">Curly braces</a></h2>
<p>
Omit the curly braces around an <code>if</code>, <code>while</code>,
<code>for</code> etc. body only when both that body and the condition
itself occupy a single line. In every other case we require
the braces. This ensures that it is trivially easy to identify a
single-<i>statement</i> loop: each has only one <i>line</i> in its body.
</p>
<pre>
while (expr) // single line body; {} is forbidden
single_line_stmt();
</pre>
<pre>
while (expr(arg1,
arg2)) // indentation makes it obvious it is single line,
single_line_stmt(); // {} is optional (not enforced either way)
</pre>
<pre>
while (expr1 &amp;&amp;
expr2) { // multi-line, at same indentation, {} required
single_line_stmt();
}
</pre>
<p>
However, the moment your loop/if/else body extends on to 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:
</p>
<pre>
while (true) // BAD! multi-line body with no braces
/* comment... */
single_line_stmt();
</pre>
<p>
Do this instead:
</p>
<pre>
while (true) { // Always put braces around a multi-line body.
/* comment... */
single_line_stmt();
}
</pre>
<p>
There is one exception: when the second body line is not at the same
indentation level as the first body line:
</p>
<pre>
if (expr)
die("a diagnostic that would make this line"
" extend past the 80-column limit"));
</pre>
<p>
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.
</p>
<p>
To reiterate, don't do this:
</p>
<pre>
if (expr) // BAD: no braces around...
while (expr_2) { // ... a multi-line body
...
}
</pre>
<p>
Do this, instead:
</p>
<pre>
if (expr) {
while (expr_2) {
...
}
}
</pre>
<p>
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 <code>if</code> or <code>else</code>
block, and the counterpart block <b>does</b> use braces. In
that case, put braces around both blocks. Also, if
the <code>else</code> block is much shorter than
the <code>if</code> block, consider negating the
<code>if</code>-condition and swapping the bodies, putting the
short block first and making the longer, multi-line block be the
<code>else</code> block.
</p>
<pre>
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 {
...
...
}
</pre>
<p>
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:
</p>
<pre>
if (!expr) {
x = y; // putting the smaller block first is more readable
} else {
...
...
}
</pre>
<p>
But if negating a complex condition is too ugly, then at least
add braces:
</p>
<pre>
if (complex expr not worth negating) {
...
...
} else {
x = y;
}
</pre>
<p>Use hanging braces for compound statements: the opening brace
of a compound statement should be on the same line as the
condition being tested. Only top-level function bodies, nested
scopes, and compound structure declarations should ever have {
on a line by itself.
</p>
<pre>
void
foo(int a, int b)
{ // correct - function body
int 2d[][] = {
{ // correct - complex initialization
1, 2,
},
};
if (a)
{ // BAD: compound brace on its own line
do_stuff();
}
{ // correct - nested scope
int tmp;
if (a &lt; b) { // correct - hanging brace
tmp = b;
b = a;
a = tmp;
}
}
}
</pre>
<h2><a id="conditions">Conditional expressions</a></h2>
<p>For readability reasons new code should avoid shortening comparisons
to 0 for numeric types. Boolean and pointer comparisions may be
shortened. All long forms are okay:
</p>
<pre>
virFooPtr foos = NULL;
size nfoos = 0;
bool hasFoos = false;
GOOD:
if (!foos)
if (!hasFoos)
if (nfoos == 0)
if (foos == NULL)
if (hasFoos == true)
BAD:
if (!nfoos)
if (nfoos)
</pre>
<p>New code should avoid the ternary operator as much as possible.
Specifically it must never span more than one line or nest:
</p>
<pre>
BAD:
char *foo = baz ?
virDoSomethingReallyComplex(driver, vm, something, baz->foo) :
NULL;
char *foo = bar ? bar->baz ? bar->baz->foo : "nobaz" : "nobar";
</pre>
<h2><a id="preprocessor">Preprocessor</a></h2>
<p>Macros defined with an ALL_CAPS name should generally be
assumed to be unsafe with regards to arguments with side-effects
(that is, MAX(a++, b--) might increment a or decrement b too
many or too few times). Exceptions to this rule are explicitly
documented for macros in viralloc.h and virstring.h.
</p>
<p>
For variadic macros, stick with C99 syntax:
</p>
<pre>
#define vshPrint(_ctl, ...) fprintf(stdout, __VA_ARGS__)
</pre>
<p>Use parenthesis when checking if a macro is defined, and use
indentation to track nesting:
</p>
<pre>
#if defined(HAVE_POSIX_FALLOCATE) &amp;&amp; !defined(HAVE_FALLOCATE)
# define fallocate(a, ignored, b, c) posix_fallocate(a, b, c)
#endif
</pre>
<h2><a id="types">C types</a></h2>
<p>
Use the right type.
</p>
<h3>Scalars</h3>
<ul>
<li>If you're using <code>int</code> or <code>long</code>, odds are
good that there's a better type.</li>
<li>If a variable is counting something, be sure to declare it with an
unsigned type.</li>
<li>If it's memory-size-related, use <code>size_t</code> (use
<code>ssize_t</code> only if required).</li>
<li>If it's file-size related, use uintmax_t, or maybe <code>off_t</code>.</li>
<li>If it's file-offset related (i.e., signed), use <code>off_t</code>.</li>
<li>If it's just counting small numbers use <code>unsigned int</code>;
(on all but oddball embedded systems, you can assume that that
type is at least four bytes wide).</li>
<li>If a variable has boolean semantics, give it the <code>bool</code> type
and use the corresponding <code>true</code> and <code>false</code> macros.
It's ok to include &lt;stdbool.h&gt;, since libvirt's use of gnulib ensures
that it exists and is usable.</li>
<li>In the unusual event that you require a specific width, use a
standard type like <code>int32_t</code>, <code>uint32_t</code>,
<code>uint64_t</code>, etc.</li>
<li>While using <code>bool</code> is good for readability, it comes with
minor caveats:
<ul>
<li>Don't use <code>bool</code> 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 <code>bool</code> in libvirt's
logical wire protocol, since XDR maps that to its lower-level <code>bool_t</code>
type, which <b>is</b> fixed-size.</li>
<li>Don't compare a bool variable against the literal, <code>true</code>,
since a value with a logical non-false value need not be <code>1</code>.
I.e., don't write <code>if (seen == true) ...</code>. Rather,
write <code>if (seen)...</code>.</li>
</ul>
</li>
</ul>
<p>
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 <code>size_t</code>,
<code>pid_t</code> or <code>off_t</code>, use matching types for any
corresponding variables.
</p>
<p>
Also, if you try to use e.g., <code>unsigned int</code> as a type, and that
conflicts with the signedness of a related variable, sometimes
it's best just to use the <b>wrong</b> type, if <i>pulling the thread</i>
and fixing all related variables would be too invasive.
</p>
<p>
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.
</p>
<h3>Pointers</h3>
<p>
Ensure that all of your pointers are <i>const-correct</i>.
Unless a pointer is used to modify the pointed-to storage,
give it the <code>const</code> 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.
</p>
<h2><a id="attribute_annotations">Attribute annotations</a></h2>
<p>
Use the following annotations to help the compiler and/or static
analysis tools understand the code better:
</p>
<table class="top_table">
<tr><th>Macro</th><th>Meaning</th></tr>
<tr><td><code>ATTRIBUTE_NONNULL</code></td><td>passing NULL for this parameter is not allowed</td></tr>
<tr><td><code>ATTRIBUTE_PACKED</code></td><td>force a structure to be packed</td></tr>
<tr><td><code>G_GNUC_FALLTHROUGH</code></td><td>allow code reuse by multiple switch cases</td></tr>
<tr><td><code>G_GNUC_NO_INLINE</code></td><td>the function is mocked in the test suite</td></tr>
<tr><td><code>G_GNUC_NORETURN</code></td><td>the function never returns</td></tr>
<tr><td><code>G_GNUC_NULL_TERMINATED</code></td><td>last parameter must be NULL</td></tr>
<tr><td><code>G_GNUC_PRINTF</code></td><td>validate that the formatting string matches parameters</td></tr>
<tr><td><code>G_GNUC_UNUSED</code></td><td>parameter is unused in this implementation of the function</td></tr>
<tr><td><code>G_GNUC_WARN_UNUSED_RESULT</code></td><td>the return value must be checked</td></tr>
</table>
<h2><a id="glib">Adoption of GLib APIs</a></h2>
<p>
Libvirt has adopted use of the
<a href="https://developer.gnome.org/glib/stable/">GLib library</a>.
Due to libvirt's long history of development, there are many APIs
in libvirt, for which GLib provides an alternative solution. The
general rule to follow is that the standard GLib solution will be
preferred over historical libvirt APIs. Existing code will be
ported over to use GLib APIs over time, but new code should use
the GLib APIs straight away where possible.
</p>
<p>
The following is a list of libvirt APIs that should no longer be
used in new code, and their suggested GLib replacements:
</p>
<dl>
<dt><code>VIR_ALLOC</code>, <code>VIR_REALLOC</code>,
<code>VIR_RESIZE_N</code>, <code>VIR_EXPAND_N</code>,
<code>VIR_SHRINK_N</code>, <code>VIR_FREE</code>,
<code>VIR_APPEND_ELEMENT</code>, <code>VIR_INSERT_ELEMENT</code>,
<code>VIR_DELETE_ELEMENT</code></dt>
<dd>Prefer the GLib APIs <code>g_new0</code>/<code>g_renew</code>/
<code>g_free</code> in most cases. There should rarely be a need
to use <code>g_malloc</code>/<code>g_realloc</code>.
Instead of using plain C arrays, it is preferrable to use
one of the GLib types, <code>GArray</code>, <code>GPtrArray</code>
or <code>GByteArray</code>. These
all use a struct to track the array memory and size together
and efficiently resize. <strong>NEVER MIX</strong> use of the
classic libvirt memory allocation APIs and GLib APIs within
a single method. Keep the style consistent, converting existing
code to GLib style in a separate, prior commit.</dd>
<dt><code>virStrerror</code></dt>
<dd>The GLib <code>g_strerror()</code> function should be used instead,
which has a simpler calling convention as an added benefit.</dd>
<table class="top_table">
<tr><th>deprecated version</th><th>GLib version</th><th>Notes</th></tr>
<tr><td><code>VIR_ALLOC(var)</code></td><td><code>g_new0(var_t, 1)</code></td>
<td>the type needs to be passed explicitly</td></tr>
<tr><td><code>VIR_ALLOC_N</code></td><td><code>g_new0(var_t, n)</code></td><td></td></tr>
<tr><td><code>VIR_REALLOC_N</code></td><td><code>g_renew(var_t, ptr, n)</code></td>
<td>the newly added memory is not zeroed</td></tr>
<tr><td><code>VIR_EXPAND_N</code></td><td><code>g_renew(var_t, ptr, n)</code></td>
<td>zero the new memory manually or use an array type</td></tr>
<tr><td><code>VIR_SHRINK_N</code></td><td><code>g_renew(var_t, ptr, n)</code></td>
<td></td></tr>
<tr><td><code>VIR_APPEND_ELEMENT</code></td><td><code>g_array_append_val</code></td>
<td><code>g_ptr_array_add</code> or <code>g_byte_array_append</code></td></tr>
<tr><td><code>VIR_INSERT_ELEMENT</code></td><td><code>g_array_insert_val</code></td>
<td><code>g_ptr_array_insert</code></td></tr>
<tr><td><code>VIR_DELETE_ELEMENT</code></td><td><code>g_array_remove_index</code></td>
<td><code>g_ptr_array_remove_index</code> or <code>g_byte_array_remove_index</code></td></tr>
<tr><td><code>VIR_FREE</code></td><td><code>g_free</code></td>
<td><code>g_free</code> does not zero the pointer</td></tr>
</table>
<p>String allocation macros and functions:</p>
<table class="top_table">
<tr><th>deprecated version</th><th>GLib version</th><th>Notes</th></tr>
<tr><td><code>virAsprintf</code></td><td><code>g_strdup_printf</code></td><td></td></tr>
<tr><td><code>virVasprintf</code></td><td><code>g_strdup_vprint</code></td>
<td>use <code>g_vasprintf</code> if you really need to know the returned length</td></tr>
<tr><td><code>virStrerror</code></td><td><code>g_strerror</code></td>
<td>the error strings are cached globally so no need to free it</td></tr>
</table>
</dl>
<p>
The following libvirt APIs have been deleted already:
</p>
<dl>
<dt><code>VIR_AUTOPTR</code>, <code>VIR_AUTOCLEAN</code>, <code>VIR_AUTOFREE</code></dt>
<dd>The GLib macros <code>g_autoptr</code>, <code>g_auto</code> and
<code>g_autofree</code> must be used
instead in all new code. In existing code, the GLib macros must
never be mixed with libvirt macros within a method, nor should
they be mixed with <code>VIR_FREE</code>. If introducing GLib macros to an
existing method, any use of libvirt macros must be converted
in an independent commit.
</dd>
<dt><code>VIR_DEFINE_AUTOPTR_FUNC</code>, <code>VIR_DEFINE_AUTOCLEAN_FUNC</code></dt>
<dd>The GLib macros <code>G_DEFINE_AUTOPTR_CLEANUP_FUNC</code> and
<code>G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC</code> must be used in all
new code. Existing code should be converted to the
new macros where relevant. It is permissible to use
<code>g_autoptr</code>, <code>g_auto</code> on an object whose cleanup function
is declared with the libvirt macros and vice-versa.
</dd>
<dt><code>VIR_AUTOUNREF</code></dt>
<dd>The GLib macros <code>g_autoptr</code> and <code>G_DEFINE_AUTOPTR_CLEANUP_FUNC</code>
should be used to manage autoclean of virObject classes.
This matches usage with GObject classes.</dd>
<dt><code>VIR_STRDUP</code>, <code>VIR_STRNDUP</code></dt>
<dd>Prefer the GLib APIs <code>g_strdup</code> and <code>g_strndup</code>.</dd>
</dl>
<table class="top_table">
<tr><th>deleted version</th><th>GLib version</th><th>Notes</th></tr>
<tr><td><code>VIR_AUTOPTR</code></td><td><code>g_autoptr</code></td><td></td></tr>
<tr><td><code>VIR_AUTOCLEAN</code></td><td><code>g_auto</code></td><td></td></tr>
<tr><td><code>VIR_AUTOFREE</code></td><td><code>g_autofree</code></td><td>The GLib version does not use parentheses</td></tr>
<tr><td><code>VIR_AUTOUNREF</code></td><td><code>g_autoptr</code></td><td>The cleanup function needs to be defined</td></tr>
<tr><td><code>VIR_DEFINE_AUTOPTR_FUNC</code></td><td><code>G_DEFINE_AUTOPTR_CLEANUP_FUNC</code></td><td></td></tr>
<tr><td><code>VIR_DEFINE_AUTOCLEAN_FUNC</code></td><td><code>G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC</code></td><td></td></tr>
<tr><td><code>VIR_STEAL_PTR</code></td><td><code>g_steal_pointer</code></td>
<td><code>a = f(&amp;b)</code> instead of <code>f(a, b)</code></td></tr>
<tr><td><code>VIR_RETURN_PTR</code></td><td><code>return g_steal_pointer</code></td><td></td></tr>
<tr><td><code>ARRAY_CARDINALITY</code></td><td><code>G_N_ELEMENTS</code></td><td></td></tr>
<tr><td><code>ATTRIBUTE_FALLTHROUGH</code></td><td><code>G_GNUC_FALLTHROUGH</code></td><td></td></tr>
<tr><td><code>ATTRIBUTE_FMT_PRINTF</code></td><td><code>G_GNUC_PRINTF</code></td><td></td></tr>
<tr><td><code>ATTRIBUTE_NOINLINE</code></td><td><code>G_GNUC_NO_INLINE</code></td><td></td></tr>
<tr><td><code>ATTRIBUTE_NORETURN</code></td><td><code>G_GNUC_NORETURN</code></td><td></td></tr>
<tr><td><code>ATTRIBUTE_RETURN_CHECK</code></td><td><code>G_GNUC_WARN_UNUSED_RESULT</code></td><td></td></tr>
<tr><td><code>ATTRIBUTE_SENTINEL</code></td><td><code>G_GNUC_NULL_TERMINATED</code></td><td></td></tr>
<tr><td><code>ATTRIBUTE_UNUSED</code></td><td><code>G_GNUC_UNUSED</code></td><td></td></tr>
<tr><td><code>VIR_STRDUP</code></td><td><code>g_strdup</code></td><td></td></tr>
<tr><td><code>VIR_STRNDUP</code></td><td><code>g_strndup</code></td><td></td></tr>
</table>
<h2><a id="file_handling">File handling</a></h2>
<p>
Usage of the <code>fdopen()</code>, <code>close()</code>, <code>fclose()</code>
APIs is deprecated in libvirt code base to help avoiding double-closing of files
or file descriptors, which is particularly dangerous in a multi-threaded
application. Instead of these APIs, use the macros from virfile.h
</p>
<ul>
<li><p>Open a file from a file descriptor:</p>
<pre>
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 */
</pre></li>
<li><p>Close a file descriptor:</p>
<pre>
if (VIR_CLOSE(fd) &lt; 0) {
virReportSystemError(errno, "%s", _("failed to close file"));
}
</pre></li>
<li><p>Close a file:</p>
<pre>
if (VIR_FCLOSE(file) &lt; 0) {
virReportSystemError(errno, "%s", _("failed to close file"));
}
</pre></li>
<li><p>Close a file or file descriptor in an error path, without losing
the previous <code>errno</code> value:</p>
<pre>
VIR_FORCE_CLOSE(fd);
VIR_FORCE_FCLOSE(file);
</pre>
</li>
</ul>
<h2><a id="string_comparision">String comparisons</a></h2>
<p>
Do not use the strcmp, strncmp, etc functions directly. Instead use
one of the following semantically named macros
</p>
<ul>
<li><p>For strict equality:</p>
<pre>
STREQ(a,b)
STRNEQ(a,b)
</pre>
</li>
<li><p>For case insensitive equality:</p>
<pre>
STRCASEEQ(a,b)
STRCASENEQ(a,b)
</pre>
</li>
<li><p>For strict equality of a substring:</p>
<pre>
STREQLEN(a,b,n)
STRNEQLEN(a,b,n)
</pre>
</li>
<li><p>For case insensitive equality of a substring:</p>
<pre>
STRCASEEQLEN(a,b,n)
STRCASENEQLEN(a,b,n)
</pre>
</li>
<li><p>For strict equality of a prefix:</p>
<pre>
STRPREFIX(a,b)
</pre>
</li>
<li><p>To avoid having to check if a or b are NULL:</p>
<pre>
STREQ_NULLABLE(a, b)
STRNEQ_NULLABLE(a, b)
</pre>
</li>
</ul>
<h2><a id="string_copying">String copying</a></h2>
<p>
Do not use the strncpy function. According to the man page, it
does <b>not</b> guarantee a NULL-terminated buffer, which makes
it extremely dangerous to use. Instead, use one of the replacement
functions provided by libvirt:
</p>
<pre>
virStrncpy(char *dest, const char *src, size_t n, size_t destbytes)
</pre>
<p>
The first two arguments have the same meaning as for strncpy,
namely the destination and source of the copy operation. Unlike
strncpy, the function will always copy exactly the number of bytes
requested and make sure the destination is NULL-terminated, as the
source is required to be; sanity checks are performed to ensure the
size of the destination, as specified by the last argument, is
sufficient for the operation to succeed. On success, 0 is returned;
on failure, a value &lt;0 is returned instead.
</p>
<pre>
virStrcpy(char *dest, const char *src, size_t destbytes)
</pre>
<p>
Use this variant if you know you want to copy the entire src
string into dest.
</p>
<pre>
virStrcpyStatic(char *dest, const char *src)
</pre>
<p>
Use this variant if you know you want to copy the entire src
string into dest <b>and</b> 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.
</p>
<pre>
dst = g_strdup(src);
dst = g_strndup(src, n);
</pre>
<p>
You should avoid using strdup or strndup directly as they do not handle
out-of-memory errors, and do not allow a NULL source.
Use <code>g_strdup</code> and <code>g_strndup</code> from GLib which
abort on OOM and handle NULL source by returning NULL.
</p>
<h2><a id="strbuf">Variable length string buffer</a></h2>
<p>
If there is a need for complex string concatenations, avoid using
the usual sequence of malloc/strcpy/strcat/snprintf functions and
make use of either the
<a href="https://developer.gnome.org/glib/stable/glib-Strings.html">GString</a>
type from GLib or the virBuffer API.
If formatting XML or QEMU command line is needed, use the virBuffer
API described in virbuffer.h, since it has helper functions for those.
</p>
<p>Typical usage is as follows:</p>
<pre>
char *
somefunction(...)
{
g_auto(virBuffer) buf = VIR_BUFFER_INITIALIZER;
...
virBufferAddLit(&amp;buf, "&lt;domain&gt;\n");
virBufferAsprintf(&amp;buf, " &lt;memory&gt;%d&lt;/memory&gt;\n", memory);
if (some_error)
return NULL; /* g_auto will free the memory used so far */
...
virBufferAddLit(&amp;buf, "&lt;/domain&gt;\n");
...
if (virBufferCheckError(&amp;buf) &lt; 0)
return NULL;
return virBufferContentAndReset(&amp;buf);
}
</pre>
<h2><a id="includes">Include files</a></h2>
<p>
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:
</p>
<pre>
/*
* Copyright notice
* ....
* ....
* ....
*
*/
#include &lt;config.h&gt; Must come first in every file.
#include &lt;stdio.h&gt; Any system includes you need.
#include &lt;string.h&gt;
#include &lt;limits.h&gt;
#if WITH_NUMACTL Some system includes aren't supported
# include &lt;numa.h&gt; 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.
{
...
</pre>
<p>
Of particular note: <b>Do not</b> include libvirt/libvirt.h,
libvirt/virterror.h, libvirt/libvirt-qemu.h, or libvirt/libvirt-lxc.h.
They are included by "internal.h" already and there are some special reasons
why you cannot include these files explicitly. One of the special cases,
"libvirt/libvirt.h" is included prior to "internal.h" in "remote_protocol.x",
to avoid exposing *_LAST enum elements.
</p>
<h2><a id="printf">Printf-style functions</a></h2>
<p>
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 virCommandAddEnvFormat in vircommand.h:
</p>
<pre>
void virCommandAddEnvFormat(virCommandPtr cmd, const char *format, ...)
G_GNUC_PRINTF(2, 3);
</pre>
<p>
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.
</p>
<p>
When printing to a string, consider using GString or virBuffer for
incremental allocations, g_strdup_printf for a one-shot allocation,
and g_snprintf for fixed-width buffers. Only use g_sprintf,
if you can prove the buffer won't overflow.
</p>
<h2><a id="errors">Error message format</a></h2>
<p>
Error messages visible to the user should be short and descriptive. All
error messages are translated using gettext and thus must be wrapped in
<code>_()</code> macro. To simplify the translation work, the error message
must not be concatenated from various parts. To simplify searching for
the error message in the code the strings should not be broken even
if they result into a line longer than 80 columns and any formatting
modifier should be enclosed by quotes or other obvious separator.
If a string used with <code>%s</code> can be NULL the NULLSTR macro must
be used.
</p>
<pre>
GOOD: virReportError(VIR_ERR_INTERNAL_ERROR,
_("Failed to connect to remote host '%s'"), hostname)
BAD: virReportError(VIR_ERR_INTERNAL_ERROR,
_("Failed to %s to remote host '%s'"),
"connect", hostname);
BAD: virReportError(VIR_ERR_INTERNAL_ERROR,
_("Failed to connect "
"to remote host '%s'),
hostname);
</pre>
<h2><a id="goto">Use of goto</a></h2>
<p>
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.
</p>
<p>
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. g_free() and most of the functions named XXXFree() in
libvirt is required to handle NULL as its arg. This does not
apply to libvirt's public APIs. 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.
Note that most of libvirt's type declarations can be marked with
either <code>g_autofree</code> or <code>g_autoptr</code> which uses
the compiler's <code>__attribute__((cleanup))</code> that calls
the appropriate free function when the variable goes out of scope.
</p>
<p>
There are a couple of signs that a particular use of goto is not
ok:
</p>
<ul>
<li>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.</li>
<li>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.</li>
<li>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.</li>
</ul>
<p>
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
<a href="http://kerneltrap.org/node/553/2131">KernelTrap</a>
</p>
<p>
When using goto, please use one of these standard labels if it
makes sense:
</p>
<pre>
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)
</pre>
<p>
Top-level labels should be indented by one space (putting them on
the beginning of the line confuses function context detection in git):
</p>
<pre>
int foo()
{
/* ... do stuff ... */
cleanup:
/* ... do other stuff ... */
}
</pre>
<h2><a id="committers">Libvirt committer guidelines</a></h2>
<p>
The AUTHORS files indicates the list of people with commit access right
who can actually merge the patches.
</p>
<p>
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.
</p>
<p>
An exception to 'review and approval on the list first' is fixing failures
to build:
</p>
<ul>
<li>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</li>
<li>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</li>
<li>
fixes for documentation and code comments can be managed
in the same way, but still make sure they get reviewed if non-trivial.
</li>
<li>(ir)regular pulls from other repositories or automated updates, such
as the .gnulib submodule updates, pulling in new translations or updating
the container images for the CI system
</li>
</ul>
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