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libvirt/docs/hacking.html.in
Eric Blake 06732e1a7d docs: mention C89 syntax preferences 
* docs/hacking.html.in (Code formatting): Document that // comment
and declaration-after-statement are discouraged.
* HACKING: Regenerate.
2011-03-30 13:51:22 -06:00

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<html>
<body>
<h1>Contributor guidelines</h1>
<ul id="toc"></ul>
<h2><a name="patches">General tips for contributing patches</a></h2>
<ol>
<li>Discuss any large changes on the mailing list first. Post patches
early and listen to feedback.</li>
<li><p>Post patches in unified diff format. A command similar to this
should work:</p>
<pre>
diff -urp libvirt.orig/ libvirt.modified/ &gt; libvirt-myfeature.patch
</pre>
<p>
or:
</p>
<pre>
git diff > libvirt-myfeature.patch
</pre>
</li>
<li>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.</li>
<li>Make sure your patches apply against libvirt GIT. Developers
only follow GIT and don't care much about released versions.</li>
<li><p>Run the automated tests on your code before submitting any changes.
In particular, configure with compile warnings set to -Werror:</p>
<pre>
./configure --enable-compile-warnings=error
</pre>
<p>
and run the tests:
</p>
<pre>
make check
make syntax-check
make -C tests valgrind
</pre>
<p>
The latter test checks for memory leaks.
</p>
<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>
Also, individual tests can be run from inside the <code>tests/</code>
directory, like:
</p>
<pre>
./qemuxml2xmltest
</pre>
</li>
<li>Update tests and/or documentation, particularly if you are adding
a new feature or changing the output of a program.</li>
</ol>
<p>
There is more on this subject, including lots of links to background
reading on the subject, on
<a href="http://et.redhat.com/~rjones/how-to-supply-code-to-open-source-projects/">
Richard Jones' guide to working with open source projects</a>
</p>
<h2><a name="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, add the following to one of one of your start-up files
(e.g., ~/.emacs), to help ensure that you get indentation right:
</p>
<pre>
;;; 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&amp;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))))
</pre>
<p>
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
filetype plugin indent on
au FileType make setlocal noexpandtab
au BufRead,BufNewFile *.am setlocal noexpandtab
match ErrorMsg /\s\+$\| \+\ze\t/
</pre>
<p>
Or if you don't want to mess your ~/.vimrc up, you can save the above
into a file called .lvimrc (not .vimrc) located at the root of libvirt
source, then install a vim script from
http://www.vim.org/scripts/script.php?script_id=1408,
which will load the .lvimrc only when you edit libvirt code.
</p>
<h2><a name="formatting">Code formatting (especially for new code)</a></h2>
<p>
With new code, we can be even more strict.
Please apply the following function (using GNU indent) to any new code.
Note that this also gives you an idea of the type of spacing we prefer
around operators and keywords:
</p>
<pre>
indent-libvirt()
{
indent -bad -bap -bbb -bli4 -br -ce -brs -cs -i4 -l75 -lc75 \
-sbi4 -psl -saf -sai -saw -sbi4 -ss -sc -cdw -cli4 -npcs -nbc \
--no-tabs "$@"
}
</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 name="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 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-<i>statement</i> loop: each has only one <i>line</i> in its body.
</p>
<p>
Omitting braces with a single-line body is fine:
</p>
<pre>
while (expr) // one-line body -> omitting curly braces is ok
single_line_stmt();
</pre>
<p>
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:
</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>
<h2><a href="types">Preprocessor</a></h2>
<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 href="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 name="memalloc">Low level memory management</a></h2>
<p>
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.
</p>
<ul>
<li><p>To allocate a single object:</p>
<pre>
virDomainPtr domain;
if (VIR_ALLOC(domain) &lt; 0) {
virReportOOMError();
return NULL;
}
</pre>
</li>
<li><p>To allocate an array of objects:</p>
<pre>
virDomainPtr domains;
size_t ndomains = 10;
if (VIR_ALLOC_N(domains, ndomains) &lt; 0) {
virReportOOMError();
return NULL;
}
</pre>
</li>
<li><p>To allocate an array of object pointers:</p>
<pre>
virDomainPtr *domains;
size_t ndomains = 10;
if (VIR_ALLOC_N(domains, ndomains) &lt; 0) {
virReportOOMError();
return NULL;
}
</pre>
</li>
<li><p>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):</p>
<pre>
virDomainPtr domains;
size_t ndomains = 0;
if (VIR_EXPAND_N(domains, ndomains, 1) &lt; 0) {
virReportOOMError();
return NULL;
}
domains[ndomains - 1] = domain;
</pre></li>
<li><p>To ensure an array has room to hold at least one more
element (this approach scales better, but requires tracking
allocation separately from usage)</p>
<pre>
virDomainPtr domains;
size_t ndomains = 0;
size_t ndomains_max = 0;
if (VIR_RESIZE_N(domains, ndomains_max, ndomains, 1) &lt; 0) {
virReportOOMError();
return NULL;
}
domains[ndomains++] = domain;
</pre>
</li>
<li><p>To trim an array of domains from its allocated size down
to the actual used size:</p>
<pre>
virDomainPtr domains;
size_t ndomains = x;
size_t ndomains_max = y;
VIR_SHRINK_N(domains, ndomains_max, ndomains_max - ndomains);
</pre></li>
<li><p>To free an array of domains:</p>
<pre>
virDomainPtr domains;
size_t ndomains = x;
size_t ndomains_max = y;
size_t i;
for (i = 0; i &lt; ndomains; i++)
VIR_FREE(domains[i]);
VIR_FREE(domains);
ndomains_max = ndomains = 0;
</pre>
</li>
</ul>
<h2><a name="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 particulary dangerous in a multi-threaded
applications. Instead of these APIs, use the macros from files.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 name="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>
</ul>
<h2><a name="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
functionally equivalent functions:
</p>
<pre>
virStrncpy(char *dest, const char *src, size_t n, size_t destbytes)
</pre>
<p>
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.
</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. Note that this is a macro, so arguments could
be evaluated more than once. This is equivalent to
virStrncpy(dest, src, strlen(src), destbytes)
</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. This is equivalent to
virStrncpy(dest, src, strlen(src), sizeof(dest)).
</p>
<h2><a name="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 the virBuffer API described in buf.h
</p>
<p>Typical usage is as follows:</p>
<pre>
char *
somefunction(...)
{
virBuffer buf = VIR_BUFFER_INITIALIZER;
...
virBufferAddLit(&amp;buf, "&lt;domain&gt;\n");
virBufferVSprintf(&amp;buf, " &lt;memory&gt;%d&lt;/memory&gt;\n", memory);
...
virBufferAddLit(&amp;buf, "&lt;/domain&gt;\n");
...
if (virBufferError(&amp;buf)) {
virBufferFreeAndReset(&amp;buf);
virReportOOMError();
return NULL;
}
return virBufferContentAndReset(&amp;buf);
}
</pre>
<h2><a name="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 HAVE_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 or
libvirt/virterror.h. It is included by "internal.h" already and there
are some special reasons why you cannot include these files
explicitly.
</p>
<h2><a name="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 virAsprintf, in util.h:
</p>
<pre>
int virAsprintf(char **strp, const char *fmt, ...)
ATTRIBUTE_FORMAT(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 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.
</p>
<h2><a name="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. 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.
</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>
<h2><a name="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. Try to look for warnings too; for example,
configure with
</p>
<pre>
--enable-compile-warnings=error
</pre>
<p>
which adds -Werror to compile flags, so no warnings get missed
</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>
</ul>
</body>
</html>
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