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
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
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
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))))
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.
Omit the curly braces around an if, while,
for etc. body only
when that body occupies a single line. In every other case we require
the braces. This ensures that it is trivially easy to identify a
single-statement loop: each has only one line in its body.
Omitting braces with a single-line body is fine:
while (expr) // one-line body -> omitting curly braces is ok
single_line_stmt();
However, the moment your loop/if/else body extends onto a second line, for whatever reason (even if it's just an added comment), then you should add braces. Otherwise, it would be too easy to insert a statement just before that comment (without adding braces), thinking it is already a multi-statement loop:
while (true) // BAD! multi-line body with no braces
/* comment... */
single_line_stmt();
Do this instead:
while (true) { // Always put braces around a multi-line body.
/* comment... */
single_line_stmt();
}
There is one exception: when the second body line is not at the same indentation level as the first body line:
if (expr)
die("a diagnostic that would make this line"
" extend past the 80-column limit"));
It is safe to omit the braces in the code above, since the further-indented second body line makes it obvious that this is still a single-statement body.
To reiterate, don't do this:
if (expr) // BAD: no braces around...
while (expr_2) { // ... a multi-line body
...
}
Do this, instead:
if (expr) {
while (expr_2) {
...
}
}
However, there is one exception in the other direction, when even a
one-line block should have braces. That occurs when that one-line,
brace-less block is an else block, and the corresponding
then block does use braces. In that case, either
put braces around the else block, or negate the
if-condition and swap the bodies, putting the
one-line block first and making the longer, multi-line block be the
else block.
if (expr) {
...
...
}
else
x = y; // BAD: braceless "else" with braced "then"
This is preferred, especially when the multi-line body is more than a few lines long, because it is easier to read and grasp the semantics of an if-then-else block when the simpler block occurs first, rather than after the more involved block:
if (!expr)
x = y; // putting the smaller block first is more readable
else {
...
...
}
If you'd rather not negate the condition, then at least add braces:
if (expr) {
...
...
} else {
x = y;
}
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
Use the right type.
int or long, odds are
good that there's a better type.size_t (use
ssize_t only if required).off_t.off_t.unsigned int;
(on all but oddball embedded systems, you can assume that that
type is at least four bytes wide).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.int32_t, uint32_t,
uint64_t, etc.bool is good for readability, it comes with
minor caveats:
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.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.
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.
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 to have one less element:
virDomainPtr domains; size_t ndomains = x; size_t ndomains_max = y; VIR_SHRINK_N(domains, ndomains_max, 1);
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;
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);
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)
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)).
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");
virBufferVSprint(&buf, " <memory>%d</memory>\n", memory);
...
virBufferAddLit(&buf, "</domain>\n");
...
if (virBufferError(&buf)) {
virBufferFreeAndReset(&buf);
virReportOOMError();
return NULL;
}
return virBufferContentAndReset(&buf);
}
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.
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.
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:
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
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)
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: