Contributor guidelines

General tips for contributing patches

  1. Discuss any large changes on the mailing list first. Post patches early and listen to feedback.

  2. Post patches in unified diff format, with git rename detection enabled. You need a one-time setup of:

      git config diff.renames true

    After that, a command similar to this should work:

      diff -urp libvirt.orig/ libvirt.modified/ > libvirt-myfeature.patch

    or: 

      git diff > libvirt-myfeature.patch

    Also, for code motion patches, you may find that git diff --patience provides an easier-to-read patch. However, the usual workflow of libvirt developer is:

      git checkout master
  git pull
  git checkout -t origin -b workbranch
  Hack, committing any changes along the way

    Then, when you want to post your patches:

      git pull --rebase
  (fix any conflicts)
  git send-email --cover-letter --no-chain-reply-to --annotate \
                 --to=libvir-list@redhat.com master

    (Note that the "git send-email" subcommand may not be in the main git package and using it may require installion of a separate package, for example the "git-email" package in Fedora.) For a single patch you can omit --cover-letter, but a series of two or more patches needs a cover letter. If you get tired of typing --to=libvir-list@redhat.com designation you can set it in git config:

      git config sendemail.to libvir-list@redhat.com

    Please follow this as close as you can, especially the rebase and git send-email part, as it makes life easier for other developers to review your patch set. 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), he is advised to note differences to previous versions after the --- 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 --subject-prefix=PATCHv2 appended to git send-email (substitute v2 with the correct version if needed though).

  3. 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. Libvirt does not currently attach any meaning to Signed-off-by: lines, so it is up to you if you want to include or omit them in the commit message.

  4. 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 (including make check and make syntax-check) 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 git bisect, among other things).

  5. Make sure your patches apply against libvirt GIT. Developers only follow GIT and don't care much about released versions.

  6. Run the automated tests on your code before submitting any changes. In particular, configure with compile warnings set to -Werror. This is done automatically for a git checkout; from a tarball, use:

      ./configure --enable-werror

    and run the tests: 

      make check
  make syntax-check
  make -C tests valgrind

    Valgrind is a test that checks for memory management issues, such as leaks or use of uninitialized variables.

    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 also a ./run 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.

  7. The Valgrind test should produce similar output to make check. 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: 

    ==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)

    In this example, the virDomainDefParseXML() had an error path where the virDomainVideoDefPtr video pointer was not properly disposed. By simply adding a virDomainVideoDefFree(video); in the error path, the issue was resolved.

    Another common mistake is calling a printing function, such as VIR_DEBUG() 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. 

    ==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)

    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: 

    ==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)

    In this instance, it is acceptible to modify the tests/.valgrind.supp 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 Valgrind home page. The following trace was added to tests/.valgrind.supp in order to suppress the warning: 

    {
    dlInitMemoryLeak1
    Memcheck:Leak
    fun:?alloc
    ...
    fun:call_init.part.0
    fun:_dl_init
    ...
    obj:*/lib*/ld-2.*so*
}
  8. Update tests and/or documentation, particularly if you are adding a new feature or changing the output of a program.

There is more on this subject, including lots of links to background reading on the subject, on Richard Jones' guide to working with open source projects

Code indentation

Libvirt's C source code generally adheres to some basic code-formatting conventions. The existing code base is not totally consistent on this front, but we do prefer that contributed code be formatted similarly. In short, use spaces-not-TABs for indentation, use 4 spaces for each indentation level, and other than that, follow the K&R style.

If you use Emacs, add the following to one of one of your start-up files (e.g., ~/.emacs), to help ensure that you get indentation right: 

  ;;; When editing C sources in libvirt, use this style.
  (defun libvirt-c-mode ()
    "C mode with adjusted defaults for use with libvirt."
    (interactive)
    (c-set-style "K&R")
    (setq indent-tabs-mode nil) ; indent using spaces, not TABs
    (setq c-indent-level 4)
    (setq c-basic-offset 4))
  (add-hook 'c-mode-hook
            '(lambda () (if (string-match "/libvirt" (buffer-file-name))
                            (libvirt-c-mode))))

If you use vim, append the following to your ~/.vimrc file: 

  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/

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.

Code formatting (especially for new code)

With new code, we can be even more strict. Please apply the following function (using GNU indent) to any new code. Note that this also gives you an idea of the type of spacing we prefer around operators and keywords: 

  indent-libvirt()
  {
    indent -bad -bap -bbb -bli4 -br -ce -brs -cs -i4 -l75 -lc75 \
      -sbi4 -psl -saf -sai -saw -sbi4 -ss -sc -cdw -cli4 -npcs -nbc \
      --no-tabs "$@"
  }

Note that sometimes you'll have to post-process that output further, by piping it through expand -i, since some leading TABs can get through. Usually they're in macro definitions or strings, and should be converted anyhow.

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 /* */ comments rather than //. Also, when declaring local variables, the prevailing style has been to declare them at the beginning of a scope, rather than immediately before use.

Bracket spacing

The keywords if, for, while, and switch must have a single space following them before the opening bracket. E.g. 

      if(foo)   // Bad
      if (foo)  // Good

Function implementations must not have any whitespace between the function name and the opening bracket. E.g. 

      int foo (int wizz)  // Bad
      int foo(int wizz)   // Good

Function calls must not have any whitespace between the function name and the opening bracket. E.g. 

      bar = foo (wizz);  // Bad
      bar = foo(wizz);   // Good

Function typedefs must not have any whitespace between the closing bracket of the function name and opening bracket of the arg list. E.g. 

      typedef int (*foo) (int wizz);  // Bad
      typedef int (*foo)(int wizz);   // Good

There must not be any whitespace immediately following any opening bracket, or immediately prior to any closing bracket. E.g. 

      int foo( int wizz );  // Bad
      int foo(int wizz);    // Good

Curly braces

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

Omitting braces with a single-line body is fine: 

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

However, the moment your loop/if/else body extends 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: 

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

Do this instead: 

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

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

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

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

To reiterate, don't do this: 

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

Do this, instead: 

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

However, there is one exception in the other direction, when even a one-line block should have braces. That occurs when that one-line, brace-less block is an if or else block, and the counterpart block does use braces. In that case, put braces around both blocks. Also, if the else block is much shorter than the if block, consider negating the if-condition and swapping the bodies, putting the short block first and making the longer, multi-line block be the else block. 

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

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

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

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

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

  if (complex expr not worth negating) {
      ...
      ...
  } else {
      x = y;
  }

Preprocessor

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.

For variadic macros, stick with C99 syntax: 

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

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

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

C types

Use the right type.

Scalars

Of course, take all of the above with a grain of salt. If you're about to use some system interface that requires a type like size_t, pid_t or off_t, use matching types for any corresponding variables.

Also, if you try to use e.g., unsigned int as a type, and that conflicts with the signedness of a related variable, sometimes it's best just to use the wrong type, if pulling the thread and fixing all related variables would be too invasive.

Finally, while using descriptive types is important, be careful not to go overboard. If whatever you're doing causes warnings, or requires casts, then reconsider or ask for help.

Pointers

Ensure that all of your pointers are const-correct. Unless a pointer is used to modify the pointed-to storage, give it the const attribute. That way, the reader knows up-front that this is a read-only pointer. Perhaps more importantly, if we're diligent about this, when you see a non-const pointer, you're guaranteed that it is used to modify the storage it points to, or it is aliased to another pointer that is.

Low level memory management

Use of the malloc/free/realloc/calloc APIs is deprecated in the libvirt codebase, because they encourage a number of serious coding bugs and do not enable compile time verification of checks for NULL. Instead of these routines, use the macros from viralloc.h.

File handling

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

String comparisons

Do not use the strcmp, strncmp, etc functions directly. Instead use one of the following semantically named macros

String copying

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

  virStrncpy(char *dest, const char *src, size_t n, size_t destbytes)

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

  virStrcpy(char *dest, const char *src, size_t destbytes)

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

  virStrcpyStatic(char *dest, const char *src)

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

  VIR_STRDUP(char *dst, const char *src);
  VIR_STRNDUP(char *dst, const char *src, size_t n);

You should avoid using strdup or strndup directly as they do not report out-of-memory error, and do not allow a NULL source. Use VIR_STRDUP or VIR_STRNDUP macros instead, which return 0 for NULL source, 1 for successful copy, and -1 for allocation failure with the error already reported. In very specific cases, when you don't want to report the out-of-memory error, you can use VIR_STRDUP_QUIET or VIR_STRNDUP_QUIET, but such usage is very rare and usually considered a flaw.

Variable length string buffer

If there is a need for complex string concatenations, avoid using the usual sequence of malloc/strcpy/strcat/snprintf functions and make use of the virBuffer API described in buf.h

Typical usage is as follows:

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

     ...

     virBufferAddLit(&buf, "<domain>\n");
     virBufferAsprintf(&buf, "  <memory>%d</memory>\n", memory);
     ...
     virBufferAddLit(&buf, "</domain>\n");

     ...

     if (virBufferError(&buf)) {
         virBufferFreeAndReset(&buf);
         virReportOOMError();
         return NULL;
     }

     return virBufferContentAndReset(&buf);
  }

Include files

There are now quite a large number of include files, both libvirt internal and external, and system includes. To manage all this complexity it's best to stick to the following general plan for all *.c source files: 

  /*
   * Copyright notice
   * ....
   * ....
   * ....
   *
   */

  #include <config.h>             Must come first in every file.

  #include <stdio.h>              Any system includes you need.
  #include <string.h>
  #include <limits.h>

  #if WITH_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, 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.

Printf-style functions

Whenever you add a new printf-style function, i.e., one with a format string argument and following "..." in its prototype, be sure to use gcc's printf attribute directive in the prototype. For example, here's the one for virAsprintf, in util.h: 

  int virAsprintf(char **strp, const char *fmt, ...)
      ATTRIBUTE_FORMAT(printf, 2, 3);

This makes it so gcc's -Wformat and -Wformat-security options can do their jobs and cross-check format strings with the number and types of arguments.

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

Use of goto

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

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

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

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)

Libvirt committer guidelines

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

The general rule for committing a patch is to make sure it has been reviewed properly in the mailing-list first, usually if a couple of people gave an ACK or +1 to a patch and nobody raised an objection on the list it should be good to go. If the patch touches a part of the code where you're not the main maintainer, or where you do not have a very clear idea of how things work, it's better to wait for a more authoritative feedback though. Before committing, please also rebuild locally, run 'make check syntax-check', and make sure you don't raise errors. Try to look for warnings too; for example, configure with 

  --enable-compile-warnings=error

which adds -Werror to compile flags, so no warnings get missed

An exception to 'review and approval on the list first' is fixing failures to build: