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. Official upstream repository is kept in git (https://libvirt.org/git/libvirt.git) and is browsable along with other libvirt-related repositories (e.g. libvirt-python) online.

  3. Patches to translations are maintained via the zanata project. 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.

  4. The simplest way to send patches is to use the git-publish tool. All libvirt-related repositories contain a config file that tells git-publish to use the correct mailing list and subject prefix.

    Alternatively, you may send patches using git send-email.

    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
    

    More hints on compiling can be found here. When you want to post your patches:

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

    For a single patch you can omit --cover-letter, but a series of two or more patches needs a cover letter.

    Note that the git send-email 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 git send-email, you might need to configure it to point it to your SMTP server with something like:

      git config --global sendemail.smtpServer stmp.youremailprovider.net
    

    If you get tired of typing --to=libvir-list@redhat.com all the time, you can configure that to be automatically handled as well:

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

    As a rule, patches should be sent to the mailing list only: all developers are subscribed to libvir-list and read it regularly, so please don't CC individual developers unless they've explicitly asked you to.

    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.

    If everything went well, your patch should show up on the libvir-list archives 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. Note that, if you are not already a subscriber, your very first post to the mailing list will be subject to moderation, and it's not uncommon for that to take around a day.

    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), they are 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).

  5. 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.

  6. Contributors to libvirt projects must assert that they are in compliance with the Developer Certificate of Origin 1.1. 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.

  7. 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).

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

  9. Run the automated tests on your code before submitting any changes. That is:

      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.

    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:

      make check VIR_TEST_EXPENSIVE=1
    

    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
    

    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:

      VIR_TEST_DEBUG=1 VIR_TEST_RANGE=3-5,7-20,^16 ./run tests/qemuxml2argvtest
    

    Also, individual tests can be run from inside the tests/ directory, like:

      ./qemuxml2xmltest
    

    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.

      VIR_TEST_REGENERATE_OUTPUT=1 ./qemuxml2argvtest
    

    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.

    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 VIR_TEST_FILE_ACCESS environment variable. Then VIR_TEST_FILE_ACCESS_OUTPUT environment variable can alter location where the file is stored.

      VIR_TEST_FILE_ACCESS=1 VIR_TEST_FILE_ACCESS_OUTPUT="/tmp/file_access.txt" ./qemuxml2argvtest
    
  10. 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 acceptable 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*
    }
    
  11. Update tests and/or documentation, particularly if you are adding a new feature or changing the output of a program.

  12. Don't forget to update the release notes by changing docs/news.xml 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 docs/news.xml should be updated in its own commit not to get in the way of backports.

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.

Language Usage

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.

The preferred languages at this time are:

Languages that should not be used for any new contributions:

Tooling

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:

Naming conventions

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.

File names

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.

Enum type & field names

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.

    typedef enum _virSocketType virSocketType;
    enum _virSocketType {
        VIR_SOCKET_TYPE_IPV4,
        VIR_SOCKET_TYPE_IPV6,
    };
Struct type names

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.

    typedef struct _virHashTable virHashTable;
    typedef virHashTable *virHashTablePtr;
    struct _virHashTable {
       ...
    };
Function names

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'.

Macro names

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.

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, 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:

  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/

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

Commas

Commas should always be followed by a space or end of line, and never have leading space; this is enforced during 'make syntax-check'.

      call(a,b ,c);// Bad
      call(a, b, c); // Good

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.

      enum {
          VALUE_ONE,
          VALUE_TWO // Bad
      };
      enum {
          VALUE_THREE,
          VALUE_FOUR, // Good
      };

Semicolons

Semicolons should never have a space beforehand. Inside the condition of a for 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 while). While not enforced, loop counters generally use post-increment.

      for (i = 0 ;i < limit ; ++i) { // Bad
      for (i = 0; i < limit; i++) { // Good
      for (;;) { // ok
      while (1) { // Better

Empty loop bodies are better represented with curly braces and a comment, although use of a semicolon is not currently rejected.

      while ((rc = waitpid(pid, &st, 0) == -1) &&
             errno == EINTR); // ok
      while ((rc = waitpid(pid, &st, 0) == -1) &&
             errno == EINTR) { // Better
          /* nothing */
      }

Curly braces

Omit the curly braces around an if, while, for 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-statement loop: each has only one line in its body.

  while (expr)             // single line body; {} is forbidden
      single_line_stmt();
  while (expr(arg1,
              arg2))      // indentation makes it obvious it is single line,
      single_line_stmt(); // {} is optional (not enforced either way)
  while (expr1 &&
         expr2) {         // multi-line, at same indentation, {} required
      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;
  }

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.

  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 < b) {       // correct - hanging brace
              tmp = b;
              b = a;
              a = tmp;
          }
      }
  }

Conditional expressions

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:

   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)

New code should avoid the ternary operator as much as possible. Specifically it must never span more than one line or nest:

BAD:
    char *foo = baz ?
                virDoSomethingReallyComplex(driver, vm, something, baz->foo) :
                NULL;

    char *foo = bar ? bar->baz ? bar->baz->foo : "nobaz" : "nobar";

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.

Attribute annotations

Use the following annotations to help the compiler and/or static analysis tools understand the code better:

Macro Meaning
ATTRIBUTE_NONNULL passing NULL for this parameter is not allowed
ATTRIBUTE_PACKED force a structure to be packed
G_GNUC_FALLTHROUGH allow code reuse by multiple switch cases
G_GNUC_NO_INLINE the function is mocked in the test suite
G_GNUC_NORETURN the function never returns
G_GNUC_NULL_TERMINATED last parameter must be NULL
G_GNUC_PRINTF validate that the formatting string matches parameters
G_GNUC_UNUSED parameter is unused in this implementation of the function
G_GNUC_WARN_UNUSED_RESULT the return value must be checked

Adoption of GLib APIs

Libvirt has adopted use of the GLib library. 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.

The following is a list of libvirt APIs that should no longer be used in new code, and their suggested GLib replacements:

VIR_ALLOC, VIR_REALLOC, VIR_RESIZE_N, VIR_EXPAND_N, VIR_SHRINK_N, VIR_FREE, VIR_APPEND_ELEMENT, VIR_INSERT_ELEMENT, VIR_DELETE_ELEMENT
Prefer the GLib APIs g_new0/g_renew/ g_free in most cases. There should rarely be a need to use g_malloc/g_realloc. Instead of using plain C arrays, it is preferrable to use one of the GLib types, GArray, GPtrArray or GByteArray. These all use a struct to track the array memory and size together and efficiently resize. NEVER MIX 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.
virStrerror
The GLib g_strerror() function should be used instead, which has a simpler calling convention as an added benefit.
deprecated version GLib version Notes
VIR_ALLOC(var) g_new0(var_t, 1) the type needs to be passed explicitly
VIR_ALLOC_N g_new0(var_t, n)
VIR_REALLOC_N g_renew(var_t, ptr, n) the newly added memory is not zeroed
VIR_EXPAND_N g_renew(var_t, ptr, n) zero the new memory manually or use an array type
VIR_SHRINK_N g_renew(var_t, ptr, n)
VIR_APPEND_ELEMENT g_array_append_val g_ptr_array_add or g_byte_array_append
VIR_INSERT_ELEMENT g_array_insert_val g_ptr_array_insert
VIR_DELETE_ELEMENT g_array_remove_index g_ptr_array_remove_index or g_byte_array_remove_index
VIR_FREE g_free g_free does not zero the pointer

String allocation macros and functions:

deprecated version GLib version Notes
virAsprintf g_strdup_printf
virVasprintf g_strdup_vprint use g_vasprintf if you really need to know the returned length
virStrerror g_strerror the error strings are cached globally so no need to free it

The following libvirt APIs have been deleted already:

VIR_AUTOPTR, VIR_AUTOCLEAN, VIR_AUTOFREE
The GLib macros g_autoptr, g_auto and g_autofree 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 VIR_FREE. If introducing GLib macros to an existing method, any use of libvirt macros must be converted in an independent commit.
VIR_DEFINE_AUTOPTR_FUNC, VIR_DEFINE_AUTOCLEAN_FUNC
The GLib macros G_DEFINE_AUTOPTR_CLEANUP_FUNC and G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC must be used in all new code. Existing code should be converted to the new macros where relevant. It is permissible to use g_autoptr, g_auto on an object whose cleanup function is declared with the libvirt macros and vice-versa.
VIR_AUTOUNREF
The GLib macros g_autoptr and G_DEFINE_AUTOPTR_CLEANUP_FUNC should be used to manage autoclean of virObject classes. This matches usage with GObject classes.
VIR_STRDUP, VIR_STRNDUP
Prefer the GLib APIs g_strdup and g_strndup.
deleted version GLib version Notes
VIR_AUTOPTR g_autoptr
VIR_AUTOCLEAN g_auto
VIR_AUTOFREE g_autofree The GLib version does not use parentheses
VIR_AUTOUNREF g_autoptr The cleanup function needs to be defined
VIR_DEFINE_AUTOPTR_FUNC G_DEFINE_AUTOPTR_CLEANUP_FUNC
VIR_DEFINE_AUTOCLEAN_FUNC G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC
VIR_STEAL_PTR g_steal_pointer a = f(&b) instead of f(a, b)
VIR_RETURN_PTR return g_steal_pointer
ARRAY_CARDINALITY G_N_ELEMENTS
ATTRIBUTE_FALLTHROUGH G_GNUC_FALLTHROUGH
ATTRIBUTE_FMT_PRINTF G_GNUC_PRINTF
ATTRIBUTE_NOINLINE G_GNUC_NO_INLINE
ATTRIBUTE_NORETURN G_GNUC_NORETURN
ATTRIBUTE_RETURN_CHECK G_GNUC_WARN_UNUSED_RESULT
ATTRIBUTE_SENTINEL G_GNUC_NULL_TERMINATED
ATTRIBUTE_UNUSED G_GNUC_UNUSED
VIR_STRDUP g_strdup
VIR_STRNDUP g_strndup

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 replacement functions provided by libvirt:

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

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 <0 is returned instead.

  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.

  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.

    dst = g_strdup(src);
    dst = g_strndup(src, n);

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 g_strdup and g_strndup from GLib which abort on OOM and handle NULL source by returning NULL.

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 either the GString 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.

Typical usage is as follows:

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

     ...

     virBufferAddLit(&buf, "<domain>\n");
     virBufferAsprintf(&buf, "  <memory>%d</memory>\n", memory);
     if (some_error)
         return NULL; /* g_auto will free the memory used so far */
     ...
     virBufferAddLit(&buf, "</domain>\n");

     ...

     if (virBufferCheckError(&buf) < 0)
         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 virCommandAddEnvFormat in vircommand.h:

  void virCommandAddEnvFormat(virCommandPtr cmd, const char *format, ...)
      G_GNUC_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 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.

Error message format

Error messages visible to the user should be short and descriptive. All error messages are translated using gettext and thus must be wrapped in _() 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 %s can be NULL the NULLSTR macro must be used.

  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);

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. 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 g_autofree or g_autoptr which uses the compiler's __attribute__((cleanup)) that calls the appropriate free function when the variable goes out of scope.

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)

Top-level labels should be indented by one space (putting them on the beginning of the line confuses function context detection in git):

int foo()
{
    /* ... do stuff ... */
 cleanup:
    /* ... do other stuff ... */
}

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.

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