libvirt/docs/coding-style.rst

Coding style

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. :

typedef struct _virHashTable virHashTable;
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 https://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 -l100 -lc100 \
         -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.

The maximum permitted line length is 100 characters, but lines should aim to be approximately 80 characters.

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

Curly braces around an if, while, for etc. can be omitted if the body and the condition itself occupy only 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 optional
    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 comparisons may be shortened. All long forms are okay:

virFoo *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(WITH_POSIX_FALLOCATE) && !defined(WITH_FALLOCATE)
# define fallocate(a, ignored, b, c) posix_fallocate(a, b, c)
#endif

C types

Use the right type.

Scalars

• If you're using int or long, odds are good that there's a better type.
• If a variable is counting something, be sure to declare it with an unsigned type.
• If it's memory-size-related, use size_t (use ssize_t only if required).
• If it's file-size related, use uintmax_t, or maybe off_t.
• If it's file-offset related (i.e., signed), use off_t.
• If it's just counting small numbers use unsigned int; (on all but oddball embedded systems, you can assume that that type is at least four bytes wide).
• If a variable has boolean semantics, give it the bool type and use the corresponding true and false macros.
• In the unusual event that you require a specific width, use a standard type like int32_t, uint32_t, uint64_t, etc.
• While using bool is good for readability, it comes with minor caveats:
  • Don't use 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.
  • Don't compare a bool variable against the literal, 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.

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.

Defining Local Variables

Always define local variables at the top of the block in which they are used (before any pure code). Although modern C compilers allow defining a local variable in the middle of a block of code, this practice can lead to bugs, and must be avoided in all libvirt code. As indicated in these examples, it is okay to initialize variables where they are defined, even if the initialization involves calling another function.

GOOD:
  int
  bob(char *loblaw)
  {
      int x;
      int y = lawBlog();
      char *z = NULL;

      x = y + 20;
      ...
  }

BAD:
  int
  bob(char *loblaw)
  {
      int x;
      int y = lawBlog();

      x = y + 20;

      char *z = NULL; // <===
      ...
  }

Attribute annotations

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

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

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

• 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);

String comparisons

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)

• To avoid having to check if a or b are NULL:

  STREQ_NULLABLE(a, b)
  STRNEQ_NULLABLE(a, b)

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:

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");

   ...

   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:

• 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.
• 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.
• 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.

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 ... */
}

XML element and attribute naming

New elements and/or attributes should be short and descriptive. In general, they should reflect what the feature does instead of how exactly it is named in given hypervisor because this creates an abstraction that other drivers can benefit from (for instance if the same feature is named differently in two hypervisors). That is not to say an element or attribute can't have the same name as in a hypervisor, but proceed with caution.

Single worded names are preferred, but if more words must be used then they shall be joined in camelCase style.