gnome-devel-docs 3.28.0-1 / usr / share / help / C / programming-guidelines / c-coding-style.page

<page xmlns="http://projectmallard.org/1.0/"
      xmlns:its="http://www.w3.org/2005/11/its"
      type="topic"
      id="c-coding-style">

  <info>
    <link type="guide" xref="index#general-guidelines"/>

    <credit type="author copyright">
      <name>Federico Mena-Quintero</name>
      <email its:translate="no">federico@gnome.org</email>
      <years>2013</years>
    </credit>
    <credit type="author copyright">
      <name>The GTK+ Team</name>
    </credit>

    <include href="cc-by-sa-3-0.xml" xmlns="http://www.w3.org/2001/XInclude"/>

    <desc>Our guidelines for C code in GNOME</desc>
  </info>

  <title>C Coding Style</title>

  <p>
    This document presents the preferred coding style for C programs
    in GNOME.  While coding style is very much a matter of taste, in
    GNOME we favor a coding style that promotes consistency,
    readability, and maintainability.
  </p>

  <p>
    We present examples of good coding style as well as examples of
    bad style that is not acceptable in GNOME.  Please try to submit
    patches that conform to GNOME’s coding style; this indicates that
    you have done your homework to respect the project’s goal of
    long-term maintainability.  Patches with GNOME’s coding style will
    also be easier to review!
  </p>

  <note>
    <p>
      This document is for C code.  For other languages, check the
      <link xref="index">main page</link> of the GNOME Programming
      Guidelines.
    </p>
  </note>

  <p>
    These guidelines are heavily inspired by GTK’s CODING-STYLE
    document, the Linux Kernel’s CodingStyle, and the GNU Coding
    Standards.  These are slight variations of each other, with
    particular modifications for each project’s particular needs and
    culture, and GNOME’s version is no different.
  </p>

  <section id="most-important-rule">
    <title>The Single Most Important Rule</title>

    <p>
      The single most important rule when writing code is this:
      <em>check the surrounding code and try to imitate it</em>.
    </p>

    <p>
      As a maintainer it is dismaying to receive a patch that is
      obviously in a different coding style to the surrounding code.
      This is disrespectful, like someone tromping into a spotlessly-clean
      house with muddy shoes.
    </p>

    <p>
      So, whatever this document recommends, if there is already
      written code and you are patching it, keep its current style
      consistent even if it is not your favorite style.
    </p>
  </section>

  <section id="line-width">
    <title>Line Width</title>

    <p>
      Try to use lines of code between 80 and 120 characters long.
      This amount of text is easy to fit in most monitors with a
      decent font size.  Lines longer than that become hard to read,
      and they mean that you should probably restructure your code.
      If you have too many levels of indentation, it means that you
      should fix your code anyway.
    </p>
  </section>

  <section id="indentation">
    <title>Indentation</title>

    <p>
      In general there are two preferred indentation styles for code
      in GNOME.
    </p>

    <list type="ordered">
      <item>
	<p>
	  Linux Kernel style.  Tabs with a length of 8 characters are
	  used for the indentation, with K&amp;R brace placement:
	</p>

	<code style="valid">
for (i = 0; i &lt; num_elements; i++) {
	foo[i] = foo[i] + 42;

	if (foo[i] &lt; 35) {
		printf ("Foo!");
		foo[i]--;
	} else {
		printf ("Bar!");
		foo[i]++;
	}
}</code>
      </item>

      <item>
	<p>
	  GNU style.  Each new level is indented by 2 spaces,
	  braces go on a line by themselves, and they are indented as
	  well.
	</p>

	<code style="valid">
for (i = 0; i &lt; num_elements; i++)
  {
    foo[i] = foo[i] + 42;

    if (foo[i] &lt; 35)
      {
        printf ("Foo!");
        foo[i]--;
      }
    else
      {
        printf ("Bar!");
        foo[i]++;
      }
  }</code>
      </item>
    </list>


    <p>
      Both styles have their pros and cons.  The most important things
      is to <em>be consistent</em> with the surrounding code.  For
      example, the GTK+ library, which is GNOME’s widget toolkit, is
      written with the GNU style.  Nautilus, GNOME’s file manager, is
      written in Linux kernel style.  Both styles are perfectly
      readable and consistent when you get used to them.
    </p>

    <p>
      Your first feeling when having to study or work on a piece of
      code that doesn’t have your preferred indentation style may be,
      how shall we put it, gut-wrenching.  You should resist your
      inclination to reindent everything, or to use an inconsistent
      style for your patch.  Remember the first rule:  <em>be
      consistent</em> and respectful of that code’s customs, and your
      patches will have a much higher chance of being accepted without
      a lot of arguing about the right indentation style.
    </p>
  </section>

  <section id="tab-characters">
    <title>Tab Characters</title>

    <p>
      <em>Do not ever change the size of tabs in your editor</em>;
      leave them as 8 spaces.  Changing the size of tabs means that
      code that you didn’t write yourself will be perpetually misaligned.
    </p>

    <p>
      Instead, set the <em>indentation size</em> as appropriate for
      the code you are editing.  When writing in something other than
      Linux kernel style, you may even want to tell your editor to
      automatically convert all tabs to 8 spaces, so that there is no
      ambiguity about the intended amount of space.
    </p>
  </section>

  <section id="braces">
    <title>Braces</title>

    <p>
      Curly braces should not be used for single statement blocks:
    </p>

<code style="valid">
/* valid */
if (condition)
	single_statement ();
else
	another_single_statement (arg1);</code>

	<p>
	  The “no block for single statements” rule has only four
	  exceptions:
	</p>

	<list type="ordered">
          <item>
            <p>
              In GNU style, if either side of an if-else statement has
              braces, both sides should, to match up indentation:
            </p>

<code style="valid">
/* valid GNU style */
if (condition)
  {
    foo ();
    bar ();
  }
else
  {
    baz ();
  }</code>

<code style="invalid">
/* invalid */
if (condition)
  {
    foo ();
    bar ();
  }
else
  baz ();</code>
          </item>

	  <item>
	    <p>
	      If the single statement covers multiple lines, e.g. for functions with
	      many arguments, and it is followed by <code>else</code> or
	      <code>else if</code>:
	    </p>

<code style="valid">
/* valid Linux kernel style */
if (condition) {
	a_single_statement_with_many_arguments (some_lengthy_argument,
						another_lengthy_argument,
						and_another_one,
						plus_one);
} else
	another_single_statement (arg1, arg2);

/* valid GNU style */
if (condition)
  {
    a_single_statement_with_many_arguments (some_lengthy_argument,
                                            another_lengthy_argument,
                                            and_another_one,
                                            plus_one);
  }
else
  {
    another_single_statement (arg1, arg2);
  }</code>
          </item>

          <item>
            <p>
              If the condition is composed of many lines:
            </p>

<code style="valid">
/* valid Linux kernel style */
if (condition1 ||
    (condition2 &amp;&amp; condition3) ||
    condition4 ||
    (condition5 &amp;&amp; (condition6 || condition7))) {
	a_single_statement ();
}

/* valid GNU style */
if (condition1 ||
    (condition2 &amp;&amp; condition3) ||
    condition4 ||
    (condition5 &amp;&amp; (condition6 || condition7)))
  {
    a_single_statement ();
  }</code>

            <p>
              Note that such long conditions are usually hard to understand.  A
              good practice is to set the condition to a boolean variable, with
              a good name for that variable.  Another way is to move the long
              condition to a function.
            </p>
          </item>

          <item>
            <p>
              Nested <code>if</code>s, in which case the block should be placed
              on the outermost <code>if</code>:
            </p>

<code style="valid">
/* valid Linux kernel style */
if (condition) {
	if (another_condition)
		single_statement ();
	else
		another_single_statement ();
}

/* valid GNU style */
if (condition)
  {
    if (another_condition)
      single_statement ();
    else
      another_single_statement ();
  }</code>

<code style="invalid">
/* invalid */
if (condition)
	if (another_condition)
		single_statement ();
	else if (yet_another_condition)
		another_single_statement ();</code>
          </item>
        </list>

        <p>
          In general, new blocks should be placed on a new indentation level,
          like this:
        </p>

        <code style="valid">
int retval = 0;

statement_1 ();
statement_2 ();

{
	int var1 = 42;
	gboolean res = FALSE;

	res = statement_3 (var1);

	retval = res ? -1 : 1;
}</code>

        <p>
          While curly braces for function definitions should rest on a
          new line they should not add an indentation level:
        </p>

        <code style="valid">
/* valid Linux kernel style*/
static void
my_function (int argument)
{
	do_my_things ();
}

/* valid GNU style*/
static void
my_function (int argument)
{
  do_my_things ();
}</code>

<code style="invalid">
/* invalid */
static void
my_function (int argument) {
	do_my_things ();
}

/* invalid */
static void
my_function (int argument)
  {
    do_my_things ();
  }</code>
  </section>

  <section id="conditions">
    <title>Conditions</title>

    <p>
      Do not check boolean values for equality.  By using implicit
      comparisons, the resulting code can be read more like conversational
      English.  Another rationale is that a ‘true’ value may not be necessarily
      equal to whatever the <code>TRUE</code> macro uses.  For example:
    </p>

    <code style="invalid">
/* invalid */
if (found == TRUE)
	do_foo ();

/* invalid */
if (found == FALSE)
	do_bar ();</code>

    <code style="valid">
/* valid */
if (found)
	do_foo ();

/* valid */
if (!found)
	do_bar ();</code>

    <p>
      The C language uses the value 0 for many purposes.  As a numeric value,
      the end of a string, a null pointer and the <code>FALSE</code> boolean.
      To make the code clearer, you should write code that highlights the
      specific way 0 is used.  So when reading a comparison, it is possible to
      know the variable type.  For boolean variables, an implicit comparison is
      appropriate because it’s already a logical expression.  Other variable
      types are not logical expressions by themselves, so an explicit
      comparison is better:
    </p>

    <code style="valid">
/* valid */
if (some_pointer == NULL)
	do_blah ();

/* valid */
if (number == 0)
	do_foo ();

/* valid */
if (str != NULL &amp;&amp; *str != '\0')
	do_bar ();</code>

    <code style="invalid">
/* invalid */
if (!some_pointer)
	do_blah ();

/* invalid */
if (!number)
	do_foo ();

/* invalid */
if (str &amp;&amp; *str)
	do_bar ();</code>
  </section>

  <section id="functions">
    <title>Functions</title>

    <p>
      Functions should be declared by placing the returned value on a
      separate line from the function name:
    </p>

    <code style="valid">
void
my_function (void)
{
  …
}</code>

    <p>
      The argument list must be broken into a new line for each
      argument, with the argument names right aligned, taking into
      account pointers:
    </p>

    <code style="valid">
void
my_function (some_type_t      type,
             another_type_t  *a_pointer,
             double_ptr_t   **double_pointer,
             final_type_t     another_type)
{
  …
}</code>

    <p>
      If you use Emacs, you can use <code>M-x align</code> to do this
      kind of alignment automatically.  Just put the point and mark
      around the function’s prototype, and invoke that command.
    </p>

    <p>
      The alignment also holds when invoking a function without breaking the
      line length limit:
    </p>

    <code style="valid">
align_function_arguments (first_argument,
                          second_argument,
                          third_argument);</code>
  </section>

  <section id="whitespace">
    <title>Whitespace</title>

    <p>
      Always put a space before an opening parenthesis but never after:
    </p>

    <code style="valid">
/* valid */
if (condition)
	do_my_things ();

/* valid */
switch (condition) {
}</code>

<code style="invalid">
/* invalid */
if(condition)
	do_my_things();

/* invalid */
if ( condition )
	do_my_things ( );</code>

    <p>
      When declaring a structure type use newlines to separate logical sections
      of the structure:
    </p>

    <code style="valid">
struct _GtkWrapBoxPrivate
{
	GtkOrientation        orientation;
	GtkWrapAllocationMode mode;

	GtkWrapBoxSpreading   horizontal_spreading;
	GtkWrapBoxSpreading   vertical_spreading;

	guint16               vertical_spacing;
	guint16               horizontal_spacing;

	guint16               minimum_line_children;
	guint16               natural_line_children;

	GList                *children;
};</code>

    <p>
      Do not eliminate whitespace and newlines just because something would
      fit on a single line:
    </p>

    <code style="invalid">
/* invalid */
if (condition) foo (); else bar ();</code>

    <p>
      Do eliminate trailing whitespace on any line, preferably as a separate
      patch or commit. Never use empty lines at the beginning or at the end of
      a file.
    </p>

    <p>
      This is a little Emacs function that you can use to clean up
      lines with trailing whitespace:
    </p>

    <code>
(defun clean-line-ends ()
  (interactive)
  (if (not buffer-read-only)
      (save-excursion
	(goto-char (point-min))
	(let ((count 0))
	  (while (re-search-forward "[ 	]+$" nil t)
	    (setq count (+ count 1))
	    (replace-match "" t t))
	  (message "Cleaned %d lines" count)))))</code>
  </section>

  <section id="switch">
    <title>The <code>switch</code> Statement</title>

    <p>
      A <code>switch</code> should open a block on a new
      indentation level, and each <code>case</code> should start on
      the same indentation level as the curly braces, with the case
      block on a new indentation level:
    </p>

    <code style="valid">
/* valid Linux kernel style */
switch (condition) {
case FOO:
	do_foo ();
	break;

case BAR:
	do_bar ();
	break;
}

/* valid GNU style */
switch (condition)
  {
  case FOO:
    do_foo ();
    break;

  case BAR:
    do_bar ();
    break;
  }</code>

<code style="invalid">
/* invalid */
switch (condition) {
  case FOO: do_foo (); break;
  case BAR: do_bar (); break;
}

/* invalid */
switch (condition)
  {
  case FOO: do_foo ();
    break;
  case BAR: do_bar ();
    break;
  }

/* invalid */
switch (condition)
  {
    case FOO:
    do_foo ();
    break;
    case BAR:
    do_bar ();
    break;
  }</code>

    <p>
      It is preferable, though not mandatory, to separate the various
      cases with a newline:
    </p>

    <code style="valid">
switch (condition) {
case FOO:
	do_foo ();
	break;

case BAR:
	do_bar ();
	break;

default:
	do_default ();
}</code>

    <p>
      The <code>break</code> statement for the <code>default</code> case is not
      mandatory.
    </p>

    <p>
      If switching over an enumerated type, a <code>case</code> statement must
      exist for every member of the enumerated type. For members you do not
      want to handle, alias their <code>case</code> statements to
      <code>default</code>:
    </p>

    <code style="valid">
switch (enumerated_condition) {
case HANDLED_1:
	do_foo ();
	break;

case HANDLED_2:
	do_bar ();
	break;

case IGNORED_1:
case IGNORED_2:
default:
	do_default ();
}</code>

    <p>
      If most members of the enumerated type should not be handled, consider
      using an <code>if</code> statement instead of a <code>switch</code>.
    </p>

    <p>
      If a <code>case</code> block needs to declare new variables, the same rules as the
      inner blocks apply (see above); the <code>break</code> statement should be placed
      outside of the inner block:
    </p>

    <code style="valid">
/* valid GNU style */
switch (condition)
  {
  case FOO:
    {
      int foo;

      foo = do_foo ();
    }
    break;

  …
  }</code>
  </section>

  <section id="header-files">
    <title>Header Files</title>

    <p>
      The only major rule for headers is that the function definitions
      should be vertically aligned in three columns:
    </p>

    <code style="valid">
return_type          function_name           (type   argument,
                                              type   argument,
                                              type   argument);</code>

    <p>
      The maximum width of each column is given by the longest element
      in the column:
    </p>

    <code style="valid">
void         gtk_type_set_property (GtkType      *type,
                                    const gchar  *value,
                                    GError      **error);
const gchar *gtk_type_get_property (GtkType      *type);</code>

    <p>
      It is also possible to align the columns to the next tab:
    </p>

    <code style="valid">
void          gtk_type_set_prop           (GtkType *type,
                                           gfloat   value);
gfloat        gtk_type_get_prop           (GtkType *type);
gint          gtk_type_update_foobar      (GtkType *type);</code>

    <p>
      As before, you can use <code>M-x align</code> in Emacs to do
      this automatically.
    </p>

    <p>
      If you are creating a public library, try to export a single
      public header file that in turn includes all the smaller header
      files into it.  This is so that public headers are never
      included directly; rather a single include is used in
      applications.  For example, GTK+ uses the following in its
      header files that should not be included directly by
      applications:
    </p>

    <code style="valid">
#if !defined (__GTK_H_INSIDE__) &amp;&amp; !defined (GTK_COMPILATION)
#error "Only &lt;gtk/gtk.h&gt; can be included directly."
#endif</code>

    <p>
      For libraries, all headers should have inclusion guards (for
      internal usage) and C++ guards.  These provide the <code>extern
      "C"</code> magic that C++ requires to include plain C headers:
    </p>

    <code style="valid">
#ifndef MYLIB_FOO_H_
#define MYLIB_FOO_H_

#include &lt;gtk/gtk.h&gt;

G_BEGIN_DECLS

…

G_END_DECLS

#endif /* MYLIB_FOO_H_ */</code>
  </section>

  <section id="gobject">
    <title>GObject Classes</title>

    <p>
      GObject class definitions and implementations require some
      additional coding style notices, and should always be
      <link xref="namespacing#gobject">correctly namespaced</link>.
    </p>

    <p>
      Typedef declarations should be placed at the beginning of the file:
    </p>

    <code style="valid">
typedef struct _GtkBoxedStruct       GtkBoxedStruct;
typedef struct _GtkMoreBoxedStruct   GtkMoreBoxedStruct;</code>

    <p>
      This includes enumeration types:
    </p>

    <code style="valid">
typedef enum
{
  GTK_SIZE_REQUEST_WIDTH_FOR_HEIGHT,
  GTK_SIZE_REQUEST_HEIGHT_FOR_WIDTH
} GtkSizeRequestMode;</code>

    <p>
      And callback types:
    </p>

    <code style="valid">
typedef void (* GtkCallback) (GtkWidget *widget,
                              gpointer   user_data);</code>

    <p>
      Instance structures should be declared using
      <code>G_DECLARE_FINAL_TYPE</code> or
      <code>G_DECLARE_DERIVABLE_TYPE</code>:
    </p>

    <code style="valid">
#define GTK_TYPE_FOO (gtk_foo_get_type ())
G_DECLARE_FINAL_TYPE (GtkFoo, gtk_foo, GTK, FOO, GtkWidget)</code>

    <p>
      For final types, private data can be stored in the object struct, which
      should be defined in the C file:
    </p>

    <code style="valid">
struct _GtkFoo
{
  GObject   parent_instance;

  guint     private_data;
  gpointer  more_private_data;
};</code>

    <p>
      For derivable types, private data must be stored in a private struct in
      the C file, configured using <code>G_DEFINE_TYPE_WITH_PRIVATE()</code>
      and accessed using a <code>_get_instance_private()</code> function:
    </p>

    <code style="valid">
#define GTK_TYPE_FOO gtk_foo_get_type ()
G_DECLARE_DERIVABLE_TYPE (GtkFoo, gtk_foo, GTK, FOO, GtkWidget)

struct _GtkFooClass
{
  GtkWidgetClass parent_class;

  void (* handle_frob)  (GtkFrobber *frobber,
                         guint       n_frobs);

  gpointer padding[12];
};</code>

    <p>
      Always use the <code>G_DEFINE_TYPE()</code>,
      <code>G_DEFINE_TYPE_WITH_PRIVATE()</code>, and
      <code>G_DEFINE_TYPE_WITH_CODE()</code> macros, or their abstract variants
      <code>G_DEFINE_ABSTRACT_TYPE()</code>,
      <code>G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE()</code>,
      and <code>G_DEFINE_ABSTRACT_TYPE_WITH_CODE()</code>; also, use the similar
      macros for defining interfaces and boxed types.
    </p>

    <p>
      Interface types should always have the dummy typedef for cast
      purposes:
    </p>

    <code style="valid">
typedef struct _GtkFooable          GtkFooable;</code>

    <p>
      The interface structure should have ‘Interface’ postfixed to the
      dummy typedef:
    </p>

    <code style="valid">
typedef struct _GtkFooableInterface     GtkFooableInterface;</code>

    <p>
      Interfaces must have the following macros:
    </p>

    <table>
      <thead>
	<tr>
	  <td><p>Macro</p></td>
	  <td><p>Expands to</p></td>
	</tr>
      </thead>
      <tbody>
	<tr>
	  <td><p><code>GTK_TYPE_<var>iface_name</var></code></p></td>
	  <td><p><code><var>iface_name</var>_get_type</code></p></td>
	</tr>
	<tr>
	  <td><p><code>GTK_<var>iface_name</var></code></p></td>
	  <td><p><code>G_TYPE_CHECK_INSTANCE_CAST</code></p></td>
	</tr>
	<tr>
	  <td><p><code>GTK_IS_<var>iface_name</var></code></p></td>
          <td><p><code>G_TYPE_CHECK_INSTANCE_TYPE</code></p></td>
	</tr>
	<tr>
	  <td><p><code>GTK_<var>iface_name</var>_GET_IFACE</code></p></td>
          <td><p><code>G_TYPE_INSTANCE_GET_INTERFACE</code></p></td>
	</tr>
      </tbody>
    </table>

  </section>

  <section id="memory-allocation">
    <title>Memory Allocation</title>

    <p>
      When dynamically allocating data on the heap use <code>g_new()</code>.
    </p>

    <p>
      Public structure types should always be returned after being
      zero-ed, either explicitly for each member, or by using
      <code>g_new0()</code>.
    </p>

    <p>
      See <link xref="memory-management"/> for more details.
    </p>
  </section>

  <section id="macros">
    <title>Macros</title>

    <p>
      Try to avoid private macros unless strictly necessary. Remember
      to <code>#undef</code> them at the end of a block or a series of functions
      needing them.
    </p>

    <p>
      Inline functions are usually preferable to private macros.
    </p>

    <p>
      Public macros should not be used unless they evaluate to a
      constant.
    </p>
  </section>

  <section id="public-api">
    <title>Public API</title>

    <p>
      Avoid exporting variables as public API, since this is
      cumbersome on some platforms. It is always preferable to add
      getters and setters instead.  Also, beware global variables in
      general.
    </p>
  </section>

  <section id="private-api">
    <title>Private API</title>

    <p>
      Non-exported functions that are needed in more than one source file
      should be prefixed with an underscore (‘_’), and declared in a
      private header file.  For example, <code>_mylib_internal_foo()</code>.
    </p>

    <p>
      Underscore-prefixed functions are never exported.
    </p>

    <p>
      Non-exported functions that are only needed in one source file
      should be declared static.
    </p>
  </section>
</page>