Federico Mena-Quintero
federico@gnome.org 2013
Η ομάδα GTK+
Οι οδηγίες μας για κώδικα C στο GNOME
Ελληνική μεταφραστική ομάδα GNOME
team@gnome.gr 2016
Θάνος Τρυφωνίδης
tomtryf@gnome.org 2016

# Στυλ κώδικα C

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.

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!

This document is for C code. For other languages, check the main page of the GNOME Programming Guidelines.

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.

# The Single Most Important Rule

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

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.

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.

# Πλάτος γραμμής

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.

# Εσοχή

Γενικά υπάρχουν δύο προτεινόμενα στυλ εσοχής κώδικα για το GNOME.

Το στυλ Linux Kernel. Στηλοθέτες με μήκος 8 χαρακτήρων για εσοχή, με τοποθέτηση αγκίστρων K&R:

for (i = 0; i < num_elements; i++) { foo[i] = foo[i] + 42;

if (foo[i] < 35) { printf ("Foo!"); foo[i]--; } else { printf ("Bar!"); foo[i]++; } }

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

for (i = 0; i < num_elements; i++) { foo[i] = foo[i] + 42;

if (foo[i] < 35) { printf ("Foo!"); foo[i]--; } else { printf ("Bar!"); foo[i]++; } }

Both styles have their pros and cons. The most important things is to be consistent 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.

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: be consistent 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.

# Χαρακτήρες στηλοθέτη

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

Instead, set the indentation size 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.

# Braces

Curly braces should not be used for single statement blocks:

/* valid */ if (condition) single_statement (); else another_single_statement (arg1);

The “no block for single statements” rule has only four exceptions:

In GNU style, if either side of an if-else statement has braces, both sides should, to match up indentation:

/* valid GNU style */ if (condition) { foo (); bar (); } else { baz (); }
/* invalid */ if (condition) { foo (); bar (); } else baz ();

If the single statement covers multiple lines, e.g. for functions with many arguments, and it is followed by `else` or `else if`:

/* 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); }

If the condition is composed of many lines:

/* valid Linux kernel style */ if (condition1 || (condition2 && condition3) || condition4 || (condition5 && (condition6 || condition7))) { a_single_statement (); }

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

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.

Nested `if`s, in which case the block should be placed on the outermost `if`:

/* 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 (); }
/* invalid */ if (condition) if (another_condition) single_statement (); else if (yet_another_condition) another_single_statement ();

In general, new blocks should be placed on a new indentation level, like this:

int retval = 0;

statement_1 (); statement_2 ();

{ int var1 = 42; gboolean res = FALSE;

res = statement_3 (var1);

retval = res ? -1 : 1; }

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

/* 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 (); }
/* invalid */ static void my_function (int argument) { do_my_things (); }

/* invalid */ static void my_function (int argument) { do_my_things (); }

# Συνθήκες

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 `TRUE` macro uses. For example:

/* invalid */ if (found == TRUE) do_foo ();

/* invalid */ if (found == FALSE) do_bar ();
/* valid */ if (found) do_foo ();

/* valid */ if (!found) do_bar ();

The C language uses the value 0 for many purposes. As a numeric value, the end of a string, a null pointer and the `FALSE` 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:

/* valid */ if (some_pointer == NULL) do_blah ();

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

/* valid */ if (str != NULL && *str != '\0') do_bar ();
/* invalid */ if (!some_pointer) do_blah ();

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

/* invalid */ if (str && *str) do_bar ();

# Συναρτήσεις

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

void my_function (void) { … }

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

void my_function (some_type_t type, another_type_t *a_pointer, double_ptr_t **double_pointer, final_type_t another_type) { … }

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

The alignment also holds when invoking a function without breaking the line length limit:

align_function_arguments (first_argument, second_argument, third_argument);

# Κενά διαστήματα

Προσθέστε πάντα ένα διάστημα πριν το άνοιγμα μιας παρένθεσης αλλά ποτέ μετά:

/* valid */ if (condition) do_my_things ();

/* valid */ switch (condition) { }
/* invalid */ if(condition) do_my_things();

/* invalid */ if ( condition ) do_my_things ( );

Όταν ορίζετε μια δομή χρησιμοποιήστε νέες γραμμές για να διαχωρίσετε τα λογικά κομμάτια της δομής:

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; };

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

/* invalid */ if (condition) foo (); else bar ();

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.

Παρακάτω ακολουθεί μια συνάρτηση Emacs με την οποία μπορείτε να καθαρίσετε τις γραμμές με κενά διαστήματα στο τέλος αυτής.

(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)))))

# Η συνθήκη `switch`

A `switch` should open a block on a new indentation level, and each `case` should start on the same indentation level as the curly braces, with the case block on a new indentation level:

/* 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; }
/* 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; }

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

switch (condition) { case FOO: do_foo (); break;

case BAR: do_bar (); break;

default: do_default (); }

The `break` statement for the `default` case is not mandatory.

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

switch (enumerated_condition) { case HANDLED_1: do_foo (); break;

case HANDLED_2: do_bar (); break;

case IGNORED_1: case IGNORED_2: default: do_default (); }

If most members of the enumerated type should not be handled, consider using an `if` statement instead of a `switch`.

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

/* valid GNU style */ switch (condition) { case FOO: { int foo;

foo = do_foo (); } break;

… }

# Κεφαλίδες

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

return_type function_name (type argument, type argument, type argument);

The maximum width of each column is given by the longest element in the column:

void gtk_type_set_property (GtkType *type, const gchar *value, GError **error); const gchar *gtk_type_get_property (GtkType *type);

It is also possible to align the columns to the next tab:

void gtk_type_set_prop (GtkType *type, gfloat value); gfloat gtk_type_get_prop (GtkType *type); gint gtk_type_update_foobar (GtkType *type);

As before, you can use `M-x align` in Emacs to do this automatically.

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:

#if !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION) #error "Only <gtk/gtk.h> can be included directly." #endif

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

#ifndef MYLIB_FOO_H_ #define MYLIB_FOO_H_

#include <gtk/gtk.h>

G_BEGIN_DECLS

…

G_END_DECLS

#endif /* MYLIB_FOO_H_ */

# Κλάσεις GObject

GObject class definitions and implementations require some additional coding style notices, and should always be correctly namespaced.

Typedef declarations should be placed at the beginning of the file:

typedef struct _GtkBoxedStruct GtkBoxedStruct; typedef struct _GtkMoreBoxedStruct GtkMoreBoxedStruct;

This includes enumeration types:

typedef enum { GTK_SIZE_REQUEST_WIDTH_FOR_HEIGHT, GTK_SIZE_REQUEST_HEIGHT_FOR_WIDTH } GtkSizeRequestMode;

Και τύπους επανάκλησης:

typedef void (* GtkCallback) (GtkWidget *widget, gpointer user_data);

Instance structures should be declared using `G_DECLARE_FINAL_TYPE` or `G_DECLARE_DERIVABLE_TYPE`:

#define GTK_TYPE_FOO (gtk_foo_get_type ()) G_DECLARE_FINAL_TYPE (GtkFoo, gtk_foo, GTK, FOO, GtkWidget)

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

struct _GtkFoo { GObject parent_instance;

guint private_data; gpointer more_private_data; };

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

#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]; };

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

Interface types should always have the dummy typedef for cast purposes:

typedef struct _GtkFooable GtkFooable;

The interface structure should have ‘Interface’ postfixed to the dummy typedef:

typedef struct _GtkFooableInterface GtkFooableInterface;

Interfaces must have the following macros:

Μακροεντολή | Expands to
--- | ---
`GTK_TYPE_iface_name` | `iface_name_get_type`
`GTK_iface_name` | `G_TYPE_CHECK_INSTANCE_CAST`
`GTK_IS_iface_name` | `G_TYPE_CHECK_INSTANCE_TYPE`
`GTK_iface_name_GET_IFACE` | `G_TYPE_INSTANCE_GET_INTERFACE`

# Memory Allocation

When dynamically allocating data on the heap use `g_new()`.

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

Για περισσότερες λεπτομέρειες δείτε .

# Μακροεντολές

Try to avoid private macros unless strictly necessary. Remember to `#undef` them at the end of a block or a series of functions needing them.

Inline functions are usually preferable to private macros.

Public macros should not be used unless they evaluate to a constant.

# Δημόσιο API

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.

# Ιδιωτικό API

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, `_mylib_internal_foo()`.

Underscore-prefixed functions are never exported.

Non-exported functions that are only needed in one source file should be declared static.