K&R style

The K&R style (Kernighan & Ritchie Style), and the closely related "one true brace style" in hacker jargon^[5][6] (abbreviated as 1TBS^[7]), are commonly used in C, C++, and other curly-brace programming languages. It was the style used in the original Unix kernel, Kernighan and Ritchie's book The C Programming Language, as well as Kernighan and Plauger's book The Elements of Programming Style.

When following K&R, each function has its opening brace at the next line on the same indentation level as its header, the statements within the braces are indented, and the closing brace at the end is on the same indentation level as the header of the function at a line of its own.

Multi-line blocks inside a function, however, have their opening braces at the same line as their respective control statements; closing braces remain in a line of their own, unless followed by a keyword else or while. Such non-aligned braces are nicknamed "Egyptian braces" (or "Egyptian brackets") for their resemblance to arms in some fanciful poses of ancient Egyptians.^[8][9][10] In K&R, single-line blocks do not have braces, which is a possible source of easy-to-miss bugs when merging code (for example, see the goto fail bug).

int main(int argc, char *argv[])
{
    ...
    while (x == y) {
        something();
        something_else();

        if (some_error)
            do_correct(); // In K&R a single-statement block does not have braces.
        else
            continue_as_usual();
    }

    final_thing();
    ...
}

The C Programming Language does not explicitly specify this style, though it is followed consistently throughout the book. From the book:

The position of braces is less important, although people hold passionate beliefs. We have chosen one of several popular styles. Pick a style that suits you, then use it consistently.

In old versions of the C language, argument types needed to be declared on the subsequent line (i.e., just after the header of the function):

/* Original pre-ISO C style without function prototypes */
int main(argc, argv)
    int   argc;
    char  *argv[];
{
    ...
}

bool is_negative(x) {
    if (x < 0) {
        return(true);
    } else {
        return(false);
    }
}

int power(int x, int y)
{
        int result;

        if (y < 0) {
                result = 0;
        } else {
                result = 1;
                while (y-- > 0)
                        result *= x;
        }
        return result;
}

    if (x < 0) {
        puts("Negative");
        negative(x);
    }
    else {
        puts("Non-negative");
        nonnegative(x);
    }

    class Vector {
    public:
        Vector(int s) :elem(new double[s]), sz(s) { }   // construct a Vector
        double& operator[](int i) { return elem[i]; }   // element access: subscripting
        int size() { return sz; }
    private:
        double * elem;    // pointer to the elements
        int sz;           // number of elements
    };

while (x == y) {
        something();
        something_else();
}
final_thing();

if (data != NULL && res > 0) {
        if (JS_DefineProperty(cx, o, "data",
            STRING_TO_JSVAL(JS_NewStringCopyN(cx, data, res)),
            NULL, NULL, JSPROP_ENUMERATE) != 0) {
                QUEUE_EXCEPTION("Internal error!");
                goto err;
        }
        PQfreemem(data);
} else {
        if (JS_DefineProperty(cx, o, "data", OBJECT_TO_JSVAL(NULL),
            NULL, NULL, JSPROP_ENUMERATE) != 0) {
                QUEUE_EXCEPTION("Internal error!");
                goto err;
        }
}

static JSBool
pgresult_constructor(JSContext *cx, JSObject *obj, uintN argc,
    jsval *argv, jsval *rval)
{

        QUEUE_EXCEPTION("PGresult class not user-instantiable");

        return (JS_FALSE);
}

Allman style

The Allman style is named after Eric Allman. It is also sometimes termed BSD style since Allman wrote many of the utilities for BSD Unix (although this should not be confused with the different "BSD KNF style"; see above).

This style puts the brace associated with a control statement on the next line, indented to the same level as the control statement. Statements within the braces are indented to the next level.^[22]

while (x == y)
{
    something();
    something_else();
}

final_thing();

This style is similar to the standard indentation used by the Pascal languages and Transact-SQL, where the braces are equivalent to the keywords begin and end.

(* Example Allman code indentation style in Pascal *)
procedure dosomething(x, y: Integer);
begin
    while x = y do
    begin
        something();
        something_else();
    end;
end;

Consequences of this style are that the indented code is clearly set apart from the containing statement by lines that are almost all whitespace and the closing brace lines up in the same column as the opening brace. Some people feel this makes it easy to find matching braces. The blocking style also delineates the block of code from the associated control statement. Commenting out or removing a control statement or block of code, or code refactoring, are all less likely to introduce syntax errors via dangling or missing braces. Also, it is consistent with brace placement for the outer-function block.

For example, the following is still correct syntactically:

// while (x == y)
{
    something();
    something_else();
}

As is this:

// for (int i=0; i < x; i++)
// while (x == y)
if (x == y)
{
    something();
    something_else();
}

Even like this, with conditional compilation:

    int c;
#ifdef HAS_GETCH
    while ((c = getch()) != EOF)
#else
    while ((c = getchar()) != EOF)
#endif
    {
        do_something(c);
    }

Whitesmiths style

The Whitesmiths style, also sometimes termed Wishart style, was originally used in the documentation for the first commercial C compiler, the Whitesmiths Compiler. It was also popular in the early days of Windows, since it was used in three influential Windows programming books, Programmer's Guide to Windows by Durant, Carlson & Yao, Programming Windows by Petzold, and Windows 3.0 Power Programming Techniques by Norton & Yao.

Whitesmiths, along with Allman, have been the most common bracing styles with equal popularity according to the Jargon File.^[5]

This style puts the brace associated with a control statement on the next line, indented. Statements within the braces are indented to the same level as the braces.

Like Ratliff style, the closing brace is indented the same as statements within the braces.^[23]

while (x == y)
    {
    something();
    something_else();
    }

final_thing();

The advantages of this style are similar to those of the Allman style. Blocks are clearly set apart from control statements. The alignment of the braces with the block emphasizes that the full block is conceptually, and programmatically, one compound statement. Indenting the braces emphasizes that they are subordinate to the control statement. The ending brace no longer lines up with the statement, but instead with the opening brace.

An example:

if (data != NULL && res > 0)
    {
    if (!JS_DefineProperty(cx, o, "data", STRING_TO_JSVAL(JS_NewStringCopyN(cx, data, res)), NULL, NULL, JSPROP_ENUMERATE))
        {
        QUEUE_EXCEPTION("Internal error!");
        goto err;
        }
    PQfreemem(data);
    }
else if (!JS_DefineProperty(cx, o, "data", OBJECT_TO_JSVAL(NULL), NULL, NULL, JSPROP_ENUMERATE))
    {
    QUEUE_EXCEPTION("Internal error!");
    goto err;
    }

else if are treated as statement, much like the #elif preprocessor statement.

GNU style

Like the Allman and Whitesmiths styles, GNU style puts braces on a line by themselves, indented by two spaces, except when opening a function definition, where they are not indented.^[24] In either case, the contained code is indented by two spaces from the braces.

Popularised by Richard Stallman, the layout may be influenced by his background of writing Lisp code.^[25] In Lisp, the equivalent to a block (a progn) is a first-class data entity, and giving it its own indentation level helps to emphasize that, whereas in C, a block is only syntax. This style can also be found in some ALGOL and XPL programming language textbooks from the 1960s and 1970s.^[26][27] Although not directly related to indentation, GNU coding style also includes a space before the bracketed list of arguments to a function.

static char *
concat (char *s1, char *s2)
{
  while (x == y)
    {
      something ();
      something_else ();
    }
  final_thing ();
}

^[24]

This style combines the advantages of Allman and Whitesmiths, thereby removing the possible Whitesmiths disadvantage of braces not standing out from the block. One disadvantage is that the ending brace no longer lines up with the statement it conceptually belongs to. Another possible disadvantage is that it might waste space by using two visual levels of indents for one conceptual level, but in reality this is unlikely because, in systems with single-level indentation, each level is usually at least 4 spaces, same as 2 * 2 spaces in GNU style.

The GNU Coding Standards recommend this style, and nearly all maintainers of GNU project software use it.

The GNU Emacs text editor and the GNU systems' indent command will reformat code according to this style by default.^[28] Those who do not use GNU Emacs, or similarly extensible/customisable editors, may find that the automatic indentation settings of their editor are unhelpful for this style. However, many editors defaulting to KNF style cope well with the GNU style when the tab width is set to two spaces; likewise, GNU Emacs adapts well to KNF style by simply setting the tab width to eight spaces. In both cases, automatic reformatting destroys the original spacing, but automatic line indenting will work properly.

Steve McConnell, in his book Code Complete, advises against using this style: he marks a code sample which uses it with a "Coding Horror" icon, symbolizing especially dangerous code, and states that it impedes readability.^[23] The Linux kernel coding style documentation also strongly recommends against this style, urging readers to burn a copy of the GNU coding standards as a "great symbolic gesture".^[29]

Horstmann style

The 1997 edition of Computing Concepts with C++ Essentials by Cay S. Horstmann adapts Allman by placing the first statement of a block on the same line as the opening brace. This style is also used in examples in Jensen and Wirth's Pascal User Manual and Report.^[30]

while (x == y)
{   something();
    something_else();
    //...
    if (x < 0)
    {   printf("Negative");
        negative(x);
    }
    else
    {   printf("Non-negative");
        nonnegative(x);
    }
}
final_thing();

This style combines the advantages of Allman by keeping the vertical alignment of the braces for readability, and identifying blocks easily, with the saving of a line of the K&R style. However, the 2003 edition now uses Allman style throughout.^[31]

Pico style

This is the style used most commonly in the language Pico by its designers. Pico lacks return statements, and uses semicolons as statement separators instead of terminators. It yields this syntax:^[32]

stuff(n):
{ x: 3 * n;
  y: do_stuff(x);
  y + x }

The advantages and disadvantages are similar to those of saving screen real estate with K&R style. An added advantage is that the starting and closing braces are consistent in application (both share space with a line of code), relative to K&R style, where one brace shares space with a line of code and one brace has a line alone.

Ratliff style

In the book Programmers at Work,^[33] C. Wayne Ratliff discussed using the style below. The style begins much like 1TBS but then the closing brace lines up with the indentation of the nested block. Ratliff was the original programmer behind the popular dBase-II and -III fourth-generation programming languages. He indicated that it was originally documented in material from Digital Research Inc. This style has sometimes been termed banner style,^[34] possibly for the resemblance to a banner hanging from a pole. In this style, which is to Whitesmiths as K&R is to Allman, the closing control is indented the same as the last item in the list (and thus properly loses salience)^[23] The style can make visual scanning easier for some, since the headers of any block are the only thing exdented at that level (the theory being that the closing control of the prior block interferes with the visual flow of the next block header in the K&R and Allman styles). Kernighan and Plauger use this style in the Ratfor code in Software Tools.^[35]

 // In C
 for (i = 0; i < 10; i++) {
     if (i % 2 == 0) {
         do_something(i);
         }
     else {
         do_something_else(i);
         }
     }

or, in a markup language...

{|
|-
| lots of stuff...
      more stuff
      || alternative for short lines || etc. 
|}

{|
|-
... etc
|}

Styles from languages other than C

The following styles aren't conventionally used for C code, but are derived from language families which possess major differences in syntax and semantics from those in the C family. They may theoretically appear in C code written as part of a project mostly written in one of these other languages (for example, as part of a FFI), where maintaining a consistent "look and feel" to the project's codebase may locally override considerations of using conventional C style.

// In C
for (i = 0; i < 10; i++)
    {if (i % 2 == 0)
        {do_something(i);}
     else
        {do_something_else(i);
         do_third_thing(i);}}

# In Python
for i in range(10):
    if i % 2 == 0:
        do_something(i)
    else:
        do_something_else(i)
        do_third_thing(i)

;; In Lisp
(dotimes (i 10)
  (if (= (rem i 2) 0)
      (do-something i)
    (progn
      (do-something-else i)
      (do-third-thing i))))

braceless = do
  text <- getContents
  let
    firstWord = head $ words text
    bigWord = map toUpper firstWord
  putStrLn bigWord

braceful = do
  { text <- getContents
  ; let
      { firstWord = head $ words text
      ; bigWord = map toUpper firstWord
      }
  ; putStrLn bigWord
  }

life←{⊃1⍵∨.∧3 4=+/+⌿¯1 0 1∘.⊖¯1 0 1⌽¨⊂⍵}

Losing track of blocks

In some situations, there is a risk of losing track of block boundaries. This is often seen in large sections of code containing many compound statements nested to many levels of indentations. By the time the programmer scrolls to the bottom of a huge set of nested statements, they may have lost track of which control statements go where. However, overly-long code could have other causes, such as being too complex, and a programmer facing this problem might instead consider whether code refactoring would help in the longer term.

Programmers who rely on counting the opening braces may have difficulty with indentation styles such as K&R, where the starting brace is not visually separated from its control statement. Programmers who rely more on indentations will gain more from styles that are vertically compact, such as K&R, because the blocks are shorter.

To avoid losing track of control statements such as for, a large indentation can be used, such as an 8-unit-wide hard tab, along with breaking up large functions into smaller and more readable functions. Linux is done this way, while using the K&R style.

In text editors of the vi family, one means to track block boundaries is to position the text cursor over one of the braces, and press the % key. The cursor then jumps to the opposing brace. Since the text cursor's next key (viz., the n key) retained directional positioning information (whether the up or down key was formerly pressed), the dot macro (the . key) could then be used to place the text cursor on the next brace,^[40] given a suitable coding style. Instead, inspecting the block boundaries using the % key can be used to enforce a coding standard.

Another way is to use inline comments added after the closing brace:

for (int i = 0; i < total; i++) {
    foo(bar);
} //for (i)

if (x < 0) {
   bar(foo);
} //if (x < 0)

The major disadvantage of this method is maintaining duplicate code in multiple locations.

Another solution is implemented in a folding editor, which can hide or reveal blocks of code via their indentation level or compound-statement structure. Many editors will also highlight matching brackets or braces when the cursor is positioned next to one.

Statement insertion

K&R style prevents another common error suffered when using the standard Unix line editor, ed. A statement mistakenly inserted between the control statement and the opening brace of the loop block turns the body of the loop into a single trip.

for (int i = 0; i < 10; i++)
    whoops(bar);   /* repeated 10 times, with i from 0 to 9 */
{
    only_once();   /* Programmer intended this to be done 10 times */
} //for (i) ← This comment is no longer valid, and is very misleading!

K&R style avoids this problem by keeping the control statement and the opening brace on the same line.

Tabs and spaces

• Tabs versus Spaces: An Eternal Holy War by Jamie Zawinski
• Why I prefer no tabs in source code by Adam Spiers
• Why I love having tabs in source code (archived)
• Elastic tabstops – the solution to the tabs-versus-spaces issue