path: root/doc/coding-style

WRITING C CODE
==============

The code in the krb5 source tree largely follows BSD KNF
(/usr/share/misc/style on NetBSD) except that it uses a four column
basic offset.  The style described here is a synthesis of BSD KNF and
the GNU coding standards for the C language.  The formatting described
in the "Formatting Your Source Code" section of the GNU coding
standards is mostly what we want, except we use BSD brace style and
BSD-ish conventions for the spacing around operators.

Formatting style for C code
---------------------------

In general, use a four column basic offset, tab stops at eight
columns.  Indents should be tabified, i.e. continuous tabs followed by
spaces if necessary if the indent isn't a multiple of eight columns.
The "bsd" style in emacs cc-mode mostly does the right thing.  You can
use "M-x c-set-style" "bsd" to get this.  Alternatively, you can use
the "krb5" style that is included here.

Labels, including case labels, are outdented by four columns.
Continuations of statements are indented by an additional four
columns.  Continuations of argument lists or parenthesized expressions
should line up with the column after the opening parenthesis.

The maximum width should be 79 columns.  If you need more than this,
consider rewriting the code so that it fits in 79 columns, since
control flow that is nested deeply enough to require excessive width
is also likely to be difficult to understand if not broken up.
Exceptions may be made for long strings, though ANSI C string
concatenation should work around that as well.

Function names for definitions should start on column zero, on the
line following their return type name, e.g.

	char *
	foo(int a)
	{
	    /* ... */
	}

[just pretend that's really at column zero]

The opening brace of a function definition should also be on column
zero.

Braces that open substatements, such as those following "if", "else",
"while", "for", "do", and "switch", should be on the same line as the
begining of the statement.  This is sometimes called "hanging" braces.
The corresponding closing brace should be at the same indentation
level as the beginning of the statement.

The "while" keyword in a do-while construct should sit on the same
line as the closing brace of the substatement following "do":

	do {
	    foo();
	} while (0);

If there is an "if" statement immediately following an "else" keyword,
it should go on the same line immediately after the "else":

	if (x) {
	    foo();
	} else if (y) {
	    bar();
	}

Comments to the right of code start in column 32.  Comments not to the
right of code are indented at the prevailing indent for the
surrounding code.  Make the comments complete sentences.  If you need
more than one line, make them into block comments, like this:

	/*
	 * This is a block comment.  It should consist of complete
	 * sentences.
	 *
	 * Paragraphs should be separated by blank lines so that emacs
	 * fill commands will work properly.
	 */

Really important single-line comments should also be done in block
form:

	/*
	 * This is a really important one-line comment.
	 */

In order to get the start and end delimiters for block comments to
stay when you use emacs to fill paragraphs in the comments, set both
the c-hanging-comment-starter-p and the c-hanging-comment-ender-p
variables to nil.  This will be done by the tentative "krb5" style for
the emacs cc-mode.

Spaces go after keywords, but not after function names.  Do not,
however, put a space after sizeof if its argument is
parenthesized. [XXX from BSD KNF but do we want to do this?]  Don't
put a space after a cast operator, either.  Spaces also do not go
before parentheses that are argument lists for function calls if there
is a function pointer being dereferenced somewhere.  Spaces go after
commas in argument lists, as well as commas that are comma operators.
Spaces also go between parts in a for loop, except for "forever" type
loops.  Use for statements rather than while statements to create
forever loops.

	if (x) {
	    p = calloc(1024, sizeof(int));
	}
	cp = (elem->fp)(1024);
	for (i = 0; i < 10; i++);
	for (;;) {
	    /* ... */
	}

Binary operators get spaces, unary ones do not.  Prefix and postfix
operators also do not get spaces.

	x = --a + b / c - d++;
	y = p->z.v[x];

In general, do not parenthesize the argument of "return".  

Coding practices for C
----------------------

Do not use assignments as truth values.  Rather than this:

	/* bad style */
	if ((retval = krb5_foo()))
	    /* ... */;

do this:

	retval = krb5_foo();
	if (retval)
	    /* ... */;

This makes the code easier to read, and also makes it easier to use
debuggers.  It may be excusable to put assignments into the
conditional espression of a "while" statement, though, like:

	while ((ch = getopt(argc, argv, "abn")) != -1)
	    /* ... */;

Using assignments as truth values in conditional expressions may make
code particularly impenetrable.

There are at least three types of "zero" known to C.  These are the
integer zero (0), the null pointer constant (NULL), and the character
constant zero ('\0').  Use them in their correct contexts.  (Purists
will point out that 0 is a valid null pointer constant; still, do not
use 0 to specify a null pointer constant.  For further unconfusion,
read the section on null pointer constants in the C FAQ.)  Do not use
a lone variable as a truth value unless it's of integer type.  Thus:

	int i;
	char *cp;
	/* ... */
	if (i)
	    /* ... */;
	if (cp != NULL) {
	    while (*cp != '\0')
		/* ... */;
	}

Do not cast uses of NULL unless you're calling a function with a
variable number of arguments, in which case you should cast it to to
the appropriate pointer type.  Likewise, do not cast the return value
from malloc() and friends; the prototype should declare them properly
as returning a void * and thus shouldn't require an explicit cast.

Do not assume that realloc(NULL, size) will do the right thing, or
that free(NULL) will do the right thing.  ANSI guarantees that it
will, but some old libraries (hopefully becoming obsolete) don't.
Also, don't assume that malloc(0) will return a non-NULL pointer.
Typically, though, the output of malloc(0) will be safe to pass to
realloc() and free().

In any case, reading the secion in the C FAQ on null pointers is
highly recommended to remove confusion regarding null pointers in C,
since this is a subject of much confusion to even experienced
programmers.  In particular, if you do not understand why using
calloc() to allocate a struct that contains pointer fields or why
calling memset() to initialize such a struct to all-bytes-zero is
wrong, reread that section again.  (Note that there are *lots* of
examples of code in the krb5 source tree that erroneously calls
memset() to zero a struct, and we should fix these somehow
eventually.)

Control flow statements that have a single statement as their body
should nevertheless have braces around ther bodies if the body is more
than one line long, especially in the case of stacked multiple if-else
clauses; use:

	if (x) {
	    if (y)
		foo();
	    else
		bar();
	}

instead of:

	/* bad style */
	if (x)
	    if (y)
		foo();
	    else
		bar();

which, while legible to the compiler, may confuse human readers and
make the code less maintainable, especially if new branches get added
to any of the clauses.

Also, you should almost never intersperse conditional compilation
directives with control flow statements, as some combination of
#define'd symbols may result in statements getting eaten by dangling
bits of control flow statements.  When it is not possible to avoid
this questionable practice (you really should rewrite the relevant
code section), make use of redundant braces to ensure that a compiler
error will result in preference to incorrect runtime behavior (such as
inadvertantly providing someone with a root shell).

Do not intersperse conditional compilation directives with control
flow statements in such a way that confuses emacs cc-mode.  Not only
does emacs get confused, but the code becomes more difficult to read
and maintain.  Therefore, avoid code like this:

	    /* bad style */
	    if (x) {
		f();
	    }
	#ifdef FOO
	    else if (y) {
	#else
	    else {
	#endif
		g();
	    }

If a function is declared to return a value, do not call "return"
without an argument or allow the flow of control to fall off the end
of the function.

Do not declare variables in an inner scope, e.g. inside the compound
substatement of an if statement, unless the complexity of the code
really demands that the variables be declared that way.  In such
situations, the function could probably stand to be broken up into
smaller chunks anyway.  Do not declare variables in an inner scope
that shadow ones in an outer scope, since this leads to confusion.
Also, some debugging environments, such as gdb under Solaris, can't
see variables declared in an inner scope, so declaring such variables
will make maintenance more difficult as well.

Parenthesize expressions that may be confusing, particularly where C's
precedences are broken.  For example, the shift operators have lower
precedence than the +, -, *, /, and % operators.  Perhaps the most
familiar C precedence quirk is that equality and relational operators
are of higher precedence than assignment operators.  Less well known
is that the bitwise operators are of a lower precedence than equality
and relational operators.

The sizeof operator takes either a unary expression or a parenthesized
type name.  It is not necessary to parenthesize the operand of sizeof
if it is applied to a unary expression.  [XXX do we want to force
parenthesization of expressions used as operands to sizeof?]  The
sizeof operator does not evaluate its operand if it is a unary
expression, so usages such as

	s = sizeof ++foo;

should be avoided for the sake of sanity and readability.

Strive to make your code capable of compiling using "gcc -Wall
-Wmissing-prototypes -Wtraditional -Wcast-qual -Wcast-align
-Wconversion -Waggregate-return -pedantic" [XXX need to rethink this
somewhat] without generating any errors or warnings.  Do not, however,
compile using the "-ansi" flag to gcc, since that can result in odd
behavior with header files on some systems, causing some necessary
symbols to not be defined.

Namespaces
----------

The C standard reserves a bunch of namespaces for the implementation.
Don't stomp on them.  For practical purposes, any identifier with a
leading underscore should not be used.  (Technically, ^_[a-z].* are
reserved only for file scope, so should be safe for things smaller
than file scope, but it's better to be paranoid in this case.)

POSIX reserves typedef names ending with _t as well.

Recall that errno is a reserved identifier, and is permitted to be a
macro.  Therefore, do not use errno as the name of a field.

Reserved namespaces are somewhat more restricted than this; read the
appropriate section of the C standard if you have questions.

If you're writing new library code, pick a short prefix and stick with
it for any identifier with external linkage.  If for some reason a
library needs to have external symbols that should not be visible to
the application, pick another (related) prefix to use for the internal
globals.  This applies to typedef names, tag names, and preprocessor
identifiers as well.

For the krb5 library, the prefix for public global symbols is
"krb5_".  It is a matter of debate whether to use "krb5__" or
"krb5int_" as a prefix for library internal globals.  There are
admittedly a number of places where we leak thing into the namespace;
we should try to fix these.

Header files should also not leak symbols.  Usually using the upcased
version of the prefix you've picked will suffice, e.g. "KRB5_" as a
CPP symbol prefix corresponding to "krb5_".

Emacs cc-mode style
-------------------

Putting the following code in your .emacs file will result in mostly
the right thing happening with respect to formatting style.  Note that
you may want to turn on auto-newline feature of cc-mode, though that
seems to have some bugs with brace-elseif-brace handling at least in
the version of cc-mode that comes with emacs 20.3.

	(defconst krb5-c-style
	  '("bsd" 
	    (c-cleanup-list
	     brace-elseif-brace brace-else-brace defun-close-semi)
	    (c-comment-continuation-stars . "* ")
	    (c-electric-pound-behavior alignleft)
	    (c-hanging-braces-alist
	     (brace-list-open)
	     (class-open after)
	     (substatement-open after)
	     (block-close . c-snug-do-while)
	     (extern-lang-open after))
	    (c-hanging-colons-alist
	     (case-label after)
	     (label after))
	    (c-hanging-comment-starter-p)
	    (c-hanging-comment-ender-p)
	    (c-indent-comments-syntactically-p . t)
	    (c-label-minimum-indentation . 0)
	    (c-special-indent-hook)))
	(defun krb5-c-hook ()
	  (c-add-style "krb5" krb5-c-style t))
	(add-hook 'c-mode-common-hook 'krb5-c-hook)

indent.pro settings
-------------------

The following settings for the indent program should produce a
reasonable approximation to the C coding style described here, though
some manual cleanup may be necessary.  Note that the gindent installed
in the gnu locker does not currently handle -psl correctly though.

-bap
-br
-ce
-ci4
-cli0
-d0
-di8
-i4
-ip
-l79
-nbc
-ncdb
-ndj
-nfc1
-lp
-npcs
-psl
-sc
-sob