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Coding style

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__NOTOC__
 
The C language '''Coding style''' described here is based on the BSD coding style, with some additional elements from the GNU coding standards and the SunOS coding standards.
 
The C language '''Coding style''' described here is based on the BSD coding style, with some additional elements from the GNU coding standards and the SunOS coding standards.
   
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standards is mostly what we want, except we use BSD brace style and
 
standards is mostly what we want, except we use BSD brace style and
 
BSD-ish conventions for the spacing around operators.
 
BSD-ish conventions for the spacing around operators.
 
=== Formatting style for C code ===
 
 
(deleted moved content)
 
   
 
=== Coding practices for C ===
 
=== Coding practices for C ===
   
Assume, for most purposes, working ANSI/ISO C ('89, not '99) support,
 
both for internal use and for applications compiling against Kerberos
 
header files and libraries. Some exceptions are noted below.
 
 
Do not use assignments as truth values. Rather than this:
 
 
<pre>
 
/* bad style */
 
if ((retval = krb5_foo()))
 
/* ... */;
 
</pre>
 
 
do this:
 
 
<pre>
 
/* better style */
 
retval = krb5_foo();
 
if (retval)
 
/* ... */;
 
</pre>
 
 
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:
 
 
<pre>
 
while ((ch = getopt(argc, argv, "abn")) != -1)
 
/* ... */;
 
</pre>
 
 
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'). Yes, these are usually all technically the
 
integer zero. 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:
 
 
<pre>
 
int i;
 
char *cp;
 
/* ... */
 
if (i)
 
/* ... */;
 
if (cp != NULL) {
 
while (*cp != '\0')
 
/* ... */;
 
}
 
</pre>
 
 
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 section 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 members 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 their bodies if the body is
 
more than one line long, especially in the case of stacked multiple
 
if-else clauses; use:
 
 
<pre>
 
if (x) {
 
if (y)
 
foo();
 
else
 
bar();
 
}
 
</pre>
 
 
instead of:
 
 
<pre>
 
/* bad style */
 
if (x)
 
if (y)
 
foo();
 
else
 
bar();
 
</pre>
 
 
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:
 
 
<pre>
 
/* bad style */
 
if (x) {
 
f();
 
}
 
#ifdef FOO
 
else if (y) {
 
#else
 
else {
 
#endif
 
g();
 
}
 
</pre>
 
 
Put comments after conditional compilation directives such as "#else"
 
and "#endif". Make them correspond to the sense of the value that
 
controls the compilation of the section they are closing, i.e.
 
 
<pre>
 
#ifdef FOO
 
/* ... */
 
#else /* !FOO */
 
/* ... */
 
#endif /* !FOO */
 
</pre>
 
 
Also, in the case of more complex conditional compilation directives,
 
write the comments like this:
 
 
<pre>
 
#if defined(FOO) || defined(BAR)
 
/* ... */
 
#else /* !(defined(FOO) || defined(BAR)) */
 
/* ... */
 
#endif /* !(defined(FOO) || defined(BAR)) */
 
</pre>
 
 
If you are writing a do-while loop that has only one statement in its
 
body, put braces around it anyway, since the while clause may be
 
mistaken for a while loop with an empty body. Don't do this:
 
 
<pre>
 
/* bad style */
 
do
 
foo();
 
while (x);
 
</pre>
 
 
Instead, write this:
 
 
<pre>
 
/* better style */
 
do {
 
foo();
 
} while (x);
 
</pre>
 
 
While it is syntactically correct to call through a function pointer
 
without applying a dereference operator to it, do not write code that
 
does this. It is easier to see that the function call is actually
 
taking place through a function pointer if you write an explicit
 
dereference. However, do not explicitly take the address of a
 
function in order to assign it to a function pointer, since a function
 
name degrades into a pointer. Thus:
 
 
 
<pre>
 
int (*fp)(void);
 
int foofunc(void);
 
fp = foofunc;
 
x = (*fp)();
 
</pre>
 
 
In general, do not take the address of an array. It does not return a
 
pointer to the first element; it returns a pointer to the array
 
itself. These are often identical when cast to an integral type, but
 
they are inherently of different types themselves. Functions that
 
take array types or pointers to array types as arguments can be
 
particularly trouble-prone.
 
 
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.
 
 
Always declare the return type of a function, even if it returns int.
 
Yes, this means declaring main() to return int, since main() is
 
required to return int by the standard. If a function is not supposed
 
to return a value, declare it as returning void rather than omitting
 
the return type, which will default the return type to int.
 
 
Try to use ANSI C prototype-style function definitions in preference
 
to K&R style definitions. When using K&R style function definitions,
 
declare all the argument types, even those that are int, but beware of
 
any narrow types in the argument list.
 
 
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, but still, always parenthesize
 
the operand of the sizeof operator. The sizeof operator does not
 
evaluate its operand if it is a unary expression, so usages such as
 
 
<pre>
 
s = sizeof(++foo);
 
</pre>
 
 
should be avoided for the sake of sanity and readability.
 
 
Don't pass around structures except by address. We may relax this
 
restriction for non-API function, though.
 
 
For new functions, input parameters should go before output parameters
 
in the call signature. There are exceptions, such as a context-like
 
parameter.
 
 
Every function should have block comment preceding it describing
 
briefly in complete sentences what it does, what inputs and outputs it
 
has, and what error codes it can return. It should also describe any
 
unsual aspects of the function. At some point we will want to put
 
some of this information into a machine-parsable form.
 
 
Macros should have all-uppercase names. If it is necessary to use
 
multiple statements, use braces, and wrap the whole thing in a
 
do-while(0) construct, such as
 
 
<pre>
 
#define FOOMACRO(x, y) do { \
 
foo = (x) + (y); \
 
f(y); \
 
} while (0)
 
</pre>
 
 
Leave off the semicolon at the end of a function-like macro, so that
 
it can be mostly used like a call to a function without a return
 
value. Line up the backslashes to make it more readable. Use M-x
 
c-backslash-region in emacs to do neat lined-up backslashes.
 
Parenthesize uses of arguments in the replacement text of a macro in
 
order to prevent weird interactions.
 
 
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 structure member,
 
etc.
 
 
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_".
 
Use "krb5int_" as a prefix for library internal globals. Avoid using
 
"__" in symbol names, as it may confuse C++ implementations. 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_". In general, do not define
 
macros that are lowercase, in order to avoid confusion and to prevent
 
namespace collisions.
 
 
The C standard only guarantees six case-insensitive characters to be
 
significant in external identifiers; this is largely regarded as
 
obsolescent even in 1989 and we will ignore it. It does, however,
 
only guarantee 31 case-sensitive characters to be signficant in
 
internal identifiers, so do not use identifiers that differ beyond the
 
31st character. This is unlikely to be a problem, though.
 
   
 
=== Aspects of C style in GNU coding std but not here ===
 
=== Aspects of C style in GNU coding std but not here ===
Line 449: Line 119:
 
in the gnu locker does not currently handle -psl correctly though.
 
in the gnu locker does not currently handle -psl correctly though.
   
  +
<pre>
 
-bap
 
-bap
 
-br
 
-br
Line 468: Line 139:
 
-sc
 
-sc
 
-sob
 
-sob
+
</pre>
== MAKEFILES ==
 
 
[XXX needs to be written]
 
 
== TEST SUITES ==
 
 
[XXX needs to be written]
 

Revision as of 19:48, 7 January 2009

The C language Coding style described here is based on the BSD coding style, with some additional elements from the GNU coding standards and the SunOS coding standards.

External links

Old version

Old content to be moved elsewhere is below.


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.

Coding practices for C

Aspects of C style in GNU coding std but not here

  • redundant parens to force extra indent of operators of different precedences
  • redundant parens to force general extra indent of expressions that are broken between lines
  • use of ^L characters to break up source files into pages
  • nitpicking about capitalization in comments of variable names when their values are meant
  • commenting usages of static variables
  • casts to void
  • separation of word in names with underscores vs case change
  • enum vs #define'd integer constants
  • 14 char filename limits, MS-DOS filename limits
  • portability
  • system library function quirks
  • internationalization
  • mmap()

Aspects of C style in BSD KNF but not here

  • sorting of header files
  • sorting of struct members
  • separating struct tag decl and struct typedef
  • sorting of var decl
  • lining up var names in decls
  • newline after decls
  • usage of __P
  • usage of getopt
  • not initializing vars in decls
  • stdarg/varargs handling

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
-ip4
-l79
-nbc
-ncdb
-ndj
-nfc1
-lp
-npcs
-psl
-sc
-sob
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