BIND 9 Coding Style

Last modified: October 20, 2014

BIND 9 Coding Style

BIND 9 is principally written in C, with some additional code written in PythonPerl and Bourne shell. Style guidelines for each of these are below.

C

Compiler

An ANSI standard C compiler and library are assumed. Feel free to use any ANSI C feature.

Warnings

Given a reasonable set of things to warn about (e.g. -W -Wall for gcc), the goal is to compile with no warnings.

Copyright Notices

All source files should have a copyright. The copyright year(s) should be kept current. The files and the copyright year(s) should be listed in util/copyrights. When an existing file is updated in the source repository, its copyright notice and dates are updated automatically.

Line Formatting

Use tabs for indentation. Spaces before statements are only allowed when needed to line up a continued expression. In the following example, spaces used for indentation are indicated with "_":

    if (i == 0) {
            printf("this is going to be %s very long %s statementn",
            _______"a", "printf");
    }

Text editors should be configured with tabstop set to 8 characters, and tabs should not be expanded to into spaces. The following vim settings conform well to BIND 9 C style:

    set showmatch
    set showmode
    set autoindent
    set expandtab

    filetype plugin on
    let c_syntax_for_h = 1
    autocmd FileType c,cc,cpp set cindent
    autocmd FileType c,cc,cpp set cino=(0:0l1
    autocmd FileType c,cc,cpp set fo=rotcq
    autocmd FileType c,cc,cpp set noexpandtab ts=8
    autocmd FileType python set ts=4 sw=4

    filetype indent on

Vertical Whitespace

Vertical whitespace is encouraged for improved code legibility: closely related statements should be grouped, and then the groups separated with a single empty line. There should never be two or more empty lines adjacent to one another.

Line Length

Lines should be no longer than 79 characters, even if it requires violating indentation rules to make them fit. Since ANSI C is assumed, the best way to deal with strings that extend past column 79 is to break them into two or more sections separated from each other by a newline and indentation:

                              puts("This string got very far to the "
                                   "right and wrapped.  ANSI catenation "
                                   "rules will turn this into one "
                                   "long string.");

Comments

Comments should be used whenever they improve the readability or comprehensibility of the code. Comments describing public functions are usually in the header file below the function prototype; comments describing static functions are above the function declaration.

Comments may be single-line or multiline. A single-line comment should be at the end of the line if there is other text on the line, and should start in the same column as other nearby end-of-line comments. The comment should be at the same indentation level as the code it is referring to.

Multiline comments should start with "/*" on a line by itself. Subsequent lines should have " *" lined-up with the "*" above. The end of the comment should be " */" on a line by itself, again with the "*" lined-up with the one above. Comments should start with a capital letter and end with a period.

Good:

    /*
     * Private variables.
     */

    static int              a               /* Description of 'a'. */
    static int              b               /* Description of 'b'. */
    static char *           c               /* Description of 'c'. */

The following lint and lint-like comments should be used where appropriate:

    /* ARGSUSED */
    /* FALLTHROUGH */
    /* NOTREACHED */
    /* VARARGS */

Header files

.h files should not rely on other files having been included. .h files should prevent multiple inclusion. The OS is assumed to prevent multiple inclusion of its .h files.

A header file defining a public interface is generally placed in the source tree two levels below the C file that implements the interface. For example, the include file defining the interface for lib/dns/zone.c is inlib/dns/include/dns/zone.h. (The second “dns” in the path enables the file to be included via "#include <dns/zone.h>".)

Public header files should include interface documentation in Doxygen format.

Private header files, describing interfaces that are for internal use within a library but not for public use, are kept in the source tree at the same level as their related C files, and often have "_p" in their names, e.g.lib/isc/task_p.h.

Header files that define modules should have a structure like the following. Note that <isc/lang.h> MUST be included by any public header file using the ISCLANGBEGINDECLS and ISCLANGENDDECLS macros, so the correct name-mangling happens for function declarations when C++ programs include the file. <isc/lang.h> SHOULD be included for private header files or for public files that do not declare any functions.

    /*
     * Copyright (C) 2014  Internet Software Consortium.
     *
     * Permission to use, copy, modify, and distribute this
     * software for any purpose with or without fee is hereby
     * granted, provided that the above copyright notice and this
     * permission notice appear in all copies.
     *
     * THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE
     * CONSORTIUM DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS
     * SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
     * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL INTERNET
     * SOFTWARE CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT,
     * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
     * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
     * WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
     * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE
     * USE OR PERFORMANCE OF THIS SOFTWARE.
     */

    #ifndef ISC_WHATEVER_H
    #define ISC_WHATEVER_H 1

    /*****
     ***** Module Info
     *****/

    /*
     * (Module name here.)
     *
     * (One line description here.)
     *
     * (Extended description and notes here.)
     *
     * MP:
     *  (Information about multiprocessing considerations
     *  here, e.g. locking requirements.)
     *
     * Reliability:
     *  (Any reliability concerns should be mentioned here.)
     *
     * Resources:
     *  (A rough guide to how resources are used by this module.)
     *
     * Security:
     *  (Any security issues are discussed here.)
     *
     * Standards:
     *  (Any standards relevant to the module are listed here.)
     */

    /***
     *** Imports
     ***/

    /* #includes here. */
    #include <isc/lang.h>

    /***
     *** Types
     ***/

    /* (Type definitions here.) */

    /***
     *** Functions
     ***/
    ISC_LANG_BEGINDECLS
    /* (Function declarations here, with full prototypes.) */
    ISC_LANG_ENDDECLS

    #endif /* ISC_WHATEVER_H */

Including Interfaces (.h files)

The first file to be included in a C source file must be config.h. The config.h file must never be included by any public header file (that is, any header file that will be installed by "make install").

Try to include only necessary files, not everything under the sun. Operating-system-specific files should not be included by most modules; if they are needed, they should be used with #ifdef and controlled byconfigure.

Statements

There should be at most one statement per line. The comma operator should not be used to form compound statements.

Bad:

    if (i > 0) {
        printf("yesn"); i = 0; j = 0;
        x = 4, y *= 2;
    }

Functions

The use of ANSI C function prototypes is required.

The return type of the function should be listed on a line by itself when specifying the implementation of the function. The opening curly brace should occur on the same line as the argument list, unless the argument list is more than one line long:

    static inline void
    func1(int i) {
        /* whatever */
    }

    int
    func2(int first_argument, int next_argument,
          int last_argument)
    {
        /* whatever */
    }

To suppress compiler warnings, unused function arguments must be declared within the function via the UNUSED() macro.

In the function body, local variable declarations must be at the beginning of the function, followed by any REQUIRE() statements, then UNUSED() declarations, then all other code, in that order. These sections should be separated by blank lines.

Curly Braces

Curly Braces do not get their own indentation.

An opening brace does not start a new line. The statements enclosed by the braces should not be on the same line as the opening or closing brace. A closing brace should be the only thing on the line, unless it’s part of an else clause.

Generally speaking, when a control statement (e.g., iffor or while) has only a single action associated with it, then no bracing is used around the statement. Exceptions include when the compiler would complain about an ambiguous else clause, or when extra bracing improves readability or safety.

Good:

   static void
   f(int i) {
           if (i > 0) {
                   printf("yesn");
                   i = 0;
           } else
                   printf("non");
   }

Bad:

   void f(int i)
     {
       if(i<0){i=0;printf("was negativen");}
       if (i > 0)
         {
           printf("yesn");
           i = 0;
         }}

Spaces

  • DO put a space between operators like =+==, etc.
  • DO put a space after ,.
  • DO put a space after ; in a for statement.
  • DO put spaces after C reserved words such as ifforwhile, and do.
  • DO put a space after return, and parenthesize the return value.
  • Do NOT put a space between a variable or function name and ( or [.
  • Do NOT put a space after the sizeof operator name, and DO parenthesize its argument: malloc(4 * sizeof(long)).
  • Do NOT put a space immediately after a ( or immediately before a ), unless it improves readability. The same goes for [ and ].
  • Do NOT put a space before ++ or -- when used in post-increment/ decrement mode, or after them when used in pre-increment/decrement mode.
  • Do NOT put a space before ; when terminating a statement or in a for statement.
  • Do NOT put a space after * when used to dereference a pointer, or on either side of ->.
  • Do NOT put a space after ~.
  • The | operator may either have a space on both sides or it may have no spaces, depending on readability. Either way, if the | operator is used more than once in a statement, then the spacing must be consistent.

Return Values

If a function returns a value, it should be cast to (void) if you don’t care what the value is, except for printf and its variants, fputcfwrite (when writing text), fflushmemmovememsetstrcpystrncpy, and strcat.

Certain functions will return values or not depending on the operating system or even compiler flags; these include openlog and srandom. The return value of these should not be used nor cast to (void).

All error conditions must be handled.

Mixing of error status and valid results within a single type should be avoided.

Good:

    os_result_t result;
    os_descriptor_t s;

    result = os_socket_create(AF_INET, SOCK_STREAM, 0, &s);
    if (result != OS_R_SUCCESS) {
        /* Do something about the error. */
        return;
    }

Not so good:

    int s;

    /*
     * Obviously using interfaces like socket() (below) is allowed
     * since otherwise you couldn't call operating system routines; the
     * point is not to write more interfaces like them.
     */
    s = socket(AF_INET, SOCK_STREAM, 0);
    if (s < 0) {
        /* Do something about the error using errno. */
        return;
    }

Integral Types

Careful thought should be given to whether an integral type should be signed or unsigned, and to whether a specific size is required. int should be used for generic variables (e.g. iteration counters, array subscripts). Other than for generic variables, if a negative value isn’t meaningful, the variable should be unsigned. Assignments and comparisons between signed and unsigned integers should be avoided; suppressing the warnings with casts is not desireable.

Typedefs are provided to specify particular sizes of integral variables, e.g., isc_uint32_t (unsigned 32-bit integer), isc_int16_t (signed 16-bit integer). These may be used when unsigned long or short could be ambiguous.

Clear Success or Failure

A function should report success or failure, and do so accurately. It should never fail silently. Use of design by contract (described under “system architecture”) can help here.

When a function is designed to return results to the caller by assigning to caller variables through pointer arguments, it should perform the assignment only if it succeeds, and leave the variables unmodified if it fails. AREQUIRE() statement should be used to ensure that the pointer is in a sane state when the function is called.

The isc_result_t is provided for use by result codes. See the results section of the developer information page for more details.

Testing Bits

Bit testing should be as follows:

Good:

    /* Test if flag set. */
    if ((flags & FOO) != 0) {

    }
    /* Test if flag clear. */
    if ((flags & BAR) == 0) {

    }
    /* Test if both flags set. */
    if ((flags & (FOO|BAR)) == (FOO|BAR)) {

    }

Bad:

    /* Test if flag set. */
    if (flags & FOO) {

    }
    /* Test if flag clear. */
    if (! (flags & BAR)) {

    }

Testing for Zero or Non-zero

Explicit testing against zero is required for numeric, non-boolean variables.

Good:

    int i = 10;

    /* ... */

    if (i != 0) {
        /* Do something. */
    }

Bad:

    int i = 10;

    /* ... */

    if (i) {
        /* Do something. */
    }

Null Pointer

The null pointer value should be referred to as NULL, not 0.

Testing to see whether a pointer is NULL should be an explicit comparison; do not treat a pointer variable as if it were a boolean.

Good:

    char *c = NULL;

    /* ... */

    if (c != NULL) {
        /* Do something. */
    }

Bad:

    char *c = NULL;

    /* ... */

    if (c) {
        /* Do something. */
    }

The Ternary Operator

The ?: operator should mostly be avoided. It is tolerated when deciding what value to pass as a parameter to a function, such as frequently happens with printf, and also when a simple (non-compound) value is being used in assignment or as part of a calculation.

If a statement containing a ternary operator spills over more than one line, put the ? and : at the beginning of the following lines with two additional spaces of indent.

Using the ternary operator to specify a return value is very rarely permissible, and never when returning result codes.

Good:

    printf("%c is%s a number.n", c, isdigit(c) ? "" : " NOT");
    l = (l1 < l2) ? l1 : l2;
    s = (a_very_long_variable < an_even_longer_variable)
          ? "true"
          : "false";
    if (gp.length + (go < 16384 ? 2 : 3) >= name->length) {
        /* whatever */
    }

Okay:

    return ((length1 < length2) ? -1 : 1);

Bad:

    return (success ? ISC_R_SUCCESS : ISC_R_FAILURE);

Assignment in Parameters

Variables should not have their values assigned or changed when being passed as parameters, except perhaps for the increment and decrement operators.

Bad:

    isc_mem_get(mctx, size = 20);

Okay:

    fputc(c++, stdout);

Invalidating Pointers

When the data a pointer points to has been freed, or is otherwise no longer valid, the pointer should be set to NULL unless the pointer is part of a structure which is itself going to be freed immediately.

Good:

    char *text;

    /* text is initialized here. */

    isc_mem_free(mctx, text);
    text = NULL;

Public Interface Namespace

All public interfaces to functions, macros, typedefs, and variables provided by the library, should use names of the form {library}_{module}_{what}, such as:

    isc_buffer_t                            /* typedef */
    dns_name_setbuffer(name, buffer)        /* function */
    ISC_LIST_HEAD(list)                     /* macro */
    isc_commandline_argument                /* variable */

Structures which are typedef‘d generally have the name of the typedef sans the final _t:

    typedef struct dns_rbtnode dns_rbtnode_t;
    struct dns_rbtnode {
        /* ... members ... */
    }

In some cases, structures are specific to a single C file and are opaque outside that file. In these cases, the typedef occurs in the associated header file, but the structure definition in the C file itself. Examples of this include the zone object dns_zone_t; the structure is only acessable via get/set functions in lib/dns/zone.c. Other times, structure members can be accessed from outside the C file where they are implemented; examples include dns_view_t. Which way to implement a particular object is up to the developer’s discretion.

Generally speaking, macros are defined with all capital letters, but this is not universally consistent (eg, numerous isc_buffer_{foo} macros).

The {module} and {what} segments of the name do not have underscores separating natural word elements, as demonstrated in isc_commandline_argument and dns_name_setbuffer above. The {module} part is usually the same as the basename of the source file, but sometimes other {module} interfaces appear within one file, such as dns_label_* interfaces in lib/dns/name.c. However, in the public libraries the file name must be the same as some module interface provided by the file; e.g., dns_rbt_* interfaces would not be declared in a file named redblack.c (in lieu of any other dns_redblack_* interfaces in the file).

The one notable exception to this naming rule is the interfaces provided by <isc/util.h>. There’s a large caveat associated with the public description of this file that it is hazardous to use because it pollutes the general namespace.

When the signature of a public function needs to change, the old function name should be retained for backward compatibility, if at all possible. For example, when dns_zone_setfile() needed to include a file format parameter, it was changed to dns_zone_setfile2(); the original function name became a wrapper for the new function, calling it with the default value of the format parameter:

    isc_result_t
    dns_zone_setfile(dns_zone_t *zone, const char *file) {
            return (dns_zone_setfile2(zone, file, dns_masterformat_text);
    }

    isc_result_t
    dns_zone_setfile2(dns_zone_t *zone, const char *file,
                      dns_masterformat_t format)
    {
            ...
    }

Shared Private Interfaces

When a module provides an interface for internal use by other modules in the library or by unit tests, it should use the same naming convention described for the public interfaces, except {library} and {module} are separated by a double-underscore. This indicates that the name is internal, its API is not as formal as the public API, and thus it might change without any sort of notice. Examples of this usage includedns__zone_loadpending() and isc__taskmgr_ready().

In many cases, a public interface is instantiated by a private back-end implementation. The double-underscore naming style is sometimes used in that situation; for example, isc_task_attach() calls the attach function provided by a task API implementation; in BIND 9, this function is provided by isc__task_attach().

Other times, private interface implementations are static functions that are pointed to by “method” tables. For example, the dns_db interface is implemented in several places, including lib/dns/rbtdb.c (the red-black tree database used for internal storage of zones and cache data) and lib/dns/sdlz.c (an interface to DLZ modules). An object of type dns_dbmethods_t is created for each of these, containing function pointers to the local implementations of each of the dns_db API functions. The dns_db_findnode() function is provided by static functions called findnode() in each file, and so on.

Initialization

When an object is allocated from the heap, all fields in the object must be initialized.

Dead Code Pruning

Source which becomes obsolete should be removed, not just disabled with #if 0 ... #endif.

Portability

When using a C library function, consider whether all operating systems support it. Is it in the POSIX standard? If so, how long has it been there? (BIND is still run on some operating systems released in the 1990s.) Is its behavior the same on all platforms? Is its signature the same? Are integer parameters the same size and signedness? Does it alwasy return the same values on success, and set the same errno codes on failure?

If there is a chance the library call may not be completely portable, edit configure.in to check for it on the local system and only call it from within a suitable #ifdef. If the function is nonoptional, it may be necessary to add your own implentation of it (or copy one from a source with a BSD-compatible license).

BIND provides portable internal versions of many common library calls. Some are designed to ensure that library calls have standardized ISC result codes instead of using potentially nonwportable errno values; these include the file operations in isc_file and isc_stdio. Others, such as isc_tm_strptime(), are needed to ensure consistent cross-platform behavior. Others simply provide needed functions on platforms that don’t have them: for example, isc_string_strlcpy() is an implementation of the BSD-specific strlcpy() function. On Linux and systems without a strlcpy() function, it is #defined to isc_string_strlcpy()

In some cases, UNIX and Windows implementations of functions are kept in separate files, such as lib/isc/unix/file.c and lib/isc/win32/file.c.

Some notes on standard functions

  • Always use memmove() rather than memcpy().
  • If using snprintf() in a source file, be sure it includes <isc/print.h>

Log messages

Error and warning messages should be logged through the logging system. Debugging printfs may be used during development, but must be removed when the debugging is finished.

Log messages do not start with a capital letter, nor do they end in a period, and they are not followed by newlines.

When variable text such as a file name or domain name occurs as part of a log message, it should be enclosed in single quotes, as in “zone ‘%s’ is lame”.

When the variable text forms a separate phrase, such as when it separated from the rest of the message by a colon, it can be left unquoted:

isc_log_write(... "open: %s: %s", filename, isc_result_totext(result));

File names (__FILE__), line numbers (__LINE__), function names, memory addresses, and other references to program internals may be used in debugging messages and in messages to report programming errors detected at runtime. They may not be used in messages that indicate errors in the program’s inputs or operation.

Python

BIND 9 contains some optional tools written in Python, in the bin/python subdirectory. Python scripts are stored in the git repository as {toolname}.py.in; and {toolname}.py will be generated by configure (which determines, among other things, the path to the Python interpreter).

For Python coding, we abide by the Python style guidelines described here, with a few modifications:

  • The __init__() method should always be the first one declared in a class definition, like so:
    class Foo:
        # constructor definition here
        def __init__(self):
            ...
        # other functions may follow
        def bar(self):
            ...
            Close all file and socket objects
    
  • All Python standard library objects that have an underlying file descriptor (fd) should be closed explicitly using the .close() method.
  • In cases where a file is opened and closed in a single block, it is often preferable to use the with statement:
    with open('filename') as f:
        do_something_with(f)
    

Perl

Perl is NOT required for building, installing, or using the BIND 9 name server. However, BIND 9 may use Perl for its system test environment, for certain optional server add-on components, and in some cases for generating source files (such as bind9.xsl.h, converted from bind9.xsl) which are then committed to to the git repository.

Perl 5 is assumed; Perl scripts do not need to work in Perl 4.

Perl source code should follow the conventions for C source code where applicable.

Bourne Shell

Shell scripts must be as portable as possible and should therefore conform strictly to POSIX standards. Shell extensions such as those introduced in Bash should be avoided. Some pitfalls to avoid:

  • To capture the output of a command, use `backquotes` rather than $(parentheses)
  • For arithmetical computation, use `expr {expression}`, not $((expression))
  • To text string length use `expr $string : ".*"` rather than `expr length $string`
  • To test for the presence of a string in a file without printing anything to stdout, use "grep string filename > /dev/null 2>&1", rather than "grep -q string filename".
  • To test for file existence use "test -f" rather than "test -e"
  • Don’t use newline (n) when calling echo. Either use another echo statement, or use "cat << EOF".
  • To set a variable from outside awk, use "awk '{...}' var=value" rather than "awk -vvar=value '{...}'"