## C Language

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.

## C Source Code

### Copyright

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.

### Line Formatting

#### Indentation

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

    	printf("this is going to be %s very long %s statement\n",
    	_______"a", "printf");

#### Vertical Whitespace

Vertical whitespace is also encouraged for improved code legibility by grouping closely related statements and then separating them with a single empty line. There should not, however, be more than one empty adjacent line anywhere.

#### Line Length

Lines should not be longer than 79 characters, even if it requires violating the indentation rules to do so. Since ANSI 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 "
                                           "left and wrapped.  ANSI catenation "
                                           "rules will turn this into one "
                                           "long string.");

### Comments

Comments should be used anytime they improve the readability of the code.

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 text 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 */

### .h 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.

.h files that define modules should have a structure like the following. Note that <isc/lang.h> should be included by any public header file to get the ISC_LANG_BEGINDECLS and ISC_LANG_ENDDECLS macros used 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) 1996-2016  Internet Systems Consortium, Inc. ("ISC")
     *
     * This Source Code Form is subject to the terms of the Mozilla Public
     * License, v. 2.0. If a copy of the MPL was not distributed with this
     * file, You can obtain one at http://mozilla.org/MPL/2.0/.
     */

    #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 */

### C Source

#### 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.

Include UNIX "sys" .h files before ordinary C includes.

#### 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("yes\n"); 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.

Good:

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

    int
    g(int i, /* other args here */
      int last_argument)
    {
    	return (i * i);
    }

To suppress compiler warnings, unused function arguments are declared using the `UNUSED()` macro.

In the function body, local variable declarations are followed by any `REQUIRE()`s, `UNUSED()` declarations, and other code, in this order. These sections are 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 (`if, for` 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 the readability (a judgement call biased toward not having the braces).

Good:

    static void
    f(int i) {
    	if (i > 0) {
    		printf("yes\n");
    		i = 0;
    	} else
    		printf("no\n");
    }

Bad:

    void f(int i)
      {
        if(i<0){i=0;printf("was negative\n");}
        if (i > 0)
          {
            printf("yes\n");
            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 a space after 'return', and also 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 also parenthesize its argument, as in `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.
- Do not put a space after a cast.

#### 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, `fputc`, `fwrite` (when writing text), `fflush`, `memcpy`, `memmove`, `memset`, `strcpy`, `strncpy`, and `strcat`.

Certain functions return values or not depending on the operating system or even compiler flags; these include 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_descriptor_t		s;
    	os_result_t		result;

    	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.

#### Casting

Casting should be avoided when possible. When it is necessary, there should be no space between the cast and what is being cast.

Bad (obviously for more than one reason ...):

    	(void) malloc(SMBUF);

#### 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 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.

#### 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)) {

    	}

#### Pointers

##### Null Pointer

The null pointer value should be referred to with "NULL", not with "0". Testing to see whether a pointer is NULL should be explicit.

Good:

    	char *c = NULL;

    	/* ... */

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

##### 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. */

    	free(text);
    	text = NULL;

#### 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. */
    	}

#### 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. In particular, using the ternary operator to specify a return value is verboten.

Good:

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

Bad:

    	return (success ? ISC_R_SUCESS : 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:

    	malloc(size = 20);

Ok:

    	fputc(c++, stdout);

### Namespace

#### Public Interfaces

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 */

however, structures which are typedef'd generally have the name of the typedef sans the final _t:

    	struct dns_rbtnode {
            	/* ... members ... */
    	}

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.

#### Shared Private Interfaces

When a module provides an interface for internal use by other modules in the library, 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.

### 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.

### 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. The `UNEXPECTED_ERROR()` macro is obsolete and should not be used in new code.

Log messages do not start with a capital letter, nor do they end in a period.

When variable text such as a file name or domain name occurs as part of an English phrase, 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. E.g., `isc_log_write(... "open: %s: %s", filename, isc_result_totext(result));`

Function names, line numbers, 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.

## Perl source code

Perl must not be required for building, installing, or using the BIND 9 name server. It may be used for things like test scripts and optional server add-on components.

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.