Recommended C Style and Coding Standards

Diomidis Spinellis
Department of Management Science and Technology
Athens University of Economics and Business
Athens, Greece
dds@aueb.gr

Author List

L.W. Cannon
R.A. Elliott
L.W. Kirchhoff
J.H. Miller
J.M. Milner
R.W. Mitze
E.P. Schan
N.O. Whittington
Bell Labs

Henry Spencer
Zoology Computer Systems
University of Toronto

David Keppel
EECS, UC Berkeley
CS&E, University of Washington

Mark Brader
SoftQuad Incorporated
Toronto

Diomidis Spinellis
Department of Technology and Management
Athens University of Economics and Business
Athens, Greece
dds@aueb.gr (mailto:dds@aueb.gr)

Introduction

This document is a modified version of a document from a committee formed at AT&T's Indian Hill labs to establish a common set of coding standards and recommendations for the Indian Hill community. The scope of this work is C coding style. Good style should encourage consistent layout, improve portability, and reduce errors. This work does not cover functional organization, or general issues such as the use of gotos. We have tried to combine previous work [1,6,8] on C style into a uniform set of standards that should be appropriate for any project using C, although parts are biased towards particular systems. The opinions in this document do not reflect the opinions of all authors. Please reflect comments and suggestions to the last author. Of necessity, these standards cannot cover all situations. Experience and informed judgement count for much. Programmers who encounter unusual situations should consult either experienced C programmers or code written by experienced C programmers (preferably following these rules).

Ultimately, the goal of these standards is to increase portability, reduce maintenance, and above all improve clarity.

Many of the style choices here are somewhat arbitrary. Mixed coding style is harder to maintain than bad coding style. When changing existing code it is better to conform to the style (indentation, spacing, commenting, naming conventions) of the existing code than it is to blindly follow this document. This is particularly relevant when coding Microsoft Windows programs which depend on the Microsoft style of declarations and coding.

``To be clear is professional; not to be clear is unprofessional.'' - Sir Ernest Gowers.

File Organization

A file consists of various sections that should be separated by several blank lines. Although there is no maximum length limit for source files, files with more than about 1000 lines are cumbersome to deal with. The editor may not have enough temp space to edit the file, compilations will go more slowly, etc. Many rows of asterisks, for example, present little information compared to the time it takes to scroll past, and are discouraged. Lines longer than 79 columns are not handled well by all terminals or windows and should be avoided if possible. Excessively long lines which result from deep indenting are often a symptom of poorly-organized code.

File Naming Conventions

File names are made up of a base name, and an optional period and suffix. The first character of the name should be a letter and all characters (except the period) should be lower-case letters and numbers. The base name should be eight or fewer characters and the suffix should be three or fewer characters (four, if you include the period). These rules apply to both program files and default files used and produced by the program (e.g., ``rogue.sav'').

Some compilers and tools require certain suffix conventions for names of files [5]. The following suffixes are required:

The following conventions are universally followed:

In addition, it is conventional to use ``Makefile'' for the control file for make (for systems that support it) and ``README'' for a summary of the contents of the directory or directory tree.

Program Files

The suggested order of sections for a program file is as follows:

  1. First in the file is a prologue that tells what is in that file. A description of the purpose of the objects in the files (whether they be functions, external data declarations or definitions, or something else) is more useful than a list of the object names. The prologue also contains author(s), revision control information, copyright message, references, etc.
    /*
     * bitmap -- Routines that operate on square bitmaps
     *
     * (C) Copyright Yoyodyne Enterprises.  All rights reserved.
     *
     * Author: John Smith
     *
     * $Header$
     *
     */
  2. Any header file includes should be next. If the include is for a non-obvious reason, the reason should be commented. In most cases, system include files like stdio.h should be included before user include files.
  3. Any defines and typedefs that apply to the file as a whole are next. One normal order is to have ``constant'' macros first, then ``function'' macros, then typedefs and enums.
  4. Next come the global (external) data declarations, usually in the order: externs, non-static globals, static globals. If a set of defines applies to a particular piece of global data (such as a flags word), the defines should be immediately after the data declaration or embedded in structure declarations, indented to put the defines one level deeper than the first keyword of the declaration to which they apply.
  5. The functions come last, and should be in some sort of meaningful order. Like functions should appear together. A ``depth-first'' (functions defined as soon as possible before their calls) is preferred over a ``breadth-first'' approach (functions on a similar level of abstraction together). Considerable judgement is called for here. If defining large numbers of essentially-independent utility functions, consider alphabetical order.

Header Files

Header files are files that are included in other files prior to compilation by the C preprocessor. Some, such as stdio.h, are defined at the system level and must included by any program using the standard I/O library. Header files are also used to contain data declarations and defines that are needed by more than one program. Header files should be functionally organized, i.e., declarations for separate subsystems should be in separate header files. Also, if a set of declarations is likely to change when code is ported from one machine to another, those declarations should be in a separate header file.

Avoid private header filenames that are the same as library header filenames. The statement #include """math.h""" will include the standard library math header file if the intended one is not found in the current directory. If this is what you want to happen, comment this fact. Don't use absolute pathnames for header files. Use the <name> construction for getting them from a standard place, or define them relative to the current directory. The ``include-path'' option of the C compiler (-I on many systems) is the best way to handle extensive private libraries of header files; it permits reorganizing the directory structure without having to alter source files.

Header files that declare functions or external variables should be included in the file that defines the function or variable. That way, the compiler can do type checking and the external declaration will always agree with the definition.

Defining variables in a header file is often a poor idea. Frequently it is a symptom of poor partitioning of code between files. Also, some objects like typedefs and initialized data definitions cannot be seen twice by the compiler in one compilation. On some systems, repeating uninitialized declarations without the extern keyword also causes problems. Repeated declarations can happen if include files are nested and will cause the compilation to fail.

Header files should not be nested. The prologue for a header file should, therefore, describe what other headers need to be #included for the header to be functional. In extreme cases, where a large number of header files are to be included in several different source files, it is acceptable to put all common #includes in one include file.

It is common to put the following into each .h file to prevent accidental double-inclusion.

#ifndef EXAMPLE_H
#define EXAMPLE_H
/* body of example.h file */
/* ...  */
#endif /* EXAMPLE_H */

This double-inclusion mechanism should not be relied upon, particularly to perform nested includes.

Other Files

It is conventional to have a file called ``README'' to document both ``the bigger picture'' and issues for the program as a whole. For example, it is common to include a list of all conditional compilation flags and what they mean. It is also common to list files that are machine dependent, etc.

Comments

``When the code and the comments disagree, both are probably wrong.'' - Norm Schryer

The comments should describe what is happening, how it is being done, what parameters mean, which globals are used and which are modified, and any restrictions or bugs. Avoid, however, comments that are clear from the code, as such information rapidly gets out of date. Comments that disagree with the code are of negative value. Short comments should be what comments, such as ``compute mean value'', rather than how comments such as ``sum of values divided by n''. C is not assembler; putting a comment at the top of a 3-10 line section telling what it does overall is often more useful than a comment on each line describing micrologic.

Comments should justify offensive code. The justification should be that something bad will happen if unoffensive code is used. Just making code faster is not enough to rationalize a hack; the performance must be shown to be unacceptable without the hack. The comment should explain the unacceptable behavior and describe why the hack is a ``good'' fix.

Comments that describe data structures, algorithms, etc., should be in block comment form with the opening /* in columns 1-2, a * in column 2 before each line of comment text, and the closing */ in columns 2-3.

/*
 *      Here is a block comment.
 *      The comment text should be tabbed or spaced over uniformly.
 *      The opening slash-star and closing star-slash are alone on a line.
 */

Note that grep '^. *' will catch all block comments in the file. Some automated program-analysis packages use different characters before comment lines as a marker for lines with specific items of information. In particular, a line with a `-' in a comment preceding a function is sometimes assumed to be a one-line summary of the function's purpose. Very long block comments such as drawn-out discussions and copyright notices often start with /* in columns 1-2, no leading * before lines of text, and the closing */ in columns 1-2. Block comments inside a function are appropriate, and they should be tabbed over to the same tab setting as the code that they describe. One-line comments alone on a line should be indented to the tab setting of the code that follows.

if (argc > 1) {
        /* Get input file from command line. */
        if (freopen(argv[1], "r"stdin) == NULL) {
                perror (argv[1]);
        }
}

Very short comments may appear on the same line as the code they describe, and should be tabbed over to separate them from the statements. If more than one short comment appears in a block of code they should all be tabbed to the same tab setting.

if (a == EXCEPTION) {
        b = TRUE;                       /* special case */
else {
        b = isprime(a);                 /* works only for odd a */
}

Declarations

Global declarations should begin in column 1. All external data declaration should be preceded by the extern keyword. If an external variable is an array that is defined with an explicit size, then the array bounds must be repeated in the extern declaration unless the size is always encoded in the array (e.g., a read-only character array that is always null-terminated). Repeated size declarations are particularly beneficial to someone picking up code written by another.

The ``pointer'' qualifier, `*', should be with the variable name rather than with the type.

char            *s, *t, *u;
instead of
char*   s, t, u;
which is wrong, since `t' and `u' do not get declared as pointers.

Unrelated declarations, even of the same type, should be on separate lines. A comment describing the role of the object being declared should be included, with the exception that a list of #defined constants do not need comments if the constant names are sufficient documentation. The names, values, and comments are usually tabbed so that they line up underneath each other. Use the tab character rather than blanks (spaces). For structure and union template declarations, each element should be alone on a line with a comment describing it. The opening brace ({) should be on the same line as the structure tag, and the closing brace (}) should be in column 1.

struct boat {
        int             wllength;       /* water line length in meters */
        int             type;           /* see below */
        long            sailarea;       /* sail area in square mm */
};

/* defines for boat.type */
#define KETCH   (1)
#define YAWL    (2)
#define SLOOP   (3)
#define SQRIG   (4)
#define MOTOR   (5)

These defines are sometimes put right after the declaration of type, within the struct declaration, with enough tabs after the `#' to indent define one level more than the structure member declarations. When the actual values are unimportant, the enum facility is better.

enum bt { KETCH=1, YAWL, SLOOP, SQRIG, MOTOR };
struct boat {
        int             wllength;       /* water line length in meters */
        enum bt         type;           /* what kind of boat */
        long            sailarea;       /* sail area in square mm */
};

Any variable whose initial value is important should be explicitly initialized, or at the very least should be commented to indicate that C's default initialization to zero is being relied upon. The empty initializer, ``{}'', should never be used. Structure initializations should be fully parenthesized with braces. Constants used to initialize longs should be explicitly long. Use capital letters; for example two long ``2l'' looks a lot like ``21'', the number twenty-one.

int             x = 1;
char            *msg = "message";
struct boat     winner[] = {
        { 40, YAWL, 6000000L },
        { 28, MOTOR, 0L },
        { 0 },
};

In any file which is part of a larger whole rather than a self-contained program, maximum use should be made of the static keyword to make functions and variables local to single files. Variables in particular should be accessible from other files only when there is a clear need that cannot be filled in another way. Such usage should be commented to make it clear that another file's variables are being used; the comment should name the other file. If your debugger hides static objects you need to see during debugging, declare them as STATIC and #define STATIC as needed.

The most important types should be highlighted by typedeffing them, even if they are only integers, as the unique name makes the program easier to read (as long as there are only a few things typedeffed to integers!). Avoid typedeffing structures and unions, as this hides the fact that an object is composite from the code reader.

The return type of functions should always be declared. Always use function prototypes. One common mistake is to omit the declaration of external math functions that return double. The compiler then assumes that the return value is an integer and the bits are dutifully converted into a (meaningless) floating point value.

``C takes the point of view that the programmer is always right.'' - Michael DeCorte

Function Declarations

Each function should be preceded by a block comment prologue that gives a short description of what the function does and (if not clear) how to use it. Discussion of non-trivial design decisions and side-effects is also appropriate. Avoid duplicating information clear from the code.

The function return type should be alone on a line, (optionally) indented one stop. ``Tabstops'' can be blanks (spaces) inserted by your editor in clumps of 2, 4, or 8. Do not default to int; if the function does not return a value then it should be given return type void. If the value returned requires a long explanation, it should be given in the prologue; otherwise it can be on the same line as the return type, tabbed over. The function name (and the formal parameter list) should be alone on a line, in column 1. Destination (return value) parameters should generally be first (on the left). All local declarations and code within the function body should be tabbed over one stop. The opening brace of the function body should be alone on a line beginning in column 1.

Each parameter should be declared (do not default to int). In general the role of each variable in the function should be described. This may either be done in the function comment or, if each declaration is on its own line, in a comment on that line. Loop counters called ``i'', string pointers called ``s'', and integral types called ``c'' and used for characters are typically excluded. If a group of functions all have a like parameter or local variable, it helps to call the repeated variable by the same name in all functions. (Conversely, avoid using the same name for different purposes in related functions.) Like parameters should also appear in the same place in the various argument lists.

Comments for parameters and local variables should be tabbed so that they line up underneath each other. Local variable declarations should be separated from the function's statements by a blank line.

Be careful when you use or declare functions that take a variable number of arguments (``varargs''). Always use the ``stdarg.h'' header definitions and do not rely on item order on the stack.

If the function uses any external variables (or functions) that are not declared globally in the file, these should have their own declarations in the function body using the extern keyword.

Avoid local declarations that override declarations at higher levels. In particular, local variables should not be redeclared in nested blocks. Although this is valid C, the potential confusion is enough that lint will complain about it when given the -h option.

Whitespace

int i;main(){for(;i["]<i;++i){--i;}"];read('-'-'-',i+++"hell\
o, world!\n",'/'/'/'));}read(j,i,p){write(j/p+p,i---j,i/i);}

- Dishonorable mention, Obfuscated C Code Contest, 1984.
Author requested anonymity.

Use vertical and horizontal whitespace generously. Indentation and spacing should reflect the block structure of the code; e.g., there should be at least 2 blank lines between the end of one function and the comments for the next.

A long string of conditional operators should be split onto separate lines.

if (foo->next==NULL && totalcount<needed && needed<=MAX_ALLOT
        && server_active(current_input)) { ...
Might be better as
if (foo->next == NULL
        && totalcount < needed && needed <= MAX_ALLOT
        && server_active(current_input))
{
        ...
Similarly, elaborate for loops should be split onto different lines.
for (curr = *listp, trail = listp;
        curr != NULL;
        trail = &(curr->next), curr = curr->next )
{
        ...
Other complex expressions, particularly those using the ternary ?: operator, are best split on to several lines, too.
c = (a == b)
        ? d + f(a)
        : f(b) - d;
Keywords that are followed by expressions in parentheses should be separated from the left parenthesis by a blank. (The sizeof operator is an exception.) Blanks should also appear after commas in argument lists to help separate the arguments visually. On the other hand, macro definitions with arguments must not have a blank between the name and the left parenthesis, otherwise the C preprocessor will not recognize the argument list.

Examples

/*
 *      Determine if the sky is blue by checking that it isn't night.
 *      CAVEAT: Only sometimes right.  May return TRUE when the answer
 *      is FALSE.  Consider clouds, eclipses, short days.
 *      NOTE: Uses `hour' from `hightime.c'.  Returns `int' for
 *      compatibility with the old version.
 */
int                                             /* true or false */
skyblue(void)
{
        extern int      hour;           /* current hour of the day */

        return (hour >= MORNING && hour <= EVENING);
}
/*
 *      Find the last element in the linked list
 *      pointed to by nodep and return a pointer to it.
 *      Return NULL if there is no last element.
 */
node_t *
tail(node_t *nodep)
{
        node_t  *np;            /* advances to NULL */
        node_t  *lp;            /* follows one behind np */

        if (nodep == NULL)
                return (NULL);
        for (np = lp = nodep; np != NULL; lp = np, np = np->next)
                ;       /* VOID */
        return (lp);
}

Simple Statements

There should be only one statement per line unless the statements are very closely related.

case FOO:         oogle (zork);  boogle (zork);  break;
case BAR:         oogle (bork);  boogle (zork);  break;
case BAZ:         oogle (gork);  boogle (bork);  break;
The null body of a for or while loop should be alone on a line and commented so that it is clear that the null body is intentional and not missing code.
while (*dest++ = *src++)
        ;       /* VOID */

Do not default the test for non-zero, i.e.

if (f() != FAIL)
is better than
if (f())
even though FAIL may have the value 0 which C considers to be false. An explicit test will help you out later when somebody decides that a failure return should be -1 instead of 0. Explicit comparison should be used even if the comparison value will never change; e.g., ``if (!(bufsize % sizeof(int)))'' should be written instead as ``if ((bufsize % sizeof(int)) == 0)'' to reflect the numeric (not boolean) nature of the test. A frequent trouble spot is using strcmp to test for string equality, where the result should never ever be defaulted. The preferred approach is to define a macro STREQ.
#define STREQ(a, b) (strcmp((a), (b)) == 0)

The non-zero test is often defaulted for predicates and other functions or expressions which meet the following restrictions:

It is common practice to declare a boolean type ``bool'' in a global include file. The special names improve readability immensely.

typedef int     bool;
#define FALSE   0
#define TRUE    1
or
typedef enum { NO=0, YES } bool;

Even with these declarations, do not check a boolean value for equality with 1 (TRUE, YES, etc.); instead test for inequality with 0 (FALSE, NO, etc.). Most functions are guaranteed to return 0 if false, but only non-zero if true. Thus,

if (func() == TRUE) { ...
must be written
if (func() != FALSE) { ...
It is even better (where possible) to rename the function/variable or rewrite the expression so that the meaning is obvious without a comparison to true or false (e.g., rename to isvalid()).
if (isvalid()) { ...

There is a time and a place for embedded assignment statements. In some constructs there is no better way to accomplish the results without making the code bulkier and less readable.

while ((c = getchar()) != EOF) {
        process the character
}
The ++ and -- operators count as assignment statements. So, for many purposes, do functions with side effects. Using embedded assignment statements to improve run-time performance is also possible. However, one should consider the tradeoff between increased speed and decreased maintainability that results when embedded assignments are used in artificial places. For example,
a = b + c;
d = a + r;
should not be replaced by
d = (a = b + c) + r;
even though the latter may save one cycle. In the long run the time difference between the two will decrease as the optimizer gains maturity, while the difference in ease of maintenance will increase as the human memory of what's going on in the latter piece of code begins to fade.

Goto statements should be used sparingly, as in any well-structured code. The main place where they can be usefully employed is to break out of several levels of switch, for, and while nesting, although the need to do such a thing may indicate that the inner constructs should be broken out into a separate function, with a success/failure return code.

        for (...) {
                while (...) {
                        ...
                        if (disaster)
                                goto error;
            
                }
        }
        ...
error:
        clean up the mess
When a goto is necessary the accompanying label should be alone on a line and tabbed one stop to the left of the code that follows. The goto should be commented (possibly in the block header) as to its utility and purpose. Continue should be used sparingly and near the top of the loop. Break is less troublesome.

Compound Statements

A compound statement is a list of statements enclosed by braces. There are many common ways of formatting the braces. Please be consistent with our local standard. When editing someone else's code, always use the style used in that code.

control {
                statement;
                statement;
}

The style above is called ``K&R style'', and is preferred if you haven't already got a favorite. With K&R style, the else part of an if-else statement and the while part of a do-while statement should appear on the same line as the close brace. With most other styles, the braces are always alone on a line.

When a block of code has several labels (unless there are a lot of them), the labels are placed on separate lines. The fall-through feature of the C switch statement, (that is, when there is no break between a code segment and the next case statement) must be commented for future maintenance. A lint-style comment/directive is best.

switch (expr) {
case ABC:
case DEF:
        statement;
        break;
case UVW:
        statement;
        /*FALLTHROUGH*/
case XYZ:
        statement;
        break;
}

Here, the last break is unnecessary, but is required because it prevents a fall-through error if another case is added later after the last one. The default case, if used, should be last and does not require a break if it is last.

Whenever an if-else statement has a compound statement for either the if or else section, the statements of both the if and else sections should both be enclosed in braces (called fully bracketed syntax).

if (expr) {
        statement;
else {
        statement;
        statement;
}
Braces are also essential in if-if-else sequences with no second else such as the following, which will be parsed incorrectly if the brace after (ex1) and its mate are omitted:
if (ex1) {
        if (ex2) {
                funca();
        }
else {
        funcb();
}

An if-else with else if should be written with the else conditions left-justified.

if (STREQ (reply, "yes")) {
        statements for yes
        ...
else if (STREQ (reply, "no")) {
        ...
else if (STREQ (reply, "maybe")) {
        ...
else {
        statements for default
        ...
}
The format then looks like a generalized switch statement and the tabbing reflects the switch between exactly one of several alternatives rather than a nesting of statements.

Do-while loops should always have braces around the body.

Forever loops should be coded using the for(;;) construct, and not the while(1) construct. Do not use braces for single statement blocks.

for (;;)
        function();

Sometimes an if causes an unconditional control transfer via break, continue, goto, or return. The else should be implicit and the code should not be indented.

if (level > limit)
        return (OVERFLOW)
normal();
return (level);
The ``flattened'' indentation tells the reader that the boolean test is invariant over the rest of the enclosing block.

Operators

Unary operators should not be separated from their single operand. Generally, all binary operators except `.' and `->' should be separated from their operands by blanks. Some judgement is called for in the case of complex expressions, which may be clearer if the ``inner'' operators are not surrounded by spaces and the ``outer'' ones are.

If you think an expression will be hard to read, consider breaking it across lines. Splitting at the lowest-precedence operator near the break is best. Since C has some unexpected precedence rules, expressions involving mixed operators should be parenthesized. Too many parentheses, however, can make a line harder to read because humans aren't good at parenthesis-matching.

There is a time and place for the binary comma operator, but generally it should be avoided. The comma operator is most useful to provide multiple initializations or operations, as in for statements. Complex expressions, for instance those with nested ternary ?: operators, can be confusing and should be avoided if possible. There are some macros like getchar where both the ternary operator and comma operators are useful. The logical expression operand before the ?: should be parenthesized and both return values must be the same type.

Naming Conventions

Individual projects will no doubt have their own naming conventions. There are some general rules however.

In general, global names (including enums) should have a common prefix identifying the module that they belong with. Globals may alternatively be grouped in a global structure. Typedeffed names often have ``_t'' appended to their name.

Avoid names that might conflict with various standard library names. Some systems will include more library code than you want. Also, your program may be extended someday.

Also note the following (from [15]):

``Length is not a virtue in a name; clarity of expression is. A global variable rarely used may deserve a long name, maxphysaddr say. An array index used on every line of a loop needn't be named any more elaborately than i. Saying index or elementnumber is more to type (or calls upon your text editor) and obscures the details of the computation. When the variable names are huge, it's harder to see what's going on. This is partly a typographic issue; consider

for(i=0 to 100)
        array[i]=0
vs.
for(elementnumber=0 to 100)
        array[elementnumber]=0;
The problem gets worse fast with real examples. Indices are just notation, so treat them as such.''

``Pointers also require sensible notation. np is just as mnemonic as nodepointer if you consistently use a naming convention from which np means ``node pointer'' is easily derived.''

As in all other aspects of readable programming, consistency is important in naming. If you call one variable maxphysaddr, don't call its cousin lowestaddress.''

``Finally, I prefer minimum-length but maximum-information names, and then let the context fill in the rest. Globals, for instance, typically have little context when they are used, so their names need to be relatively evocative. Thus I say maxphysaddr (not MaximumPhysicalAddress) for a global variable, but np not NodePointer for a pointer locally defined and used. This is largely a matter of taste, but taste is relevant to clarity.

I eschew embedded capital letters in names; to my prose-oriented eyes, they are too awkward to read comfortably. They jangle like bad typography.'' ``Procedure names should reflect what they do; function names should reflect what they return. Functions are used in expressions, often in things like if's, so they need to read appropriately.

if(checksize(x))
is unhelpful because we can't deduce whether checksize returns true on error or non-error; instead
if(validsize(x))
makes the point clear and makes a future mistake in using the routine less likely.''

Constants

Numerical constants should not be coded directly. The #define feature of the C preprocessor should be used to give constants meaningful names. Symbolic constants make the code easier to read. Defining the value in one place also makes it easier to administer large programs since the constant value can be changed uniformly by changing only the define. The enumeration data type is a better way to declare variables that take on only a discrete set of values, since additional type checking is often available. At the very least, any directly-coded numerical constant must have a comment explaining the derivation of the value.

Constants should be defined consistently with their use; e.g. use 540.0 for a float instead of 540 with an implicit float cast. There are some cases where the constants 0 and 1 may appear as themselves instead of as defines. For example if a for loop indexes through an array, then

for (i = 0; i < ARYBOUND; i++)
is reasonable while the code
door_t *front_door = opens(door[i], 7);
if (front_door == 0)
        error("can't open %s\n", door[i]);
is not. In the last example front_door is a pointer. When a value is a pointer it should be compared to NULL instead of 0. NULL is available as part of the standard I/O library's header file stdio.h and stdlib.h. Even simple values like 1 or 0 are often better expressed using defines like TRUE and FALSE (sometimes YES and NO read better).

Simple character constants should be defined as character literals rather than numbers. Non-text characters are discouraged as non-portable. If non-text characters are necessary, particularly if they are used in strings, they should be written using a escape character of three octal digits rather than one (e.g., `\007'). Even so, such usage should be considered machine-dependent and treated as such.

Macros

Complex expressions can be used as macro parameters, and operator-precedence problems can arise unless all occurrences of parameters have parentheses around them. There is little that can be done about the problems caused by side effects in parameters except to avoid side effects in expressions (a good idea anyway) and, when possible, to write macros that evaluate their parameters exactly once. There are times when it is impossible to write macros that act exactly like functions.

Some macros also exist as functions (e.g., getc and fgetc). The macro should be used in implementing the function so that changes to the macro will be automatically reflected in the function. Care is needed when interchanging macros and functions since function parameters are passed by value, while macro parameters are passed by name substitution. Carefree use of macros requires that they be declared carefully.

Macros should avoid using globals, since the global name may be hidden by a local declaration. Macros that change named parameters (rather than the storage they point at) or may be used as the left-hand side of an assignment should mention this in their comments. Macros that take no parameters but reference variables, are long, or are aliases for function calls should be given an empty parameter list, e.g.,

#define OFF_A() (a_global+OFFSET)
#define BORK()  (zork())
#define SP3()   if (b) { int x; av = f (&x); bv += x; }

Macros save function call/return overhead, but when a macro gets long, the effect of the call/return becomes negligible, so a function should be used instead.

In some cases it is appropriate to make the compiler insure that a macro is terminated with a semicolon.

if (x==3)
    SP3();
else
    BORK();
If the semicolon is omitted after the call to SP3, then the else will (silently!) become associated with the if in the SP3 macro. With the semicolon, the else doesn't match any if! The macro SP3 can be written safely as
#define SP3() \
        do { if (b) { int x; av = f (&x); bv += x; }} while (0)
Writing out the enclosing do-while by hand is awkward and some compilers and tools may complain that there is a constant in the ``while'' conditional. A macro for declaring statements may make programming easier.
#ifdef lint
        static int ZERO;
#else
#       define ZERO 0
#endif
#define STMT( stuff )           do { stuff } while (ZERO)
Declare SP3 with
#define SP3() \
	STMT( if (b) { int x; av = f (&x); bv += x; } )
Using STMT will help prevent small typos from silently changing programs.

Except for type casts, sizeof, and hacks such as the above, macros should contain keywords only if the entire macro is surrounded by braces.

Conditional Compilation

Conditional compilation is useful for things like machine-dependencies, debugging, and for setting certain options at compile-time. Beware of conditional compilation. Various controls can easily combine in unforeseen ways. If you #ifdef machine dependencies, make sure that when no machine is specified, the result is an error, not a default machine. (Use ``#error'' and indent it so it works with older compilers.) If you #ifdef optimizations, the default should be the unoptimized code rather than an uncompilable program. Be sure to test the unoptimized code.

Note that the text inside of an #ifdeffed section may be scanned (processed) by the compiler, even if the #ifdef is false. Thus, even if the #ifdeffed part of the file never gets compiled (e.g., ),"#ifdefCOMMENT" it cannot be arbitrary text.

Put #ifdefs in header files instead of source files when possible. Use the #ifdefs to define macros that can be used uniformly in the code. For instance, a header file for checking memory allocation might look like (omitting definitions for REALLOC and FREE):

#ifdef DEBUG
        extern void *mm_malloc();
#       define MALLOC(size) (mm_malloc(size))
#else
        extern void *malloc();
#       define MALLOC(size) (malloc(size))
#endif

Conditional compilation should generally be on a feature-by-feature basis. Machine or operating system dependencies should be avoided in most cases.

#ifdef 4BSD
        long t = time ((long *)NULL);
#endif
The preceding code is poor for two reasons: there may be 4BSD systems for which there is a better choice, and there may be non-4BSD systems for which the above is the best code. Instead, use define symbols such as TIME_LONG and TIME_STRUCT and define the appropriate one in a configuration file such as config.h.

Program Structure

The following are some excerpts from [15] relevant to program structure and organisation.

Complexity

Most programs are too complicated - that is, more complex than they need to be to solve their problems efficiently. Why? Mostly it's because of bad design, but I will skip that issue here because it's a big one. But programs are often complicated at the microscopic level, and that is something I can address here.

Rule 1. You can't tell where a program is going to spend its time. Bottlenecks occur in surprising places, so don't try to second guess and put in a speed hack until you've proven that's where the bottleneck is.

Rule 2. Measure. Don't tune for speed until you've measured, and even then don't unless one part of the code overwhelms the rest.

Rule 3. Fancy algorithms are slow when n is small, and n is usually small. Fancy algorithms have big constants. Until you know that n is frequently going to be big, don't get fancy. (Even if n does get big, use Rule 2 first.) For example, binary trees are always faster than splay trees for workaday problems.

Rule 4. Fancy algorithms are buggier than simple ones, and they're much harder to implement. Use simple algorithms as well as simple data structures.

The following data structures are a complete list for almost all practical programs:

Of course, you must also be prepared to collect these into compound data structures. For instance, a symbol table might be implemented as a hash table containing linked lists of arrays of characters.

Rule 5. Data dominates. If you've chosen the right data structures and organized things well, the algorithms will almost always be self-evident. Data structures, not algorithms, are central to programming. (See Brooks p. 102.)

Rule 6. There is no Rule 6.

Programming with data.

Algorithms, or details of algorithms, can often be encoded compactly, efficiently and expressively as data rather than, say, as lots of if statements. The reason is that the complexity of the job at hand, if it is due to a combination of independent details, can be encoded. A classic example of this is parsing tables, which encode the grammar of a programming language in a form interpretable by a fixed, fairly simple piece of code. Finite state machines are particularly amenable to this form of attack, but almost any program that involves the `parsing' of some abstract sort of input into a sequence of some independent `actions' can be constructed profitably as a data-driven algorithm.

Perhaps the most intriguing aspect of this kind of design is that the tables can sometimes be generated by another program - a parser generator, in the classical case. As a more earthy example, if an operating system is driven by a set of tables that connect I/O requests to the appropriate device drivers, the system may be `configured' by a program that reads a description of the particular devices connected to the machine in question and prints the corresponding tables.

One of the reasons data-driven programs are not common, at least among beginners, is the tyranny of Pascal. Pascal, like its creator, believes firmly in the separation of code and data. It therefore (at least in its original form) has no ability to create initialized data. This flies in the face of the theories of Turing and von Neumann, which define the basic principles of the stored-program computer. Code and data are the same, or at least they can be. How else can you explain how a compiler works? (Functional languages have a similar problem with I/O.)

Function pointers

Another result of the tyranny of Pascal is that beginners don't use function pointers. (You can't have function-valued variables in Pascal.) Using function pointers to encode complexity has some interesting properties.

Some of the complexity is passed to the routine pointed to. The routine must obey some standard protocol - it's one of a set of routines invoked identically - but beyond that, what it does is its business alone. The complexity is distributed.

There is this idea of a protocol, in that all functions used similarly must behave similarly. This makes for easy documentation, testing, growth and even making the program run distributed over a network - the protocol can be encoded as remote procedure calls.

I argue that clear use of function pointers is the heart of object-oriented programming. Given a set of operations you want to perform on data, and a set of data types you want to respond to those operations, the easiest way to put the program together is with a group of function pointers for each type. This, in a nutshell, defines class and method. The O-O languages give you more of course - prettier syntax, derived types and so on - but conceptually they provide little extra.

Combining data-driven programs with function pointers leads to an astonishingly expressive way of working, a way that, in my experience, has often led to pleasant surprises. Even without a special O-O language, you can get 90% of the benefit for no extra work and be more in control of the result. I cannot recommend an implementation style more highly. All the programs I have organized this way have survived comfortably after much development - far better than with less disciplined approaches. Maybe that's it: the discipline it forces pays off handsomely in the long run.

Debugging

``C Code. C code run. Run, code, run... PLEASE!!!'' - Barbara Tongue

If you use enums, the first enum constant should have a non-zero value, or the first constant should indicate an error.

enum { STATE_ERR, STATE_START, STATE_NORMAL, STATE_END } state_t;
enum { VAL_NEW=1, VAL_NORMAL, VAL_DYING, VAL_DEAD } value_t;
Uninitialized values will then often ``catch themselves''.

Check for error return values, even from functions that ``can't'' fail. Consider that close() and fclose() can and do fail, even when all prior file operations have succeeded. Write your own functions so that they test for errors and return error values or abort the program in a well-defined way. Include a lot of debugging and error-checking code and leave most of it in the finished product. Check even for ``impossible'' errors. [8]

Use the assert facility to insist that each function is being passed well-defined values, and that intermediate results are well-formed.

Build in the debug code using as few #ifdefs as possible. For instance, if ``mm_malloc'' is a debugging memory allocator, then MALLOC will select the appropriate allocator, avoids littering the code with #ifdefs, and makes clear the difference between allocation calls being debugged and extra memory that is allocated only during debugging.

#ifdef DEBUG
#       define MALLOC(size)  (mm_malloc(size))
#else
#       define MALLOC(size)  (malloc(size))
#endif

Check bounds even on things that ``can't'' overflow. A function that writes on to variable-sized storage should take an argument maxsize that is the size of the destination. If there are times when the size of the destination is unknown, some `magic' value of maxsize should mean ``no bounds checks''. When bound checks fail, make sure that the function does something useful such as abort or return an error status.

/*
 * INPUT: A null-terminated source string `src' to copy from and
 * a `dest' string to copy to.  `maxsize' is the size of `dest'
 * or UINT_MAX if the size is not known.  `src' and `dest' must
 * both be shorter than UINT_MAX, and `src' must be no longer than
 * `dest'.
 * OUTPUT: The address of `dest' or NULL if the copy fails.
 * `dest' is modified even when the copy fails.
 */
char *
copy (char *dest, size_t maxsize, char *src)
{
        char *dp = dest;

        while (maxsize-- > 0)
                if ((*dp++ = *src++) == '\0')
                        return (dest);

        return (NULL);
}

In all, remember that a program that produces wrong answers twice as fast is infinitely slower. The same is true of programs that crash occasionally or clobber valid data.

Portability

``C combines the power of assembler with the portability of assembler.''
- Anonymous, alluding to Bill Thacker.

The advantages of portable code are well known. This section gives some guidelines for writing portable code. Here, ``portable'' means that a source file can be compiled and executed on different machines with the only change being the inclusion of possibly different header files and the use of different compiler flags. The header files will contain #defines and typedefs that may vary from machine to machine. In general, a new ``machine'' is different hardware, a different operating system, a different compiler, or any combination of these. Reference [1] contains useful information on both style and portability. The following is a list of pitfalls to be avoided and recommendations to be considered when designing portable code:

ANSI C

Modern C compilers support the ANSI standard C [16]. Write code to run under standard C, and use features such as function prototypes, constant storage, and volatile storage. Standard C improves program performance by giving better information to optimizers. Standard C improves portability by insuring that all compilers accept the same input language and by providing mechanisms that try to hide machine dependencies or emit warnings about code that may be machine-dependent.

Formatting

Note that under ANSI C, the `#' for a preprocessor directive must be the first non-whitespace character on a line. Use this feature to improve the formatting of your files.

An ``#ifdef NAME'' should end with either ``#endif'' or ``#endif /* NAME */'', not with ``#endif NAME''. The comment should not be used on short #ifdefs, as it is clear from the code.

ANSI trigraphs may cause programs with strings containing ``??'' may break mysteriously.

The style for ANSI C is the same as for regular C, with two notable exceptions: storage qualifiers and parameter lists.

Because const and volatile have strange binding rules, each const or volatile object should have a separate declaration.

int const *s;           /* YES */
int const *s, *t;       /* NO */

Prototyped functions merge parameter declaration and definition in to one list. Parameters should be commented in the function comment.

/*
 * `bp': boat trying to get in.
 * `stall': a list of stalls, never NULL.
 * returns stall number, 0 => no room.
 */
int
enter_pier (boat_t const *bp, stall_t *stall)
{
        ...

Pragmas

Pragmas are used to introduce machine-dependent code in a controlled way. Obviously, pragmas should be treated as machine dependencies. Unfortunately, the syntax of ANSI pragmas makes it impossible to isolate them in machine-dependent headers.

Pragmas are of two classes. Optimizations may safely be ignored. Pragmas that change the system behavior (``required pragmas'') may not. Required pragmas should be #ifdeffed so that compilation will abort if no pragma is selected.

Two compilers may use a given pragma in two very different ways. For instance, one compiler may use ``haggis'' to signal an optimization. Another might use it to indicate that a given statement, if reached, should terminate the program. Thus, when pragmas are used, they must always be enclosed in machine-dependent #ifdefs. Pragmas must always be #ifdefed out for non-ANSI compilers. Be sure to indent the `#' character on the #pragma, as older preprocessors will halt on it otherwise.

#if defined(__STDC__) && defined(USE_HAGGIS_PRAGMA)
        #pragma (HAGGIS)
#endif
``The `#pragma' command is specified in the ANSI standard to have an arbitrary implementation-defined effect. In the GNU C preprocessor, `#pragma' first attempts to run the game `rogue'; if that fails, it tries to run the game `hack'; if that fails, it tries to run GNU Emacs displaying the Tower of Hanoi; if that fails, it reports a fatal error. In any case, preprocessing does not continue.''
- Manual for the GNU C preprocessor for GNU CC 1.34.

Special Considerations

This section contains some miscellaneous do's and don'ts.

Make

One very useful tool is make [7]. During development, make recompiles only those modules that have been changed since the last time make was used. It can be used to automate other tasks, as well. Some common conventions include:

all
always makes all binaries
clean
remove all intermediate files
debug
make a test binary 'a.out' or 'debug'
depend
make transitive dependencies
install
install binaries, libraries, etc.
deinstall
back out of ``install''
print/list
make a hard copy of all source files
shar
make a shar of all source files
spotless
make clean, use revision control to put away sources. Note: doesn't remove Makefile, although it is a source file
source
undo what spotless did
tags
run ctags, (using the -t flag is suggested)
rdist
distribute sources to other hosts
zip
create a zip file for distribution file.c
check out the named file from revision control
In addition, command-line defines can be given to define either Makefile values (such as ``CFLAGS'') or values in the program (such as ``DEBUG'').

Project-Dependent Standards

Individual projects may wish to establish additional standards beyond those given here. The following issues are some of those that should be addressed by each project program administration group.

Conclusion

A set of standards has been presented for C programming style. Among the most important points are:

As with any standard, it must be followed if it is to be useful. If you have trouble following any of these standards don't just ignore them. Talk with an experienced programmer.

References

  1. B.A. Tague, C Language Portability, Sept 22, 1977. This document issued by department 8234 contains three memos by R.C. Haight, A.L. Glasser, and T.L. Lyon dealing with style and portability.
  2. S.C. Johnson, Lint, a C Program Checker, USENIX Unix Supplementary Documents, November 1986.
  3. R.W. Mitze, The 3B/PDP-11 Swabbing Problem, Memorandum for File, 1273-770907.01MF, September 14, 1977.
  4. R.A. Elliott and D.C. Pfeffer, 3B Processor Common Diagnostic Standards- Version 1, Memorandum for File, 5514-780330.01MF, March 30, 1978.
  5. R.W. Mitze, An Overview of C Compilation of Unix User Processes on the 3B, Memorandum for File, 5521-780329.02MF, March 29, 1978.
  6. B.W. Kernighan and D.M. Ritchie, The C Programming Language, Prentice Hall 1978, Second Ed. 1988, ISBN 0-13-110362-8.
  7. S.I. Feldman, Make - A Program for Maintaining Computer Programs, USENIX Unix Supplementary Documents, November 1986.
  8. Ian Darwin and Geoff Collyer, Can't Happen or /* NOTREACHED */ or Real Programs Dump Core, USENIX Association Winter Conference, Dallas 1985 Proceedings.
  9. Brian W. Kernighan and P. J. Plauger The Elements of Programming Style. McGraw-Hill, 1974, Second Ed. 1978, ISBN 0-07-034-207-5.
  10. J. E. Lapin Portable C and UNIX System Programming, Prentice Hall 1987, ISBN 0-13-686494-5.
  11. Ian F. Darwin, Checking C Programs with lint, O'Reilly & Associates, 1989. ISBN 0-937175-30-7.
  12. Andrew R. Koenig, C Traps and Pitfalls, Addison-Wesley, 1989. ISBN 0-201-17928-8.
  13. Samuel P. Harbison and Guy L. Steele Jr. C: A Reference Manual 1984, 1987 ISBN is 0-13-109802-0
  14. Mark Horton Portable C Software Prentice-Hall, Englewood Cliffs NJ 1990 ISBN is 0-13-868050-7
  15. Rob Pike Notes on Programming in C
  16. American National Standard for Information Systems - Programming Language - C: ANSI X3.159-1989, December, 1989. Published by the American National Standards Institute, 1430 Broadway, New York, New York 10018.

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