henry (10/17/82)
A long time ago I promised to post our local C style standard, which is
a much-annotated version of an original from the Bell folks at Indian
Hill. That posting was postponed due to persistent communication
problems and a desire to do some further work on the document. Here
it is, as -ms macros source. (The popular opinion was overwhelmingly in
favor of posting it as source; folks who don't have -ms handy will just
have to convert the document or live without.)
-----
.RP
.TL
Indian Hill C Style and Coding Standards
.br
as amended for U of T Zoology
.UX
.AU
L.W. Cannon
R.A. Elliott
L.W. Kirchhoff
J.H. Miller
J.M. Milner
R.W. Mitze
E.P. Schan
N.O. Whittington
.AI
Bell Labs
.AU
Henry Spencer
.AI
Zoology Computer Systems
University of Toronto
.AB
This document is an annotated
(by the last author) version of the original paper of the same title.
It describes a set of coding standards and recommendations
which are local standards for officially-supported
.UX
programs.
The scope is coding style, not functional organization.
.AE
.\"--------------------
.\" Footnote numbering
.ds f \\u\\n+f\\d
.nr f 0 1
.ds F \\n+F.
.nr F 0 1
.\"--------------------
.NH
Introduction
.PP
This document is a result of a committee formed at Indian Hill to establish
a common set of coding standards and recommendations for the Indian Hill
community.
The scope of this work is the coding style,
not the functional organization of programs.
The standards in this document are not specific to ESS programming
only\*f.
.FS
.IP \*F
In fact, they're pretty good general standards.
This document is presented unadulterated; U of T variations, comments,
exceptions, etc. are presented in footnotes.
.FE
We have tried to combine previous work [1,6] on C style into a uniform
set of standards that should be appropriate for any project using C\*f.
.FS
.IP \*F
Of necessity, these standards cannot cover all situations.
Experience and informed judgement count for much.
Inexperienced programmers who encounter unusual situations should
consult 1) code written by experienced C programmers following
these rules, or 2) experienced C programmers.
.FE
.NH
File Organization
.PP
A file consists of various sections that should be separated by
several blank lines.
Although there is no maximum length requirement for source files,
files with more than about 1500 lines are cumbersome to deal with.
The editor may not have enough temp space to edit the file,
compilations will go slower,
etc.
Since most of us use 300 baud terminals,
entire rows of asterisks, for example, should be discouraged\*f.
.FS
.IP \*F
This is not a problem at U of T, or most other sensible places,
but rows of asterisks are still annoying.
.FE
Also lines longer than 80 columns are not handled well by all terminals
and should be avoided if possible\*f.
.FS
.IP \*F
Excessively long lines which result from deep indenting are often
a symptom of poorly-organized code.
.FE
.PP
The suggested order of sections for a file is as follows:
.IP 1.
Any header file includes should be the first thing in the file.
.IP 2.
Immediately after the includes\*f should be a prologue that tells what
.FS
.IP \*F
A common variation, in both Bell code and ours, is to reverse the
order of sections 1 and 2.
This is an acceptable practice.
.FE
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.
.IP 3.
Any typedefs and defines that apply to the file as a whole are next.
.IP 4.
Next come the global (external) data declarations.
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\*f.
.FS
.IP \*F
Such defines should be indented to put the \fIdefine\fRs one level
deeper than the first keyword of the declaration to which they apply.
.FE
.IP 5.
The functions come last\*f.
.FS
.IP \*F
They should be in some sort of meaningful order.
Top-down is generally better than bottom-up, and a ``breadth-first''
approach (functions on a similar level of abstraction together) is
preferred over depth-first (functions defined as soon as possible
after their calls).
Considerable judgement is called for here.
If defining large numbers of essentially-independent utility
functions, consider alphabetical order.
.FE
.NH 2
File Naming Conventions
.PP
.UX
requires certain suffix conventions for names of files to be processed
by the \fIcc\fR command [5]\*f.
The following suffixes are required:
.IP \(bu
C source file names must end in \fI.c\fR
.IP \(bu
Assembler source file names must end in \fI.s\fR
.PP
In addition the following conventions are universally followed:
.FS
.IP \*F
In addition to the suffix conventions given here,
it is conventional to use `Makefile' (not `makefile') for the
control file for \fImake\fR
and
`README' for a summary of the contents of a directory or directory
tree.
.FE
.IP \(bu
Relocatable object file names end in \fI.o\fR
.IP \(bu
Include header file names end in \fI.h\fR\ \*f or \fI.d\fR
.FS
.IP \*F
Preferred.
An alternate convention that may
be preferable in multi-language environments
is to use the same suffix as an ordinary source file
but with two periods instead of one (e.g. ``foo..c'').
.FE
.IP \(bu
Ldp\*f specification file names end in \fI.b\fR
.FS
.IP \*F
No idea what this is.
.FE
.IP \(bu
Yacc source file names end in \fI.y\fR
.IP \(bu
Lex source file names end in \fI.l\fR
.NH
Header Files
.PP
Header files are files that are included in other files prior to compilation
by the C preprocessor.
Some are defined at the system level like \fIstdio.h\fR which must be 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\*f.
.FS
.IP \*F
Don't use absolute pathnames for header files.
Use the \fI<name>\fR construction for getting them from a standard
place, or define them relative to the current directory.
The \fB\-I\fR option of the C compiler is the best way to handle
extensive private libraries of header files; it permits reorganizing
the directory structure without having to alter source files.
.FE
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.
.PP
Header files should not be nested.
Some objects like typedefs and initialized data definitions cannot be
seen twice by the compiler in one compilation.
On non-\c
.UX
systems this is also true of uninitialized declarations without the
\fIextern\fR keyword\*f.
.FS
.IP \*F
It should be noted that declaring variables in a header file
is often a poor idea.
Frequently it is a symptom of poor partitioning of code between files.
.FE
This can happen if include files are nested and will cause the
compilation to fail.
.NH
External Declarations
.PP
External declarations should begin in column 1.
Each declaration should be on a separate line.
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 comments should be tabbed so that they line up underneath each
other\*f.
.FS
.IP \*F
So should the constant names and their defined values.
.FE
Use the tab character (CTRL I if your terminal doesn't have a separate key)
rather than blanks.
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 alone on a line in column 1, i.e.
.DS L
struct boat {
int wllength; /* water line length in feet */
int type; /* see below */
long sarea; /* sail area in square feet */
};
/*
* defines for boat.type\*f
*/
#define KETCH 1
#define YAWL 2
#define SLOOP 3
#define SQRIG 4
#define MOTOR 5
.DE
.FS
.IP \*F
These defines are better put right after the declaration of \fItype\fR,
within the \fIstruct\fR declaration, with enough tabs
after # to indent \fIdefine\fR one level more than
the structure member declarations.
.FE
If an external variable is initialized the equal sign should not be omitted\*f.
.FS
.IP \*F
The empty initializer, ``{}'', should never be used.
Structure
initializations should be fully parenthesized with braces.
Constants used to initialize longs should be explicitly long.
.FE
.DS
int x = 1;
char *msg = "message";
struct boat winner = {
40, /* water line length */
YAWL,
600 /* sail area */
};
.DE
\*f
.FS
.IP \*F
In any file which is part of a larger whole rather than a self-contained
program, maximum use should be made of the \fIstatic\fR 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 usages should be commented to make it clear that another file's
variables are being used; the comment should name the other file.
.FE
.NH
Comments
.PP
Comments that describe data structures, algorithms, etc., should be
in block comment form with the opening /* in column one, a * in column
2 before each line of comment text\*f, and the closing */ in columns 2-3.
.FS
.IP \*F
Some automated
program-analysis
packages use a different character in this position as
a marker for lines with specific items of information.
In particular, a line with a `-' here in a comment preceding a function
is sometimes assumed to be a one-line summary of the function's purpose.
.FE
.DS L
/*
* Here is a block comment.
* The comment text should be tabbed over\*f
* and the opening /* and closing star-slash
* should be alone on a line.
*/
.DE
.FS
.IP \*F
A common practice in both Bell and local code is to
use a space rather than a tab after the *.
This is acceptable.
.FE
.PP
Note that \fIgrep ^.\e*\fR will catch all block comments in the file.
In some cases, block comments inside a function are appropriate, and
they should be tabbed over to the same tab setting as the code that
they describe.
Short comments may appear on a single line indented over to the tab
setting of the code that follows.
.DS
if (argc > 1) {
/* Get input file from command line. */
if (freopen(argv[1], "r", stdin) == NULL)
error("can't open %s\en", argv[1]);
}
.DE
.PP
Very short comments may appear on the same line as the code they describe,
but should be tabbed over far enough 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.
.DS
if (a == 2)
return(TRUE); /* special case */
else
return(isprime(a)); /* works only for odd a */
.DE
.NH
Function Declarations
.PP
Each function should be preceded by a block comment prologue that gives
the name and a short description of what the function does\*f.
.FS
.IP \*F
Discussion of non-trivial design decisions is also appropriate,
but avoid duplicating information that is present in (and clear from)
the code.
It's too easy for such redundant information to get out of date.
.FE
If the function returns a value, the type of the value returned should
be alone on a line in column 1 (do not default to \fIint\fR).
If the function does not return a value then it should not be given
a return type.
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 formal parameters should be alone on a line
beginning in column 1.
Each parameter should be declared (do not default to \fIint\fR),
with a comment on a single line.
The opening brace of the function body should also be alone on a line
beginning in column 1.
The function name, argument declaration list, and opening brace
should be separated by a blank line\*f.
.FS
.IP \*F
Neither Bell nor local code has ever included these separating blank
lines, and it is not clear that they add anything useful.
Leave them out.
.FE
All local declarations and code within the function body should be
tabbed over at least one tab.
.PP
If the function uses any external variables, these should have their
own declarations in the function body using the \fIextern\fR keyword.
If the external variable is an array the array bounds must be repeated
in the \fIextern\fR declaration.
There should also be \fIextern\fR declarations for all functions called
by a given function.
This is particularly beneficial to someone picking up code written
by another.
If a function returns a value of type other than \fIint\fR,
it is required by the compiler that such functions be declared before
they are used.
Having the \fIextern\fR delcaration in the calling function's
declarations section avoids all such problems\*f.
.FS
.IP \*F
These rules tend to produce a lot of clutter.
Both Bell and local practice frequently omits \fIextern\fR declarations
for \fIstatic\fR variables and functions.
This is permitted.
Omission of declarations for standard library routines
is also permissible,
although if they \fIare\fR declared it is better to declare them within
the functions that use them rather than globally.
.FE
.PP
In general each variable declaration should be on a separate line with
a comment describing the role played by the variable in the function.
If the variable is external or a parameter of type pointer which is
changed by the function, that should be noted in the comment.
All such comments for parameters and local variables should be
tabbed so that they line up underneath each other.
The declarations should be separated from the function's statements
by a blank line.
.PP
A local variable should not be redeclared in nested blocks\*f.
.FS
.IP \*F
In fact, avoid any local declarations that override declarations
at higher levels.
.FE
Even though this is valid C, the potential confusion is enough
that
\fIlint\fR will complain about it when given the \fB\-h\fR option.
.NH 2
Examples
.PP
.DS L
/*
* skyblue()
*
* Determine if the sky is blue.
*/
int /* TRUE or FALSE */
skyblue()
{
extern int hour;
if (hour < MORNING || hour > EVENING)
return(FALSE); /* black */
else
return(TRUE); /* blue */
}
.DE
.DS L
/*
* tail(nodep)
*
* Find the last element in the linked list
* pointed to by nodep and return a pointer to it.
*/
NODE * /* pointer to tail of list */
tail(nodep)
NODE *nodep; /* pointer to head of list */
{
register NODE *np; /* current pointer advances to NULL */
register NODE *lp; /* last pointer follows np */
np = lp = nodep;
while ((np = np->next) != NULL)
lp = np;
return(lp);
}
.DE
.NH
Compound Statements
.PP
Compound statements are statements that contain lists of statements
enclosed in braces.
The enclosed list should be tabbed over one more than the tab position
of the compound statement itself.
The opening left brace should be at the end of the line beginning the
compound statement and the closing right brace should be alone on a
line, tabbed under the beginning of the compound statement.
Note that the left brace beginning a function body is the only occurrence
of a left brace which is alone on a line.
.NH 2
Examples
.PP
.DS
if (expr) {
statement;
statement;
}
if (expr) {
statement;
statement;
} else {
statement;
statement;
}
.DE
Note that the right brace before the \fIelse\fR and the right brace
before the \fIwhile\fR of a \fIdo-while\fR statement (below) are the
only places where a right braces appears that is not alone on a line.
.DS
for (i = 0; i < MAX; i++) {
statement;
statement;
}
while (expr) {
statement;
statement;
}
do {
statement;
statement;
} while (expr);
switch (expr) {
case ABC:
case DEF:
statement;
break;
case XYZ:
statement;
break;
default:
statement;
break\*f;
}
.DE
.FS
.IP \*F
This \fIbreak\fR is, strictly speaking, unnecessary, but it is required
nonetheless because it prevents a fall-through error if another \fIcase\fR
is added later after the last one.
.FE
Note that when multiple \fIcase\fR labels are used, they are placed
on separate lines.
The fall through feature of the C \fIswitch\fR statement should
rarely if ever be used when code is executed before falling through
to the next one.
If this is done it must be commented for future maintenance.
.DS
if (strcmp(reply, "yes") == EQUAL) {
statements for yes
...
} else if (strcmp(reply, "no") == EQUAL) {
statements for no
...
} else if (strcmp(reply, "maybe") == EQUAL) {
statements for maybe
...
} else {
statements for none of the above
...
}
.DE
The last example is a generalized \fIswitch\fR statement and the
tabbing reflects the switch between exactly one of several
alternatives rather than a nesting of statements.
.NH
Expressions
.NH 2
Operators
.PP
The old versions of equal-ops =+, =\-, =*, etc. should not be used.
The preferred use is +=, \-=, *=, etc.
All binary operators except . and \-> should be separated from their
operands by blanks\*f.
.FS
.IP \*F
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.
.FE
In addition, keywords that are followed by expressions in parentheses
should be separated from the left parenthesis by a blank\*f.
.FS
.IP \*F
\fISizeof\fR is an exception, see the discussion of function calls.
Less logically, so is \fIreturn\fR.
.FE
Blanks should also appear after commas in argument lists to help
separate the arguments visually.
On the other hand, macros with arguments and function calls should
not have a blank between the name and the left parenthesis.
In particular, the C preprocessor requires the left parenthesis
to be immediately after the macro name or else the argument list
will not be recognized.
Unary operators should not be separated from their single operand.
Since C has some unexpected precedence rules,
all expressions involving mixed operators should be fully parenthesized.
.PP
\fIExamples\fR
.DS
a += c + d;
a = (a + b) / (c * d);
strp\->field = str.fl - ((x & MASK) >> DISP);
while (*d++ = *s++)
; /* EMPTY BODY */
.DE
.NH 2
Naming Conventions
.PP
Individual projects will no doubt have their own naming conventions.
There are some general rules however.
.IP \(bu
An initial underscore should not be used for any user-created names\*f.
.FS
.IP \*F
Trailing underscores should be avoided too.
.FE
.UX
uses it for names that the user should not have to know (like
the standard I/O library)\*f.
.FS
.IP \*F
This convention is reserved for system purposes.
If you must have your own private identifiers,
begin them with a capital letter identifying the
package to which they belong.
.FE
.IP \(bu
Macro names, \fItypedef\fR names, and \fIdefine\fR names should be
all in CAPS.
.IP \(bu
Variable names, structure tag names, and function names should be in
lower case\*f.
.FS
.IP \*F
It is best to avoid names that differ only in case, like \fIfoo\fR
and \fIFOO\fR.
The potential for confusion is considerable.
.FE
Some macros (such as \fIgetchar\fR and \fIputchar\fR) are in lower case
since they may also exist as functions.
Care is needed when interchanging macros and functions since functions
pass their parameters by value whereas macros pass their arguments by
name substitution\*f.
.FS
.IP \*F
This difference also means that carefree use of macros requires care
when they are defined.
Remember that complex expressions can be used as parameters,
and operator-precedence problems can arise unless all occurrences of
parameters in the definition 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).
.FE
.NH 2
Constants
.PP
Numerical constants should not be coded directly\*f.
.FS
.IP \*F
At the very least, any directly-coded numerical constant must have a
comment explaining the derivation of the value.
.FE
The \fIdefine\fR feature of the C preprocessor should be used to
assign a meaningful name.
This will also make it easier to administer large programs since the
constant value can be changed uniformly by changing only the \fIdefine\fR.
The enumeration data type is the preferred way to handle situations where
a variable takes on only a discrete set of values, since additional type
checking is available through \fIlint\fR.
.PP
There are some cases where the constants 0 and 1 may appear as themselves
instead of as defines.
For example if a \fIfor\fR loop indexes through an array, then
.DS
for (i = 0; i < ARYBOUND; i++)
.DE
is reasonable while the code
.DS
fptr = fopen(filename, "r");
if (fptr == 0)
error("can't open %s\en", filename);
.DE
is not.
In the last example the defined constant \fINULL\fR is available as
part of the standard I/O library's header file \fIstdio.h\fR and
must be used in place of the 0.
.NH
Portability
.PP
The advantages of portable code are well known.
This section gives some guidelines for writing portable code,
where the definition of portable is taken to mean that a source file
contains portable code if it can be compiled and executed on different
machines with the only source change being the inclusion of possibly
different header files.
The header files will contain defines and typedefs that may vary from
machine to machine.
Reference [1] contains useful information on both style and portability.
Many of the recommendations in this document originated in [1].
The following is a list of pitfalls to be avoided and recommendations
to be considered when designing portable code:
.IP \(bu
First, one must recognize that some things are inherently non-portable.
Examples are code to deal with particular hardware registers such as
the program status word,
and code that is designed to support a particular piece of hardware
such as an assembler or I/O driver.
Even in these cases there are many routines and data organizations
that can be made machine independent.
It is suggested that source file be organized so that the machine-independent
code and the machine-dependent code are in separate files.
Then if the program is to be moved to a new machine,
it is a much easier task to determine what needs to be changed\*f.
.FS
.IP \*F
If you \fI#ifdef\fR dependencies,
make sure that if no machine is specified,
the result is a syntax error, \fInot\fR a default machine!
.FE
It is also possible that code in the machine-independent files
may have uses in other programs as well.
.IP \(bu
Pay attention to word sizes.
The following sizes apply to basic types in C for the machines
that will be used most at IH\*f:
.FS
.IP \*F
The 3B is a Bell Labs machine.
The VAX, not shown in the table, is similar to the 3B in these respects.
The 68000 resembles either the pdp11 or the 3B, depending on the
particular compiler.
.FE
.br
.ne 2i
.TS
center;
l c c c
l r r r.
type pdp11 3B IBM
_
char 8 8 8
short 16 16 16
int 16 32 32
long 32 32 32
.TE
In general if the word size is important, \fIshort\fR or \fIlong\fR
should be used to get 16 or 32 bit items on any of the above machines\*f.
.FS
.IP \*F
Any unsigned type other than plain \fIunsigned int\fR should be
\fItypedef\fRed, as such types are highly compiler-dependent.
This is also true of long and short types other than \fIlong int\fR
and \fIshort int\fR.
Large programs should have a central header file which supplies
\fItypedef\fRs for commonly-used width-sensitive types, to make
it easier to change them and to aid in finding width-sensitive code.
.FE
If a simple loop counter is being used where either 16 or 32 bits will
do, then use \fIint\fR, since it will get the most efficient (natural)
unit for the current machine\*f.
.FS
.IP \*F
Beware of making assumptions about the size of pointers.
They are not always the same size as \fIint\fR.
Nor are all pointers always the same size, or freely interconvertible.
Pointer-to-character is a particular trouble spot on machines which
do not address to the byte.
.FE
.IP \(bu
Word size also affects shifts and masks.
The code
.DS
x &= 0177770
.DE
will clear only the three rightmost bits of an \fIint\fR on a PDP11.
On a 3B it will also clear the entire upper halfword.
Use
.DS
x &= ~07
.DE
instead which works properly on all machines\*f.
.FS
.IP \*F
The or operator (\ |\ ) does not have these problems,
nor do bitfields (which, unfortunately, are not very portable due to
defective compilers).
.FE
.IP \(bu
Code that takes advantage of the two's complement representation of
numbers on most machines should not be used.
Optimizations that replace arithmetic operations with equivalent
shifting operations are particularly suspect.
You should weigh the time savings with the potential for obscure
and difficult bugs when your code is moved, say, from a 3B to a 1A.
.IP \(bu
Watch out for signed characters.
On the PDP-11, characters are sign extended when used in expressions,
which is not the case on any other machine.
In particular, \fIgetchar\fR is an integer-valued function (or macro)
since the value of \fIEOF\fR for the standard I/O library is \-1,
which is not possible for a character on the 3B or IBM\*f.
.FS
.IP \*F
Actually, this is not quite the real reason why \fIgetchar\fR returns
\fIint\fR, but the comment is valid: code which assumes either
that characters are signed or that they are unsigned is unportable.
It is best to completely avoid using \fIchar\fR to hold numbers.
Manipulation of characters as if they were numbers is also
often unportable.
.FE
.IP \(bu
The PDP-11 is unique among processors on which C exists in that the
bytes are numbered from right to left within a word.
All other machines (3B, IBM, Interdata 8/32, Honeywell) number the
bytes from left to right\*f.
.FS
.IP \*F
Actually, there are some more right-to-left machines now, but
the comments still apply.
.FE
Hence any code that depends on the left-right orientation of bits
in a word deserves special scrutiny.
Bit fields within structure members will only be portable so long as
two separate fields are never concatenated and treated as a unit\*f.
.FS
.IP \*F
The same applies to variables in general.
Alignment considerations and loader peculiarities make it very rash
to assume that two consecutively-declared variables are together
in memory, or that a variable of one type is aligned appropriately
to be used as another type.
.FE
[1,3]
.IP \(bu
Do not default the boolean test for non-zero, i.e.
.DS
if (f() != FAIL)
.DE
is better than
.DS
if (f())
.DE
even though \fIFAIL\fR may have the value 0 which is considered to mean
false by C\*f.
This will help you out later when somebody decides that a failure return
should be \-1 instead of 0\ \*f.
.FS
.IP \*F
A particularly notorious case is using \fIstrcmp\fR
to test for string equality, where the result should \fInever\fR
\fIever\fR be defaulted.
The preferred approach is to define a macro \fISTREQ\fR:
.DS
#define STREQ(a, b) (strcmp((a), (b)) == 0)
.DE
.FE
.FS
.IP \*F
An exception is commonly made for predicates, which are functions
which meet the following restrictions:
.IP \(bu
Has no other purpose than to return true or false.
.IP \(bu
Returns 0 for false, 1 for true, nothing else.
.IP \(bu
Is named so that the meaning of (say) a `true' return
is absolutely obvious.
Call a predicate \fIisvalid\fR or \fIvalid\fR, not \fIcheckvalid\fR.
.FE
.IP \(bu
Be suspicious of numeric values appearing in the code.
Even simple values like 0 or 1 could be better expressed using
defines like \fIFALSE\fR and \fITRUE\fR (see previous item)\*f.
.FS
.IP \*F
Actually, \fIYES\fR and \fINO\fR often read better.
.FE
Any other constants appearing in a program would be better expressed
as a defined constant.
This makes it easier to change and also easier to read.
.IP \(bu
Become familiar with existing library functions and defines\*f.
.FS
.IP \*F
But not \fItoo\fR familiar.
The internal details of library facilities, as opposed to their
external interfaces, are subject to change without warning.
They are also often quite unportable.
.FE
You should not be writing your own string compare routine, or making
your own defines for system structures\*f.
.FS
.IP \*F
Or, especially, writing your own code to control terminals.
Use the \fItermcap\fR package.
.FE
Not only does this waste your time, but it prevents your program
from taking advantage of any microcode assists or other
means of improving performance of system routines\*f.
.FS
.IP \*F
It also makes your code less readable, because the reader has to
figure out whether you're doing something special in that reimplemented
stuff to justify its existence.
Furthermore, it's a fruitful source of bugs.
.FE
.IP \(bu
Use \fIlint\fR.
It is a valuable tool for finding machine-dependent constructs as well
as other inconsistencies or program bugs that pass the compiler\*f.
.FS
.IP \*F
The use of \fIlint\fR on all programs is strongly recommended.
It is difficult to eliminate complaints about functions whose return
value is not used (in the current version of C, at least), but most
other messages from \fIlint\fR really do indicate something wrong.
The \-h, \-p, \-a, \-x, and \-c options are worth learning.
All of them will complain about some legitimate things, but they will
also pick up many botches.
.FE
.NH
Lint
.PP
\fILint\fR is a C program checker [2] that examines C source files to
detect and report type incompatibilities, inconsistencies between
function definitions and calls,
potential program bugs, etc.
It is expected that projects will require programs to use \fIlint\fR
as part of the official acceptance procedure\*f.
.FS
.IP \*F
Yes.
.FE
In addition, work is going on in department 5521 to modify \fIlint\fR
so that it will check for adherence to the standards in this document.
.PP
It is still too early to say exactly which of the standards given here
will be checked by \fIlint\fR.
In some cases such as whether a comment is misleading or incorrect there
is little hope of mechanical checking.
In other cases such as checking that the opening brace of a function
body is alone on a line in column 1, the test has already been added\*f.
.FS
.IP \*F
Little of this is relevant at U of T.
The version of \fIlint\fR that we have lacks these mods.
.FE
Future bulletins will be used to announce new additions to \fIlint\fR
as they occur.
.PP
It should be noted that the best way to use \fIlint\fR is not as a barrier
that must be overcome before official acceptance of a program, but
rather as a tool to use whenever major changes or additions to the
code have been made.
\fILint\fR
can find obscure bugs and insure portability before problems occur.
.NH
Special Considerations
.PP
This section contains some miscellaneous do's and don'ts.
.IP \(bu
Don't change syntax via macro substitution.
It makes the program unintelligible to all but the perpetrator.
.IP \(bu
There is a time and a place for embedded assignment statements\*f.
.FS
.IP \*F
The \fB++\fR and \fB\-\-\fR operators count as assignment statements.
So, for many purposes, do functions with side effects.
.FE
In some constructs there is no better way to accomplish the results
without making the code bulkier and less readable.
The \fIwhile\fR loop in section 8.1 is one example of an appropriate
place.
Another is the common code segment:
.DS
while ((c = getchar()) != EOF) {
process the character
}
.DE
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, the code:
.DS
a = b + c;
d = a + r;
.DE
should not be replaced by
.DS
d = (a = b + c) + r;
.DE
even though the latter may save one cycle.
Note that 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\*f.
.FS
.IP \*F
Note also that side effects within expressions can result in code
whose semantics are compiler-dependent, since C's order of evaluation
is explicitly undefined in most places.
Compilers do differ.
.FE
.IP \(bu
There is also a time and place for the ternary \fI?\ :\fR operator
and the binary comma operator.
The logical expression operand before the \fI?\ :\fR should be
parenthesized:
.DS
(x >= 0) ? x : \-x
.DE
Nested \fI?\ :\fR operators can be confusing and should be avoided
if possible.
There are some macros like \fIgetchar\fR where they can be useful.
The comma operator can also be useful in \fIfor\fR statements to
provide multiple initializations or incrementations.
.IP \(bu
Goto statements should be used sparingly as in any well-structured
code\*f.
.FS
.IP \*F
The \fIcontinue\fR statement is almost as bad.
\fIBreak\fR is less troublesome.
.FE
The main place where they can be usefully employed is to break out
of several levels of \fIswitch\fR, \fIfor\fR, and \fIwhile\fR
nesting\*f, e.g.
.FS
.IP \*F
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.
.FE
.DS L
for (...)
for (...) {
...
if (disaster)
goto error;
}
\&...
error:
clean up the mess
.DE
When a \fIgoto\fR is necessary the accompanying label should be alone
on a line and tabbed one tab position to the left of the associated
code that follows.
.IP \(bu
This committee recommends that programmers not rely on automatic
beautifiers for the following reasons.
First, the main person who benefits from good program style is the
programmer himself.
This is especially true in the early design of handwritten algorithms
or pseudo-code.
Automatic beautifiers can only be applied to complete, syntactically
correct programs and hence are not available when the need for
attention to white space and indentation is greatest.
It is also felt that programmers can do a better job of making clear
the complete visual layout of a function or file, with the normal
attention to detail of a careful programmer\*f.
.FS
.IP \*F
In other words, some of the visual layout is dictated by intent
rather than syntax.
Beautifiers cannot read minds.
.FE
Sloppy programmers should learn to be careful programmers instead of
relying on a beautifier to make their code readable.
Finally, it is felt that since beautifiers are non-trivial programs
that must parse the source,
the burden of maintaining them in the face of the continuing evolution
of C is not worth the benefits gained by such a program.
.NH
Project Dependent Standards
.PP
Individual projects may wish to establish additional standards beyond
those given here.
The following issues are some of those that should be adddressed by
each project program administration group.
.IP \(bu
What additional naming conventions should be followed?
In particular, systematic prefix conventions for functional grouping
of global data and also for structure or union member names can be useful.
.IP \(bu
What kind of include file organization is appropriate for the
project's particular data hierarchy?
.IP \(bu
What procedures should be established for reviewing \fIlint\fR
complaints?
A tolerance level needs to be established in concert with the \fIlint\fR
options to prevent unimportant complaints from hiding complaints about
real bugs or inconsistencies.
.IP \(bu
If a project establishes its own archive libraries, it should plan on
supplying a lint library file [2] to the system administrators.
This will allow \fIlint\fR to check for compatible use of library
functions.
.NH
Conclusion
.PP
A set of standards has been presented for C programming style.
One of the most important points is the proper use of white space
and comments so that the structure of the program is evident from
the layout of the code.
Another good idea to keep in mind when writing code is that it is
likely that you or someone else will be asked to modify it or make
it run on a different machine sometime in the future.
.PP
As with any standard, it must be followed if it is to be useful.
The Indian Hill version of \fIlint\fR will enforce those standards
that are amenable to automatic checking.
If you have trouble following any of these standards don't just ignore them.
Programmers at Indian Hill should bring their problems to the
Software Development System Group (Lee Kirchhoff, contact) in department
5522.
Programmers outside Indian Hill should contact the Processor
Application Group (Layne Cannon, contact) in department 5512\ \*f.
.FS
.IP \*F
At U of T Zoology, it's Henry Spencer in 336B.
.FE
.bp
.ce 1
\fBReferences\fR
.sp 2
.IP [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.
.IP [2]
S.C. Johnson, "Lint, a C Program Checker", Technical Memorandum,
77-1273-14, September 16, 1977.
.IP [3]
R.W. Mitze, "The 3B/PDP-11 Swabbing Problem", Memorandum for File,
1273-770907.01MF,
September 14, 1977.
.IP [4]
R.A. Elliott and D.C. Pfeffer, "3B Processor Common Diagnostic
Standards- Version 1",
Memorandum for File, 5514-780330.01MF, March 30, 1978.
.IP [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.
.IP [6]
B.W. Kernighan and D.M. Ritchie,
\fIThe C Programming Language\fR,
Prentice-Hall 1978.
.bp
.PP
.DS L
.ta 8 16 24 32 40 48 56 64 72 80
/*
* \fBThe C Style Summary Sheet\fR Block comment,
* by Henry Spencer, U of T Zoology describes file.
*/
#include <errno.h> Headers; don't nest.
typedef int SEQNO; /* ... */ Global definitions.
#define STREQ(a, b) (strcmp((a), (b)) == 0)
static char *foo = NULL; /* ... */ Global declarations.
struct bar { Static whenever poss.
SEQNO alpha; /* ... */
# define NOSEQNO 0
int beta; /* ... */ Don't assume 16 bits.
};
/*
* Many unnecessary braces, to show where. Functions.
*/
static int /* what is returned */ Don't default int.
bletch(a)
int a; /* ... */ Don't default int.
{
int bar; /* ... */
extern int errno; /* ..., changed here */
extern char *index();
if (foobar() != FAIL) { if (!isvalid()) {
return(5); errno = ERANGE;
} } else {
x = &y + z\->field;
while (x == (y & MASK)) { }
f += (x >= 0) ? x : \-x;
} for (i = 0; i < BOUND; i++) {
/* Use lint \-hpcax. */
do { }
/* Avoid nesting ?: */
} while (index(a, b) != NULL); if (STREQ(x, "foo")) {
x |= 07;
switch (...) { } else if (STREQ(x, "bar")) {
case ABC: x &= ~07;
case DEF: } else if (STREQ(x, "ugh")) {
printf("...", a, b); /* Avoid gotos */
break; } else {
case XYZ: /* and continues. */
x = y; }
/* FALLTHROUGH */
default: while ((c = getc()) != EOF)
/* Limit imbedded =s. */ ; /* NULLBODY */
break;
}
}
.DE
-----