The Ten Commandments for C Programmers (Annotated Edition)
Henry Spencer
November 25, 1992
1 Thou shalt run lint frequently and study its pronouncements with care, for
verily its perception and judgement oft exceed thine.
This is still wise counsel, although many modern compilers search
out many of the same sins, and there are often problems with lint
being aged and infirm, or una- vailable in strange lands. There are
other tools, such as Saber C, useful to similar ends.
``Frequently'' means thou shouldst draw thy daily guidance from it,
rather than hoping thy code will achieve lint's blessing by a
sudden act of repentance at the last minute. De-linting a program
which has never been linted before is often a cleaning of the
stables such as thou wouldst not wish on thy worst enemies. Some
observe, also, that careful heed to the words of lint can be quite
helpful in debugging.
``Study'' doth not mean mindless zeal to eradicate every byte of
lint output-if for no other reason, because thou just canst not
shut it up about some things-but that thou should know the cause of
its unhap- piness and understand what worri- some sign it tries to
speak of.
2 Thou shalt not follow the NULL pointer, for chaos and madness await thee at
its end.
Clearly the holy scriptures were mis-transcribed here, as the words
should have been ``null pointer'', to minimize confusion between
the concept of null pointers and the macro NULL (of which more
anon). Otherwise, the meaning is plain. A null pointer points to
regions filled with dragons, demons, core dumps, and numberless
other foul creatures, all of which delight in frolicing in thy
program if thou disturb their sleep. A null pointer doth not point
to a 0 of any type, despite some blasphemous old code which
impiously assumes this.
3 Thou shalt cast all function arguments to the expected type if they are not
of that type already, even when thou art convinced that this is unnecessary,
lest they take cruel vengeance upon thee when thou least expect it.
A programmer should understand the type structure of his language,
lest great misfortune befall him. Contrary to the heresies espoused
by some of the dwellers on the Western Shore, `int' and `long' are
not the same type. The moment of their equivalence in size and
representation is short, and the agony that awaits believers in
their interchangeability shall last forever and ever once 64-bit
machines become common.
Also, contrary to the beliefs com- mon among the more backward
inhabi- tants of the Polluted Eastern Marshes, `NULL' does not have
a pointer type, and must be cast to the correct type whenever it is
used as a function argument.
(The words of the prophet Ansi, which permit NULL to be defined as
having the type `void *', are oft taken out of context and
misunder- stood. The prophet was granting a special dispensation
for use in cases of great hardship in wild lands. Verily, a
righteous program must make its own way through the Thicket Of
Types without lazily relying on this rarely-available dispensation
to solve all its prob- lems. In any event, the great deity Dmr who
created C hath wisely endowed it with many types of pointers, not
just one, and thus it would still be necessary to convert the
prophet's NULL to the desired type.)
It may be thought that the radical new blessing of ``prototypes''
might eliminate the need for cau- tion about argument types. Not
so, brethren. Firstly, when confronted with the twisted strangeness
of variable numbers of arguments, the problem returns... and he who
has not kept his faith strong by repeated practice shall surely
fall to this subtle trap. Secondly, the wise men have observed that
reli- ance on prototypes doth open many doors to strange errors,
and some indeed had hoped that prototypes would be decreed for
purposes of error checking but would not cause implicit
conversions. Lastly, reliance on prototypes causeth great
difficulty in the Real World today, when many cling to the old ways
and the old compilers out of desire or necessity, and no man
knoweth what machine his code may be asked to run on tomorrow.
4 If thy header files fail to declare the return types of thy library
functions, thou shalt declare them thyself with the most meticulous care,
lest grievous harm befall thy program.
The prophet Ansi, in her wisdom, hath added that thou shouldst also
scourge thy Suppliers, and demand on pain of excommunication that
they produce header files that declare their library functions. For
truly, only they know the pre- cise form of the incantation
appropriate to invoking their magic in the optimal way.
The prophet hath also commented that it is unwise, and leads one
into the pits of damnation and sub- tle bugs, to attempt to declare
such functions thyself when thy header files do the job right.
5 Thou shalt check the array bounds of all strings (indeed, all arrays), for
surely where thou typest ``foo'' someone someday shall type
``supercalifragilis- ticexpialidocious''.
As demonstrated by the deeds of the Great Worm, a consequence of
this commandment is that robust produc- tion software should never
make use of gets(), for it is truly a tool of the Devil. Thy
interfaces should always inform thy servants of the bounds of thy
arrays, and servants who spurn such advice or quietly fail to
follow it should be dispatched forthwith to the Land Of Rm, where
they can do no further harm to thee.
6 If a function be advertised to return an error code in the event of
difficulties, thou shalt check for that code, yea, even though the checks
triple the size of thy code and produce aches in thy typing fingers, for if
thou thinkest ``it cannot happen to me'', the gods shall surely punish thee
for thy arrogance.
All true believers doth wish for a better error-handling mechanism,
for explicit checks of return codes are tiresome in the extreme and
the temptation to omit them is great. But until the far-off day of
deliverance cometh, one must walk the long and winding road with
patience and care, for thy Vendor, thy Machine, and thy Software
delight in surprises and think nothing of producing subtly mean-
ingless results on the day before thy Thesis Oral or thy Big Pitch
To The Client.
Occasionally, as with the ferror() feature of stdio, it is possible
to defer error checking until the end when a cumulative result can
be tested, and this often produceth code which is shorter and
clearer. Also, even the most zealous beli- ever should exercise
some judgement when dealing with functions whose failure is totally
uninteresting... but beware, for the cast to void is a two-edged
sword that sheddeth thine own blood without remorse.
7 Thou shalt study thy libraries and strive not to re- invent them without
cause, that thy code may be short and readable and thy days pleasant and
productive.
Numberless are the unwashed heathen who scorn their libraries on
vari- ous silly and spurious grounds, such as blind worship of the
Little Tin God (also known as ``Effi- ciency''). While it is true
that some features of the C libraries were ill-advised, by and
large it is better and cheaper to use the works of others than to
persist in re-inventing the square wheel. But thou should take the
greatest of care to understand what thy libraries promise, and what
they do not, lest thou rely on facilities that may vanish from
under thy feet in future.
8 Thou shalt make thy program's purpose and structure clear to thy fellow man
by using the One True Brace Style, even if thou likest it not, for thy
creativity is better used in solving problems than in creating beautiful new
impediments to understanding.
These words, alas, have caused some uncertainty among the novices
and the converts, who knoweth not the ancient wisdoms. The One True
Brace Style referred to is that demonstrated in the writings of the
First Prophets, Kernighan and Ritchie. Often and again it is
criticized by the ignorant as hard to use, when in truth it is
merely somewhat difficult to learn, and thereafter is wonderfully
clear and obvious, if perhaps a bit sensitive to mistakes.
While thou might think that thine own ideas of brace style lead to
clearer programs, thy successors will not thank thee for it, but
rather shall revile thy works and curse thy name, and word of this
might get to thy next employer. Many customs in this life persist
because they ease friction and pro- mote productivity as a result
of universal agreement, and whether they are precisely the optimal
choices is much less important. So it is with brace style.
As a lamentable side issue, there has been some unrest from the
fanatics of the Pronoun Gestapo over the use of the word ``man'' in
this Commandment, for they believe that great efforts and loud
shout- ing devoted to the ritual purification of the language will
somehow redound to the benefit of the downtrodden (whose real and
grievous woes tendeth to get lost amidst all that thunder and
fury). When preaching the gospel to the narrow of mind and short of
temper, the word ``creature'' may be sub- stituted as a suitable
pseudoBibli- cal term free of the taint of Pol- itical
Incorrectness.
9 Thy external identifiers shall be unique in the first six characters,
though this harsh discipline be irksome and the years of its necessity
stretch before thee seemingly without end, lest thou tear thy hair out and go
mad on that fateful day when thou desirest to make thy program run on an old
system.
Though some hasty zealots cry ``not so; the Millenium is come, and
this saying is obsolete and no longer need be supported'', verily
there be many, many ancient systems in the world, and it is the
decree of the dreaded god Murphy that thy next employment just
might be on one. While thou sleepest, he plot- teth against thee.
Awake and take care.
It is, note carefully, not neces- sary that thy identifiers be lim-
ited to a length of six characters. The only requirement that the
holy words place upon thee is uniqueness within the first six. This
often is not so hard as the belittlers claimeth.
10 Thou shalt foreswear, renounce, and abjure the vile heresy which claimeth
that ``All the world's a VAX'', and have no commerce with the benighted
heathens who cling to this barbarous belief, that the days of thy program may
be long even though the days of thy current machine be short.
This particular heresy bids fair to be replaced by ``All the
world's a Sun'' or ``All the world's a 386'' (this latter being a
particularly revolting invention of Satan), but the words apply to
all such without limitation. Beware, in particular, of the subtle
and terrible ``All the world's a 32-bit machine'', which is almost
true today but shall cease to be so before thy resume grows too
much longer.
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Hacker's Wisdom: The Ten Commandments for C Programmers