The Ten Commandments for C Programmers (Annotated Edition)
By Henry Spencer
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 unavailable 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
unhappiness and understand what worrisome sign it tries to speak of.
2 Thou shalt not follow the NULL pointer, for chaos and madness await thee at
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
Also, contrary to the beliefs common among the more backward
inhabitants 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
misunderstood. 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
problems. 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
It may be thought that the radical new blessing of ``prototypes''
might eliminate the need for caution 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 reliance
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 precise 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 subtle 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
As demonstrated by the deeds of the Great Worm, a consequence of this
commandment is that robust production 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
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 meaningless
results on the day before thy Thesis Oral or thy Big Pitch To The
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 believer 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 reinvent 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
various silly and spurious grounds, such as blind worship of the
Little Tin God (also known as ``Efficiency''). 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
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 promote 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
shouting 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 substituted as a suitable pseudoBiblical
term free of the taint of Political 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 plotteth against thee. Awake and take
It is, note carefully, not necessary that thy identifiers be limited
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.