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                   The Ten Commandments for C Programmers
                            (Annotated Edition)
                                                            by Henry Spencer

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.

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.

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 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.

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.

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.

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 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.

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

   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

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.

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

   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.

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.