Make your own free website on

         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

     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

  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

     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.


    Hacker's Wisdom: The Ten Commandments for C Programmers