What does the operator do in C

Question

I saw a line of C that looked like this    ErrorHasOccured          HandleError      It compiled correctly and seems to run ok  It seems like it s checking if an error has occurred  and if it has  it handles it  But I m not really sure what it s actually doing or how it s doing it  It does look like the programmer is trying express their feelings about errors   I have never seen the        before in any programming language  and I can t find documentation for it anywhere   Google doesn t help with search terms like          What does it do and how does the code sample work

User · Answer

It s a C trigraph      is    so        is the operator

User · Answer

Well  why this exists in general is probably different than why it exists in your example   It all started half a century ago with repurposing hardcopy communication terminals as computer user interfaces  In the initial Unix and C era that was the ASR-33 Teletype    This device was slow  10 cps  and noisy and ugly and its view of the ASCII character set ended at 0x5f  so it had  look closely at the pic  none of the keys              The trigraphs were defined to fix a specific problem  The idea was that C programs could use the ASCII subset found on the ASR-33 and in other environments missing the high ASCII values      Your example is actually two of      each meaning    so the result is       However  people writing C code almost by definition had modern equipment 1 so my guess is  someone showing off or amusing themself  leaving a kind of Easter egg in the code for you to find   It sure worked  it led to a wildly popular SO question      nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp ASR-33 Teletype  1  For that matter  the trigraphs were invented by the ANSI committee  which first met after C become a runaway success  so none of the original C code or coders would have used them

User · Answer

is a trigraph that translates to    So it says    ErrorHasOccured      HandleError      which  due to short circuiting  is equivalent to   if  ErrorHasOccured        HandleError      Guru of the Week  deals with C   but relevant here   where I picked this up   Possible origin of trigraphs or as  DwB points out in the comments it s more likely due to EBCDIC being difficult  again   This discussion on the IBM developerworks board seems to support that theory   From ISO IEC 9899 1999   5 2 1 1  footnote 12  h t  Random832       The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as   described in ISO IEC 646  which is a subset of the seven-bit US ASCII code set

User · Answer

As already stated        is essentially two trigraphs      and     again   mushed together that get replaced-translated to     i e the logical OR  by the preprocessor  The following table containing every trigraph should help disambiguate alternate trigraph combinations  Trigraph   Replaces                               lt              gt                                                                 -           Source  C  A Reference Manual 5th Edition So a trigraph that looks like        will eventually map to                  will get replaced by      and so on  you get the idea  Since trigraphs are substituted during preprocessing you could use cpp to get a view of the output yourself  using a silly trigr c program  void main    const char  s    quot        quot       and processing it with  cpp -trigraphs trigr c   You ll get a console output of void main    const char  s    quot    quot      As you can notice  the option -trigraphs must be specified or else cpp will issue a warning  this indicates how trigraphs are a thing of the past and of no modern value other than confusing people who might bump into them   As for the rationale behind the introduction of trigraphs  it is better understood when looking at the history section of ISO IEC 646   ISO IEC 646 and its predecessor ASCII  ANSI X3 4  largely endorsed existing practice regarding character encodings in the telecommunications industry  As ASCII did not provide a number of characters needed for languages other than English  a number of national variants were made that substituted some less-used characters with needed ones    emphasis mine  So  in essence  some needed characters  those for which a trigraph exists  were replaced in certain national variants  This leads to the alternate representation using trigraphs comprised of characters that other variants still had around

[c] What does the ??!??! operator do in C?

Examples related to c

Examples related to operators

Examples related to trigraphs