What does the C ??!??! operator do?
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?
??! 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.
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.
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.
It's a C trigraph. ??! is | , so ??!??! is the operator ||
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