What does the ??!??! operator do in C?


??! is a trigraph that translates to |. So it says:

!ErrorHasOccured() || HandleError();

which, due to short circuiting, is equivalent to:

if (ErrorHasOccured()) 

    HandleError();

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.