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

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??! 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 ยง5.2.1.1, 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.

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