Information about trigraphs -- Posted by inturi on Wednesday, January 23 2008
A trigraph is a sequence of three characters that begins with two question marks (??).
You use trigraphs to write C source files with a character set that does not contain convenient graphic representations for some punctuation characters.
(The resultant C source file is not necessarily more readable, but it is unambiguous.)
The list of all defined trigraphs is:

Character Trigraph
[ ??(
\ ??/
] ??)
^ ??'
{ ??<
| ??!
} ??>
~ ??-
# ??=

These are the only trigraphs. The translator does not alter any other sequence that begins with two question marks.

The reason is that on many platforms trigraphs are enabled by default whereas on Linux trigraphs are enabled only when -ansi flag is passed to the compiler. We should compile the test files with -ansi flag as well.