Print ?? and !! in different sequence will show different output

Question

I had found a strange output when I write the following lines in very simple way:

Code:

 printf("LOL??!\n");
 printf("LOL!!?\n");

Output: alt text

It happens even the code is compiled under both MBCS and UNICODE.

The output varies on the sequence of "?" and "!"...

Any idea?

Explained below: To overcome just escape one of the question marks. — Martin York, Oct 05 '10 at 10:39
The best part of trigraphs is that if you don't know what they are called, it is impossible to search for them. :-) (Google isn't too helpful when asked about "??") — James McNellis, Oct 05 '10 at 11:48
@JamesMcNellis A search for '??!' at [SymbolHound](http://www.symbolhound.com/) led me here :) — Cam Jackson, Apr 09 '12 at 09:54

score 16 · Answer 1 · answered Oct 05 '10 at 10:07

16

??! is a trigraph that gets replaced by |.

As a rule, you should never place two question mark characters together anywhere in a source file.

answered Oct 05 '10 at 10:07

James McNellis

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Or anywhere else, for that matter. :) – jalf Oct 05 '10 at 14:28
@jalf: I wish the ANSI C committee had followed your rule... ;-) – R.. GitHub STOP HELPING ICE Oct 05 '10 at 17:02
I mainly just wish everyone who asked questions followed it ;) – jalf Oct 06 '10 at 08:42

codaddict · Answer 2 · 2010-10-05T10:16:20.743

5

They are called Trigraph Sequences

??! is the trigraph sequence for Vertical Bar |.

The C/C++ preprocessor recognizes the trigraphs and replaces them with their equivalent character.
So by the time your code is seen by the compiler, the trigraphs are already replaced.

# grepping in the source file:
$ grep printf a.c      
  printf("foo: ??!");

# grepping the preprocessor output:
$ gcc a.c -trigraphs -E | grep printf | grep foo
  printf("foo: |");

edited Oct 05 '10 at 10:16

answered Oct 05 '10 at 10:08

codaddict

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All these examples are in strings, but it should be noted that trigraph replacement happens **everywhere**, not just in strings. – R.. GitHub STOP HELPING ICE Oct 05 '10 at 17:03

score 5 · Accepted Answer · answered Oct 06 '10 at 05:28

You may try

printf( "What?\?!\n" );

In computer programming, digraphs and trigraphs are sequences of two and three characters respectively which are interpreted as one character by the programming language.

Some compilers support an option to turn recognition of trigraphs off, or disable trigraphs by default and require an option to turn them on. Some can issue warnings when they encounter trigraphs in source files. Borland supplied a separate program, the trigraph preprocessor, to be used only when trigraph processing is desired.

score 4 · Answer 4 · answered Oct 05 '10 at 10:10

4

The ??! is known as trigraph and is replaced with | in output. Check this link

answered Oct 05 '10 at 10:10

Sachin Shanbhag

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score 2 · Answer 5 · answered Oct 05 '10 at 10:11

2

It is a special sequence of characters in a string constant that has a special meaning. Called a trigraph they were originally implemented because not all terminals supported some characters.

answered Oct 05 '10 at 10:11

Hogan

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... nowadays, instead, they are implemented just to make programming more interesting. :) – Matteo Italia Oct 05 '10 at 10:20
@Matteo : True. Programming is def not interesting. – Hogan Oct 05 '10 at 16:28

Print ?? and !! in different sequence will show different output

5 Answers5

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