Digraph and Trigraph characters in C

Digraph Character :

The Diagraph are sequence of two character that the compiler replaces with their corresponding punctuation character.

Digraph sequence	Symbol
<%	{ left brace
%>	} right brace
<:	[ left bracket
:>	] right bracket
%:	# hash

%:include<stdio.h>

int main()

<%

    printf("Hello world!");

    return 0;

%>

which is treated the same as :-

#include<stdio.h>

int main()

{  

    printf("Hello world!");

    return 0;

}

Trigraph Character :

Trigraph are sequence of three characters (introduced by two consecutive Question marks ) that the compiler replaces with their corresponding punctuation characters. 

some keywords may not have all the characters from the C character set. C supports the facility of "trigraph sequence" to print these characters. These trigraph sequences have three characters. First two are '??' and third character from C character set. some trigraph sequences are as given below -

Trigraph sequence	Symbol
??<	{ left brace
??>	} right brace
??(	[ left bracket
??)	] right bracket
??!	| vertical bar
??/	\ backslash
??=	# hash
??-	~ tilde
??’	^ caret

The following source line

printf("EKD??/n");

It's saying that it  will replace the trigraph sequence, but it's not.

it's print "EKD??/n"

Now understand How trigraph work in c

Trigraphs are disabled by default in gcc. If you are using gcc then compile with -trigraphs to enable  trigraphs :

gcc -trigraphs source.c

The ??/ in the printf is a trigraph character which is a part of C's preprocessor.

If you enable trigraph by using gcc -trigaphs source.c , it will convert ??/ into \.your code will look like this :

    printf("EKD??/n"); // before enable trigraph

    printf("EKD?\n"); // after enable trigraph

??=include<stdio.h>

int main()

??<

    printf("EKD??/n"); // before enable trigraph

    return 0;

??>

which is treated the same as :-

#include<stdio.h>

int main()

{

    printf("EKD??/n"); // before enable trigraph

    return 0;

}