How is this bracketless/braceless code valid?

Question

So I came across this piece of code:

%:include <stdio.h>

int main(int argc, char *argv<::>) <%
    printf("C program succesfully running...");
    getchar();

    return 0;
%>

Is this some compiler bug or is this something new I have not discovered yet because it is running without any problems.

Why is bracketless code a problem? The boggling thing is braceless code — Cruncher, Oct 28 '13 at 12:43

score 13 · Accepted Answer · answered Oct 28 '13 at 12:43

13

This code uses C digraphs, as explained here: http://en.wikipedia.org/wiki/Digraphs_and_trigraphs#C

The C standard (ISO/IEC 9899:201x, Punctuators §6.4.6:3) has this to say on the matter:

In all aspects of the language, the six tokens
<: :> <% %> %: %:%:
behave, respectively, the same as the six tokens
[ ] { } # ##
except for their spelling.

answered Oct 28 '13 at 12:43

David Heffernan

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Interesting, thanks for the link, It is all making sense now! – Oct 28 '13 at 12:46
why should one do that?! – Peter Miehle Oct 28 '13 at 12:53
1

@PeterMiehle From the link of this answer: `keyboards may not have keys to cover the entire character set of the language` – Oct 28 '13 at 12:54
@PeterMiehle Follow the Wikipedia link for an explanation – David Heffernan Oct 28 '13 at 12:55

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

These are punctuators and %: is the same as # and %> is the same as } etc... The C99 draft standard section 6.4.6 Punctuators says:

In all aspects of the language, the six tokens67)
 <:  :> <%   %> %:  %:%:
behave, respectively, the same as the six tokens
  [   ] {    }   #   ##

They are also sometimes refereed to as digraphs as we see in note 67:

These tokens are sometimes called ‘‘digraphs’’.

score 1 · Answer 3 · answered Oct 28 '13 at 12:45

It is all a very Zen little corner of the C language.

In the olden days, terminals and keyboards often didn't support all the characters one might want for programming; so digraphs and trigraphs were introduced.

I'm surprised any modern compiler still supports them, but there it is.

score 1 · Answer 4 · answered Oct 28 '13 at 12:46

1

Digraph, introduced in C99.

C99 6.4.6 Punctuators

In all aspects of the language, the six tokens
<: :> <% %> %: %:%:
behave, respectively, the same as the six tokens
[ ]  {} # ##
except for their spelling.

answered Oct 28 '13 at 12:46

Yu Hao

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How is this bracketless/braceless code valid?

4 Answers4

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