How is `int main(int argc, char* argv<::>)` a valid signature of main?

Question

I've seen in a site that int main(int argc, char* argv<::>) can also be used as a signature of main. Surprisingly, The following program:

int main(int argc, char* argv<::>)
{
  return 0;
}

compiles withput any warnings in GCC , as well as clang. It also compiles in C++.

So, how is it that int main(int argc, char* argv<::>) is a valid signature of main?

Answer 1

char* argv<::> is equivalent to char* argv[]. <: and :> used here are digraphs.

C11: 6.4.6 (p3):

In all aspects of the language, the six tokens⁷⁹⁾

<: :> <% %> %: %:%:

behave, respectively, the same as the six tokens

[ ] { } # ##

except for their spelling. ⁸⁰⁾

---

_{Foot note:
79) These tokens are sometimes called ‘‘digraphs’’.
80) Thus [ and <: behave differently when ‘‘stringized’’ (see 6.10.3.2), but can otherwise be freely interchanged.}

An example:

%: define  stringize(a) printf("Digraph \"%s\" retains its spelling in case of stringization.\n", %:a)    

Calling the above macro

stringize( %:);  

will print

Digraph "%:" retains its spelling in case of stringization.