Why main does not return 0 here?

Question

I was just reading

ISO/IEC 9899:201x Committee Draft — April 12, 2011

in which i found under 5.1.2.2.3 Program termination

Answer 1

That rule was added in the 1999 version of the C standard. In C90, the status returned is undefined.

You can enable it by passing -std=c99 to gcc.

As a side note, interestingly 9 is returned because it's the return of printf which just wrote 9 characters.