C Code: How does these even work?

Question

I just saw this here

#include 

int main(int argc, char *argv[printf(\"Hello, world!\\n\")]) {}

What this does is print \"Hello

Answer 1

If I figure out how the compiler parsed it, I'll update this, but at least there needs to be no guesswork as to how it compiled:

objdump --disassemble /tmp/hello (edited):

080483c4 
:
 80483c4:       55                      push   %ebp
 80483c5:       89 e5                   mov    %esp,%ebp
 80483c7:       83 e4 f0                and    $0xfffffff0,%esp
 80483ca:       83 ec 10                sub    $0x10,%esp
 80483cd:       b8 a0 84 04 08          mov    $0x80484a0,%eax
 80483d2:       89 04 24                mov    %eax,(%esp)
 80483d5:       e8 22 ff ff ff          call   80482fc 
 80483da:       c9                      leave  
 80483db:       c3                      ret    
 80483dc:       90                      nop
 80483dd:       90                      nop
 80483de:       90                      nop
 80483df:       90                      nop

Since Linux executables are based normally at 0x8048000, the address of the argument to printf is at an offset of 0x00004a0 from the start of the binary:

xxd /tmp/hello | grep 00004a0

00004a0: 4865 6c6c 6f2c 2077 6f72 6c64 210a 0000  Hello, world!...

So, the address of the string is pushed, and printf is called with that one arg. Nothing magical at that level, so all the fun stuff was done by gcc.