Why the program compiles?

I am just trying to understand this C code ( not trying to achieve any functional goal by the program). This compiles using gcc. Is this main in main(int a, char *argv[] ) format? Is it permissible to declare anything beween argument and function body (similar to char *a; here)?

#include <stdio.h>
main(u,_,a)
  char
  *a;
{
  //printf("%s\n",_,a);//just to help debugging
  //printf("%d\n",u); //just to help debugging
}