C program to invoke another C program's main() method?

[Arkimedes]

OK, I've been searching the net for quite a while and I can't figure it out. I have a simple C program, let's call it a.c

#include <stdio.h>

int main(int argc, char **argv) {
     printf("Hello World!\n");
     return 0;
}

and I have another C program (b.c) that needs to invoke a.c's main(argc, argv) method. How can this (if at all) be done?