How can I call main() function

This article gives you some basic idea: what happens when user application/program starts.
Why I am saying "some basic idea" since here I am not considering dynamic linking/loading of shared libraries.
If you want to know in depth of what happens when user application/program starts, please
read my Linker and Loader blog:
http://bhushanverma.blogspot.com/search/label/ELF%2FLinker%2FLoader%2Flibc%2FCompiler

Whenever user runs any user application, its get loaded in memory by Operating System/Loader.
Now Operating System/Loader calls user application _start entry point.
User application starts running at _start.
_start calls the user program's main function,then it calls the exit(0)system call, which cleans up the process.

User application starts running at _start as follows:

_start(args) {
ret = main(args);
exit(ret);
}

when main() returns, os calls exit() which destroy the process and returns all its resources.
Most of the above code is written in assembly.

Now lets take an example to undestand this:

$ cat main.c
#include

_start ()
{
int ret;
ret = main ();
exit (ret);
}

int
main (void)
{
printf ("hello world");
return 0;
}

Now build this c program:
$ gcc -c main.c
$ ld -main main.o -lc
$ ./main
hello world

If the above commands not works try as follows:
$ gcc -o main -nostdlib main.c -lc
$ ./main
hello world

Yes, we did it.