//-------------------------------------------------------------------
//	launch.cpp
//
//	This program illustrates use of the UNIX 'mmap()' function
//	to map a file having so-called 'binary-executable' format,
//	and then execute it by making an (indirect) function-call. 
//	A special 'linker-script' was used to override the default
//	Executable and Linkable File-format (ELF) that is normally 
//	produced by the GNU compiler tools (gcc, as, and ld).  The
//	binary-executable format used here is inteded to be loaded
//	(and to reside during execution) at virtual address zero.
//
//	programmer: ALLAN CRUSE
//	written on: 19 OCT 2004
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror() 
#include <fcntl.h>	// for open() 
#include <stdlib.h>	// for exit() 
#include <unistd.h>	// for lseek(), close() 
#include <sys/mman.h>	// for mmap(), unmap()

void (*myexec)( void );	// null function-pointer

char filename[] = "courseid.b";

int main( int argc, char **argv )
{
	//-----------------------------------------------
	// open the executable file for read-only access
	//----------------------------------------------- 
	int	fd = open( filename, O_RDONLY );
	if ( fd < 0 ) { perror( filename ); exit(1); }

	//----------------------------------------------
	// memory-map this file at virtual-address zero
	//---------------------------------------------- 
	int	size = lseek( fd, 0, SEEK_END );
	int	prot = PROT_READ | PROT_EXEC;
	int	flag = MAP_PRIVATE | MAP_FIXED;
	void	*mm = mmap( NULL, size, prot, flag, fd, 0 );
	if ( mm == MAP_FAILED ) { perror( "mmap" ); exit(1); }

	//----------------------------------------------------
	// execute the program with an indirect function-call
	//---------------------------------------------------- 
	myexec();

	//-----------------------------------------------
	// close the file, destroy the mapping, and exit
	//----------------------------------------------- 
	munmap( mm, size );
	close( fd );
	exit(0);
}