//-------------------------------------------------------------------
//	tryshm.cpp
//
//	This program illustrates use of the four standard UNIX/Linux 
//	shared-memory API functions, as prototyped in <sys/shm.h>:  
//
//	   int shmget( key_t key, int size, int shmflags ); 
//	   void *shmat( int shmid, char *shmaddr, int shmflags );
//	   void *shmdt( char *shmaddr );
//	   int shmctl( int shmid, int cmd, struct shmid_ds *buf );   
//
//	Here a child-process acquires some information and shares it
//	with its parent-process by using a 'shared' memory-segment.
//
//	programmer: ALLAN CRUSE
//	written on: 07 OCT 2004
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror() 
#include <string.h>	// for strlen(), strtok()
#include <stdlib.h>	// for exit() 
#include <unistd.h>	// for fork() 
#include <sys/wait.h>	// for wait()
#include <sys/shm.h>	// for shmget(), shmat(), shmdt(), shmctl()


#define SHM_SIZE 40	// here we only need a small-sized buffer


void child_task( int shm_handle )
{
	//--------------------------------------------------
	// first, attach the parent's shared memory-segment 
	//--------------------------------------------------
	char 	*my_shm_address = (char*)shmat( shm_handle, 0, 0 ); 
	if ( my_shm_address == (char*)-1 ) { perror( "shmat" ); exit(1); }
	//printf( "my_shm_address=%08X \n", my_shm_address );

	//------------------------------------------------------
	// next, do some work and share results with the parent
	//------------------------------------------------------ 	
	char	prompt[] = "\nPlease type your name: ";
	write( STDOUT_FILENO, prompt, strlen( prompt ) );
	read( STDIN_FILENO, my_shm_address, SHM_SIZE ); 
	
	//----------------------------------------------------
	// finally, detach the shared memory-segment and exit
	//---------------------------------------------------- 
	printf( "(now the child is terminating)\n" );
	shmdt( my_shm_address );
	exit(0);
}	




int main( void )
{
	//-------------------------------------------
	// begin by creating a shared memory-segment
	//------------------------------------------- 
	int	shm_handle;
	shm_handle = shmget( IPC_PRIVATE, SHM_SIZE, IPC_CREAT | 0666 );
	if ( shm_handle < 0 ) { perror( "shmget" ); exit(1); }
	//printf( "shm_handle=%d \n", shm_handle );

	//------------------------------
	// then start the child process
	//------------------------------
	int	status = 0, pid = fork();
	if ( pid == 0 ) child_task( shm_handle );
	
	//-----------------------------------------------
	// now attach the shared memory-segment and wait
	//-----------------------------------------------
	void *shm_address = shmat( shm_handle, NULL, 0 ); 
	if ( shm_address == (void*)-1 ) perror( "shmat" );      
	//printf( "shm_address=%08X \n", shm_address );
	wait( &status );

	//--------------------------------------------------
	// display the contents of the shared memory-memory
	//--------------------------------------------------
	char	*msg = (char*)shm_address;
	strtok( msg, "\n" );	// replaces '\n' with '\0'
	printf( "\nok, parent thinks you are \'%s\' \n", msg );
	
	//------------------------------------------------
	// then remove the shared memory-segment and exit
	//------------------------------------------------
       	printf( "(now the parent is terminating)\n" );
	shmctl( shm_handle, IPC_RMID, 0 );
	exit(0);
}