devzero.c

该代码为unix环境高级编程的源代码

#include	<sys/types.h>
#include	<sys/mman.h>
#include	<fcntl.h>
#include	"ourhdr.h"

#define	NLOOPS		1000
#define	SIZE		sizeof(long)	/* size of shared memory area */

static int	update(long *);

int
main()
{
	int		fd, i, counter;
	pid_t	pid;
	caddr_t	area;

	if ( (fd = open("/dev/zero", O_RDWR)) < 0)
		err_sys("open error");
	if ( (area = mmap(0, SIZE, PROT_READ | PROT_WRITE,
			  						MAP_SHARED, fd, 0)) == (caddr_t) -1)
		err_sys("mmap error");
	close(fd);		/* can close /dev/zero now that it's mapped */

	TELL_WAIT();
	if ( (pid = fork()) < 0) {
		err_sys("fork error");
	} else if (pid > 0) {			/* parent */
		for (i = 0; i < NLOOPS; i += 2) {
			if ( (counter = update((long *) area)) != i)
				err_quit("parent: expected %d, got %d", i, counter);
			TELL_CHILD(pid);
			WAIT_CHILD();
		}

	} else {						/* child */
		for (i = 1; i < NLOOPS + 1; i += 2) {
			WAIT_PARENT();
			if ( (counter = update((long *) area)) != i)
				err_quit("child: expected %d, got %d", i, counter);
			TELL_PARENT(getppid());
		}
	}
	exit(0);
}
static int
update(long *ptr)
{
	return( (*ptr)++ );	/* return value before increment */
}