struct namelist
{
int id;
char name[20];
};

int
main(void)
{
int fd, pid, status;
int *ptr;
struct stat stat;

// create a Posix shared memory
int flags = O_RDWR | O_CREAT;
fd = shm_open(shmfile, flags, FILE_MODE);
if (fd < 0)
{
printf("shm_open failed, errormsg=%s errno=%d", strerror(errno), errno);
return 0;
}
ftruncate(fd, size);
ptr = (int *)mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

pid = fork();
if (pid == 0) { // child process
sleep(5);
printf("Child %d: start\n", getpid());

fd = shm_open(shmfile, flags, FILE_MODE);
fstat(fd, &stat);
ptr = (int *)mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
close(fd);
struct namelist tmp;

// store total num in ptr[0];
*ptr = 3;

namelist *cur = (namelist *)(ptr+1);

// store items
tmp.id = 1;
strcpy(tmp.name, "Nellson");
*cur++ = tmp;
tmp.id = 2;
strcpy(tmp.name, "Daisy");
*cur++ = tmp;
tmp.id = 3;
strcpy(tmp.name, "Robbie");
*cur++ = tmp;

printf("wake up parent\n");
syscall(__NR_futex ,ptr, FUTEX_WAKE, 1, NULL );

exit(0);
} else{ // parent process
printf("parent start waiting\n");
syscall(__NR_futex , ptr, FUTEX_WAIT, *(int *)ptr, NULL );
printf("parent end waiting\n");

struct namelist tmp;

int total = *ptr;
printf("\nThere is %d item in the shm\n", total);

ptr++;
namelist *cur = (namelist *)ptr;

for (int i = 0; i< total; i++) {
tmp = *cur;
printf("%d: %s\n", tmp.id, tmp.name);
cur++;
}

printf("\n");
waitpid(pid, &status, 0);
}

// remvoe a Posix shared memory from system
printf("Parent %d get child status:%d\n", getpid(), status);
return 0;
}

------------------分割线------------------

root:/home/ftpuser/ipc#g++ -o futex -lrt futex.cc
root:/home/ftpuser/ipc#./futex
parent start waiting
Child 2825: start
wake up parent
parent end waiting

There is 3 item in the shm
1: Nellson
2: Daisy
3: Robbie

Parent 2824 get child status:0