simple testing of semaphores - unix

Question

I am trying to create a simple program to test semaphores. I am forking the process and tormenting the value of variable c in the critical section of each process, but the value of c I get is still 1 not 2. Even with the mmap() uncommented. Can anyone please explain to me what I am doing wrong? Any help would be appreciated. I am a total newbie in this. Thank you very much for your time.
int main()
{
int c = 0;
sem_t mutex;
sem_t mutex1;
// sem_t *mutex = (sem_t*)mmap(NULL, sizeof(sem_t*), PROT_READ|PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
sem_init(&mutex, 0, 1);
sem_init(&mutex1, 0, 1);
pid_t i;
int id = fork();
if(id == -1) {}
else if(id == 0)
{
sem_wait (&mutex);
c++;
sem_post (&mutex);
}
else
{
sem_wait (&mutex);
c++;
sem_post (&mutex);
}
cout<<c<<endl;
//system("pause");
return 0;
}
I tried it another way by making the pshared argument 1, but it still does not work.
I have also tried it sem_op but it still does not work.
int main()
{
int c = 0;
int sid =semget(1105,2, 0666 | IPC_CREAT);
pid_t i;
int id = fork();
if(id == -1)
{
}
else if(id == 0)
{
struct sembuf sb;
sb.sem_num = 0;
sb.sem_op = -1;
sb.sem_flg = 0;
if((semop(sid, &sb, 1)) == -1)
cout<<"error"<<endl;
c++;
sb.sem_num = 0;
sb.sem_op = -1;
sb.sem_flg = 0;
if((semop(sid, &sb, 1)) == -1)
cout<<"error"<<endl;
}
else if(id == 1)
{
struct sembuf sb;
if((semop(sid, &sb, 1)) == -1)
cout<<"error"<<endl;
c++;
sb.sem_num = 0;
sb.sem_op = -1;
sb.sem_flg = 0;
if((semop(sid, &sb, 1)) == -1)
cout<<"error"<<endl;
}
cout<<c<<endl;
return 0;
}

Answer 1

If you use fork() you have to share the semaphores between the forked processes. See sem_init() manual for more details.
Alternatively you can use a named semaphore, see sem_open() for details, and
also a good article on the subject.

Answer 2

Your primary misstep is that the variable c is not itself shared — each process operates on its own copy of the variable. You want something like this:
int *c;
c = mmap(NULL, sizeof(*c), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
*c = 0;
// ... later ...
++*c;
Additionally, with respect to your sem_init() example, you should:
Allocate shared memory of the correct size: sizeof(sem_t) and not sizeof(sem_t*)
Set the pshared flag during sem_init()
You likely don't need conditional logic differentiating parent from child after the fork(). After all, you want them to do the same thing.
(Separately, please do not name a POSIX semaphore "mutex." That name will mislead hurried, POSIXly-minded folk who will think you are referring to a different kind of synchronization primitive.)
With respect to your semget() example, you appear to be waiting on the semaphore twice (sb.sem_op = -1) in the child process. The post-fork() check for the parent is incorrect — you check if the returned PID is 1 (which it will never be on a typical UNIX system) rather than if the returned PID is > 0. (Again, however, you likely don't need to have parent and child do different things here.)