Semaphores not working on threads - c++

Question

I am trying to understand how semaphores work in C++ but I am having some troubles.
Here is my code:
#include <iostream>
#include <pthread.h>
#include <fcntl.h> /* For O_* constants */
#include <sys/stat.h> /* For mode constants */
#include <semaphore.h>
using namespace std;
static sem_t *sem_thread;
static pthread_t thread_id;
void * threadFunc(void *) {
cout << "threadFunc\n";
cout << "threadFunc\n";
cout << "threadFunc\n";
cout << "threadFunc\n";
cout << "threadFunc\n";
sem_post(sem_thread);
return 0;
}
int main()
{
// Init semaphores
sem_thread = sem_open("./semaphores/sem_thread", O_TRUNC, 0777, 0);
// Init thread
int rc = pthread_create(&thread_id, NULL, threadFunc, NULL);
if (rc != 0)
{
cerr << "Pthread couldn't be created. rc=" << rc << endl;
abort();
}
sem_wait(sem_thread);
cout << "Main thread\n";
cout << "Main thread\n";
cout << "Main thread\n";
cout << "Main thread\n";
cout << "Main thread\n";
sem_close(sem_thread);
sem_unlink("./semaphores/sem_thread");
return 0;
}
So I expect the program to print threadFunc first and then Main thread. However, this is what I get:
Main thread
tMhariena dtFhurneca
dt
hMraeiand Ftuhnrce
atdh
rMeaaidnF utnhcr
etahdr
eMaadiFnu ntch
rteharde
adFunc
Any idea of what's happening?

Answer 1

You're not creating the semaphore, nor checking whether it was created.
There are two problems with your call to sem_open:
you need O_CREAT, not O_TRUNC, to create it.
the name isn't valid. Named semaphores aren't kept in the filesystem.
Looking at man sem_overview, the naming convention is specified thusly:
A named semaphore is identified by a name of the form /somename;
that is, a null-terminated string of up to NAME_MAX-4 (i.e.,
251) characters consisting of an initial slash, followed by one
or more characters, none of which are slashes.