Client Server Application - Multi threaded server.

You're trying to implement the thread semaphore between the server and the vendor using sem_init() so that only one thread can read the data from a client and send the data to the vendor for some operation and then, sending back to server using shared memory and you have implemented sem_wait() before sending the data to the vendor and after reading from the server you sem_post() it, right? If it's yes, then the possible reason for the semaphore variable not reflecting in sem_wait() after you sem_post() it in the vendor is the pshared argument in the sem_init().
The pshared variable should be 1 and the most important thing to make it happen is to create a shared memory containing the thread semaphore object so that both the processes can share that semaphore object.