Question : pthread_mutex_t  vs  sem_t

Whats the difference between POSIX's and POSIX's ?  If I just wanted the semaphore/mutex properties, would I need to use pthread?  Thanks

Answer : pthread_mutex_t  vs  sem_t

semaphores can be used between different processes to synchronise access to some shared object i.e. a file or shared memory.

If you have two processes which require read/write access to some resource you need to make sure that both are not trying to change the shared resource at the same time.  You can then use a semaphore to protect the access to the shared resource i.e each process has to acquire the semaphore before being allowed to perform an update on the shared resource.  Once the update is complete the process then releases the semaphore to allow another process to access/update the shared resource.

pthread_mutex_t is a similar concept but shared between multiple threads of a single process.  If for example you have a multithreaded program which contains global data accessible/updatable by multiple threads then you would a mutex to protect the access in the same way.