one can use semaphores as both locks and condition variables
- P operation (also known as wait, sleep, or down operation): This operation decreases the value of the semaphore. If the semaphore’s value becomes negative, the process executing the P operation is blocked, i.e., added to the semaphore’s queue.
- V operation (also known as signal, wake, or up operation): This operation increases the value of the semaphore. If the resulting value is less than or equal to zero, then a process blocked in the semaphore’s queue gets unblocked.
Example
A semaphore is an object with an integer value that we can manipulate with two routines; in the POSIX standard, these routines are sem_wait() and sem_post(). Because the initial value of the semaphore determines its behavior, before calling any other routine to interact with the semaphore, we must first initialize it to some value.
#include <semaphore.h>
sem_t s;
sem_init(&s, 0, 1);
// Binary Semaphore, aka a lock
sem_wait(&s);
// critical section here
sem_post(&s);The second argument to sem_init() will be set to 0 in all of the examples we’ll see; this indicates that the semaphore is shared between threads in the same process. They can be used to synchronize access across different processes