Semaphore

Semaphores are synchronization primitives used to control access to shared resources in concurrent programming. They maintain a counter that represents the number of available resources.

Main types:

• Binary semaphores: Counter values are 0 or 1 (functioning like mutexes)
• Counting semaphores: Counter can have arbitrary non-negative values

Core operations:

• wait/P/down: Decrement counter if it's > 0, otherwise blocks until it can
• signal/V/up: Increment counter, potentially unblocking waiting processes

typedef struct {
    int value;
    /* ... */
} semaphore;

void sem_init(semaphore *sem, int value);
void sem_wait(semaphore *sem);
int  sem_trywait(semaphore *sem); // non-blocking
void sem_post(semaphore *sem);
int  sem_value(semaphore *sem, int *sval);
void sem_destroy(semaphore *sem);