Detailed explanation of various types of semaphores in Linux

Definition: Protect shared resources so that only one process (thread) owns the resource at a time

Principle: When the semaphore value is positive, it means it is idle, if it is 0 or negative, it means it is occupied

Classification: kernel semaphore and user semaphore. User semaphore is divided into POXIS semaphore and SYSTEMV semaphore. POXIS semaphore is divided into named semaphore and unnamed semaphore.

Kernel semaphore:

• #include

• void sema_init(struct semaphore *sem, int val);

• void init_MUTEX(struct semaphore *sem); //Initial value 1

• void init_MUTEX_LOCKED(struct semaphore *sem); //Initial value 0

• void down(struct semaphore *sem); //Can sleep

• int down_interruptible(struct semaphore *sem); //Can be interrupted

• int down_trylock(struct semaphore *sem); //m non-blocking

• void up(struct semaphore *sem);

SYSTEMV semaphore:

• include

• ##int semget(key_t key, int nsems, int oflag);

##int semop(int semid, struct sembuf *opsptr, size_t nops);
int semctl(int semid , int semum, int cmd,...);
POSIX unnamed semaphore

include
sem_t sem;
int sem_init(sem_t *sem, int pshared, unsigned int val); //If pshared is 0, it is shared between threads, pshared If it is 1, the parent and child processes share
int sem_wait(sem_t *sem); //Blocking
int sem_trywait(sem_t *sem); / /Non-blocking

• ##int sem_post(sem_t *sem);

• int sem_destroy(sem_t *sem);

• If shared between processes, sem must be placed in the shared memory area (mmap, shm_open, shmget). Storage in the global variables, heap, and stack of the parent process is not acceptable

• POSIX named semaphore

sem_t *sem_open(const char *name, int oflag, mode_t mode, int val);

• int sem_wait( sem_t *sem);

• int sem_trywait(sem_t *sem);

• ##int sem_post(sem_t *sem);

• int sem_close(sem_t *sem);

• int sem_unlink(const char *name);

• Each The position of open requires close and unlink, but only the last executed unlink takes effect

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