Signal Volume (SEM)

First, the signal volume: (Data operation Lock) control inter-process mutual exclusion, synchronization, etc., coordinate multiple programs to access a shared resource at the same time.

How it works: The semaphore can only have two operation waits and operation, namely: P v operation, must be atomic operation.

P (SV): If the value of SV is greater than 0 minus one, if its value is zero, it hangs;

V (SV): If there is a suspended, resume operation, if not, add one.

Second, the function prototype:

int Semget (key_t key, int nsems,int SEMFLG);

int semop (int semid,struct sembuf *sops,size_t nsops);

int semctl (int semid, int semnum, int cmd, ...);

/*

* Semid: identifier of the semaphore set; Semnum: Number of semaphores;

* cmd: The command to execute, depending on the command, the function has three or four parameters (Union)

*/

Cmd:ipc_rmid, immediately delete the signal set, wake up the blocked process;

Cmd:setval, sets the value of a single semaphore in the semaphore set.

The Cmd:getall is used to read the values of all semaphores in the semaphore set.

Union Semun

{

int Val;

struct Semid_ds *buf;

unsigned short *array;

struct Seminfo *_buf;

};//users need to define their own claims

Third, the code implementation:

Iv. Results of implementation:

Signal Volume (SEM)