Semaphore function

Semaphore header file
#include<sys/types>
#include<sys/ipc.h>
#include<sys/sem.h>

semget

If the execution succeeds, a semaphore set identifier is obtained or a semaphore set object is created , Error return -1, Cause exists errno in .

int semget(key_t key,int nsems, int semflg)
 Example ：
	semid = semget(key, 1, IPC_CREAT|0666)

Parameters key
You can customize the generation , Generally by key = ftok(".",1); produce
Parameters nsems
Is the number of semaphores in the semaphore set , This parameter is only valid when creating semaphores
Parameters semflg
Is the operation type of semaphore and the operation authority
IPC_CREAT： If the kernel has key values and key The same set of semaphores , Returns the identifier of the semaphore set ; If it doesn't exist , Create a new semaphore set
IPC_CREAT|IPC_EXCL： If the key values and... Do not exist in the kernel key Semaphore values with equal values , Then create a new message queue ; If it exists, an error is reported .

semop

Operation semaphore set

int semop( int semid, struct sembuf semoparray[], size_t nops );
void pKey(int semid)
{
    
        struct sembuf sops;

        sops.sem_num = 0; 
        sops.sem_op  = -1;      
        sops.sem_flg = SEM_UNDO;

        semop(semid,&sops,1);
}
void vKey(int semid)
{
    
        struct sembuf sops;

        sops.sem_num = 0;
        sops.sem_op  = 1;       
        sops.sem_flg = SEM_UNDO;

        semop(semid,&sops,1);
}

Parameters semoparray
sem_num：
The number of operating signals in the signal set , The number of the first signal is 0
sem_op ：
Positive numbers
Release the corresponding number of resources , take sem_op The value of is added to the value of the semaphore , If sem_flag It specifies SEM_UNDO（ Restore ） Mark , Then the corresponding semaphore semadj The value will be subtracted sem_op Value .
0
The process block knows that the semaphore value is 0, When the semaphore value is 0 Go back to ; If the value of the semaphore is not 0, Of the corresponding semaphore semzcnt Add 1, The calling thread is blocked .
negative
Perform a wait operation on the semaphore , When the current value of the semaphore semval >= -sem_op when ,semval Subtract sem_op The absolute value of , Allocate a corresponding number of resources to the thread . If specified SEM_UNDO, Of the corresponding semaphore semadj Just add sem_op The absolute value of . When semval < -sem_op when , Of the corresponding semaphore semncnt Just add 1, Calling thread blocked , until semval >= -sem_op, When this condition is met , The calling thread is awakened , Perform the corresponding allocation operation , then semncnt subtract 1.

sem_flag： Attribute flags for semaphore operations , If 0, Indicates normal operation , If IPC_WAIT, Make the operation of semaphores non blocking . This flag is specified , The calling thread will not be blocked if the semaphore value does not meet the conditions , I'm going straight back -1, And will errno Set to EAGAIN. If SEM_UNDO, The semaphore adjustment value of the process will be maintained , So that the semaphore state can be restored at the end of the process .

The following explains several values related to a single semaphore ：
semval： Current value of semaphore , It is mentioned in the structure of semaphores at the beginning of the article .
semncnt： wait for semval Change to more threads than the current value . It is mentioned in the structure of semaphores at the beginning of the article .
semzcnt： wait for semval Turn into 0 Number of threads for . It is mentioned in the structure of semaphores at the beginning of the article .
semadj： Specify the semaphore tuning value for a particular process . Only sembuf Structural sem_flag Designated as SEM_UNDO after ,semadj Will follow sem_op And update .
Make it simple ： For a process , In the specified SEM_UNDO after , For semaphores semval The modification of values will reflect semadj On , When the process terminates , The kernel will be based on semadj Value , Restore the value before the semaphore .

struct sembuf sem_opt_wait[0] = {0, -1, SEM_UNDO};

struct sembuf sem_opt_wakeup1[1] = {0, 1, SEM_UNDO};

semctl

Information for controlling semaphores

int  semctl(int _semid  ,int _semnum,int _cmd  ……);

The third parameter cmd

·IPC_STAT Read a set of semaphores semid_ds, And store it in semun Medium buf Parameters in .
·IPC_SET Set the data structure of the semaphore set semid_ds The elements in ipc_perm, Its value comes from semun Medium buf Parameters .
·IPC_RMID Remove semaphore set from memory .
·GETALL Used to read the values of all semaphores in the semaphore set .
·GETNCNT Returns the number of processes waiting for resources .
·GETPID Back to the last execution semop Process of operation PID.
·GETVAL Returns the value of a single semaphore in a semaphore set .
·GETZCNT Returns the number of processes waiting for fully idle resources .
·SETALL Set the value of all semaphores in the semaphore set .
·SETVAL Set the value of a single semaphore in the semaphore set .