Operating system semaphore mechanism

 

Semaphore mechanism

	format	wait()/P operation	signal()/V operation
Integer	int S = 1;	while(s<=0); S=S-1;	S=S+1;	May be busy waiting
Record type	type struct{   int value;   struct process *L; } S	S.value--; if(S.value<0)     //Hang the process into the waiting queue of the semaphore     //And use the block primitive to block itself     //Abandon the processor actively     block(S.L);	S.valus ++; if(S.value <=0)     //If the semaphore is less than or equal to 0 after the resource is released     //That is, there is still a process waiting for the resource, then wakeup through wakeup     wakeup(S.L);	Not busy waiting Will voluntarily give up the right to use the processor

 

A semaphore actually represents a resource, and its value represents the remaining amount of resources

P operation: request resources, block if not requested

V operation: release the resource, wake up if there is a process waiting for the resource

 

Use semaphores to achieve mutual exclusion of processes

 

记录性型号量 mutex = 1;

p1(){

    P(mutex);
    //临界区代码
    V(mutex);
}
P2(){

    P(mutex);
    //临界区代码
    V(mutex);
}

 note! ! ! Different semaphores should be set to represent different resources

P and V operations must be expressed in pairs

 

Use semaphore mechanism to achieve process synchronization

Set the initial value of the semaphore to 0

记录型信号量 S = 0;

P1(){
    代码1；
    代码2；
    V(S);
    代码3；
}

P2(){
    P(S);
    代码4；
    代码5；
}

Used to ensure that the execution of code 4 is after code 2

Perform the V operation after each "pre-operation" to tell another process that it can continue execution

Perform P operation before each "post-operation", which is used to wait for another process to execute after the executed operation