Role of wait function

Preface

Writing C When it comes to programming , adopt fork Function to create a new process , wait Function to wait for the child process to end .

So there's a problem , Under what circumstances does the parent process need to wait for the child process to finish before continuing execution ? If you need to wait for the child process to finish , If you put the operation directly into the parent process for execution, you will wake up ? Anyway, waiting is also waiting .

Of course , There is another situation , Mission A And tasks B Is a prerequisite for subsequent operations , And the two tasks can be carried out in parallel , At this point, the parent process can indeed execute a , The subprocess executes a , Then the parent process waits for the child process to complete before continuing . however , I still think this explanation is not so perfect . This can be achieved through process communication .

therefore , I think wait Functions must have other uses .

This question was answered after I rerouted the operating system .

Why?

as everyone knows , Processes exist in the process of running in the operating system PCB( Process management block ) Modular . It is the unique identifier of the process , It is generally kept PID、 Running state 、 Resource information wait .

The process is calling exit When the function exits , Will recycle the occupied resources . Be careful , stay exit Function to reclaim resources , The process is still running . therefore , There is a resource that cannot be recycled by the process itself , Namely PCB. To run a process, you need PCB, and PCB The premise of recycling is that the process is no longer running . in other words , The process itself cannot recycle itself .

that PCB You can't recycle it ? Of course not. , The wait The function came out . Since the process itself cannot reclaim itself , Then the parent process is responsible for recycling . therefore , Such a function that waits for the child process to end has a place to play .

By the way , In the subprocess exit end of execution , PCB The time before recycling , The state of a child process is called Zombie state , It's very vivid .

mac Process in :

debian Process in :

You can see , PID by 0 The process of is the parent process of all processes . stay mac in , All processes are controlled by /sbin/launchd Create , debian Medium is kthreadd. Their role is to manage and schedule processes . Parent process passed fork Create child process , After that, the subprocess passes exec Change to the process to be executed .

And this PID by 0 The process of , It is also the only way that the system does not run through fork Process created .

however , There's another problem , That is, if the parent process does not wait for the child process to end ? At the end of the subprocess , It is found that the parent process has long ended , Then it's PCB Can't recycle ? Of course not , This PID by 0 Ancestral process of , Otherwise known as root The process will regularly scan all zombie processes , And recycle it instead of its parent process .

After knowing it, I suddenly realized , そうですね!!!

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