[Linux from scratch] process creation, termination, waiting, replacement

How are processes created? How did he end it? How does the process wait? Can the replacement process also be replaced? If you have these questions then this article will answer you

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process creation

To create a process there are the following ways

1. Run directly (./run under Linux, click and run under windows and mac)
2. Call the fork() function in the program to create a child process

So let's see how the fork() function creates a process at the bottom.

Knowledge review:

The function of the fork() function is to create a child process. The return value of the parent process is the pid of the created process, and the child process is 0.

ask

Why does the same variable have two different values?

answer

This involves the underlying layer, so let's look at the underlying logic of fork()

The underlying logic of fork

ask questions

Is the process (child process) created when fork() is executed to return pid? ?

answer

Of course it is created, otherwise where did the pid come from, then when he returns the pid, then there will be a problem, then it is the parent-child process, because, , so the return value 数据，代码是共有here 进程之间是相互独立的is 那么这里就会发生写时拷贝different,且子进程是看不到(执行不到fork()前包括fork()的代码）

as the picture shows:

ask

The role of realistic copy? ? ?

answer

1. Guarantee the independence of parent and child processes
2. Save system memory resources ( 子进程不一定会修改父进程的数据)
3. Improve the success rate of fork

explain

Maybe 1 and 2 are easy to understand, but why does 3 improve efficiency? Because fork是向os申请资源的创建进程, if there is no realistic copy, then to ensure the independence of the process, 为子进程开辟一块空间the more resources you apply for, the greater the chance of failure.

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process termination

How a process exits

1. Run normally to return, or exit, _exit
2. There is a problem with the code logic, and the operation crashes
3. The external signal terminates the process (the signal blog later says that it is kill -9 at present)

Known as one of the many mysteries when learning C and C++ (return 0 is returned to whom)

retuen 0, it is actually there 程序退出码, and this is one 父进程看的, the father process needs to know the exit status of the child process (you don’t need to know it)

ask

What if I also want an exit code?

answer

You need to enter on the command echo $?to get the latest exit code

knowledge supplement

Computers are good at dealing with numbers, but people are good at dealing with character strings. The corresponding relationship between numbers and characters (artificial regulations) is shown in the figure:

normal exit

1. return
2. exit
3. _exit

The difference between return and eixt

return is used at the end of the function, and the program will exit only when the main function is executed, but exit and _exit are different管在程序的那个位置只要执行到这个语句程序就会退出

The difference between exit and _exit

exit 会做后续工作而_exit不会, the follow-up work (calling the destructor, refreshing the stream to the display...) is shown in the code:

The role of the exit code

In fact, it can be seen as assert 当作一个预判, where the logic is wrong, it can be terminated directly

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process waiting

If the parent process of the child process does not reclaim resources, the child process is a zombie process, which will occupy system resources, so waiting is to deal with the zombie process

process wait function

1. wait
2. waitpid

Interface introduction

blocking and non-blocking

There is no need to introduce too much about blocking. After all, it often happens in life ( 堵车), but non-blocking is somewhat different. It can be understood that regular spot checks will not get stuck

Non-blocking running graph:

It is not difficult to see that this is one 循环的过程, the parent process 定期checks the status of the child process, and when it is completed, it continues to execute its own code or end the process ( 类似你爸爸叫你好好写作业，害怕你玩游戏就定期过来看一下)

code:

The result is obvious, 阻塞式等待不会执行waitpid后面的代码non-blocking will execute concurrently with the child process

Get the exit code (status)

status is of int type, but only the lower 16 bits are effective bits as shown in the figure:

How to get the exit code

//位运算
cout<<(status>>8)& 0xff

//宏
WEXITSTATUS(status)

How to get the termination signal

//位运算
cout<<status & 0x7f;

//宏
WIFEXITED(status);

ask:

Does the exit code of the process matter if it is killed by a signal?

answer

It doesn't matter, because the program has not been executed normally, so what is the meaning of his exit code? right

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process replacement

Generally, after fork(), you want the child process to execute the code of the parent process together. In fact, it does not make much sense. There is another way to execute the code of other processes, then you need to use the exec function to replace the program.

Interface of exec series

• l=list (list)
• v=vector (array)
• p=path (environment variable)

Replacement case:

ask:

Why haha~~ didn't print it? ? ?

answer

The functions of the exec series are program replacements, so they are directly replaced 数据和代码全部替换, so of course the previous codes cannot be seen

as the picture shows:

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memory pool

        At the language level, such as c, c++, if it is to apply for a few bytes, it is okay if you do not apply frequently. But there will always be a situation where you need to apply for huge memory and frequent applications at once, so there will be a memory pool.

Example in life (bank loan)

If you want to start a business and need a loan, will you apply for the corresponding amount of money when you need it, or will you apply for a large sum of money first, just in case you need it? In fact, people find it troublesome. If you can do it once, you won't choose multiple times.

ask

After the parent process waits for the success of the child process, is the data of the child process really erased?

answer

In a sense, yes, but not, if 把这一块数据抹除会消耗很多系统资源, so in general, the data is set as useless, and then linked with a data structure,

as the picture shows

end

That's all for this chapter, thank you for watching