Process creation fork (), demise wait()

1、 Process identifier pid

type pid_t;; It can hold more than 30000 people at most ,16 Long long

ps command

ps axf

getpid();

getppid();

2、 The creation of processes

fork（）;

Pay attention to understanding keywords ：duplicating, Means copy 、 clone 、 Two complete copies of the same code , The position of execution is the same ;

fork The difference between parent and child processes after ：

• fork The return value of is different ;
• pid Different ,ppid Different ;
• Pending signals and file locks do not inherit ;
• Resource utilization cleared ;

init process ：1 Number , Is the ancestor of all processes

• The scheduler's scheduling policy determines which process runs first ; You can't guess which parent-child process runs first ;

• fork The buffer needs to be flushed before

Automatically refresh ： added \n When you are outputting standard output , Because of the row cache , It is equivalent to automatically flushing the buffer ;

But when you output to /tmp/out when , Will be printed twice begin. Because fork With buffer begin It is also copied to the child process ;

Manually refresh ：fork Before to add fflush（NULL）; With or without /n, Standard output or not , Will only print once Begin;

  Programming topics ：

• use 200 The subprocess determines whether twohundred numbers are prime numbers ;

Note that the subprocess ends when it finishes its work （exit、return）;

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define LEFT 30000000
#define RIGHT 30000200

int main()
{
    int i, j, mark;
    for (i = LEFT; i <= RIGHT; i++)
    {
        mark = 1;
        for (j = 2; j < i / 2; j++)
        {
            if(i%j == 0)
            {
                mark = 0;
                break;
            }
        }
        if(mark)
            printf("%d is a primer\n",i);
        exit(0);     Be sure to end the child process ;
    }
    exit(0);
}

•   Who creates who recycles ;

The child process can be regarded as a resource created by the parent process , If the parent process does not end （ Nor recycle resources ）, Child processes can become zombie processes ;

The child process that the parent process ends will become an orphan process , from init（） management ;

The main harm of zombie process is Occupy pid resources ;

vfork（）;

• fork（） Of ” Copy while writing “ technology ：

1. Who writes who copies ;
2. When read-only, the parent and child processes share physical memory ;

3. Processes die and resources are freed

wait();

waitpid ();

Allocation algorithm ：

• Block method ;
• Cross allocation algorithm ; The general crossover algorithm is better than the block method ;
• pool

4.exec function

5、 User and group permissions

6. Observation class ： Interpreter files

7、 system（）

8、 Process accounting

9、 Process time

10、