Process creation and recycling

Process creation and recycling

fork： Create child process

• fork() Will return twice , Return... In child process 0, Returns the... Of the child process in the parent process pid, return -1 Indicates creation failed
• Once created , Child processes and parent processes share data （ Only pcb Some of the data in are different , Such as pid,ppid）. Share while reading , When writing copy .

Orphan process VS Zombie process

• Orphan process ： End of parent process , Subprocess running --------> The parent process of the child process is set to init (pid = 1) ---------------> At the end of the subprocess init Recycling
• Zombie process ： End of subprocess , The parent process runs ---------> The parent process did not end and the child process was not recycled （wait）---------------> Zombie process

Subprocess recycle

• wait： Wait for any subprocess to finish , And recycle . Block waiting
• waitpid： Specify a child process to wait for completion and recycle , Blocking and non blocking can be set

/********************************************************* Copyright  2022 Shengkai Liu. All rights reserved. FileName: wait.c Author: Shengkai Liu Date: 2022-05-31 ***********************************************************/

#include <unistd.h> // fork, getpid
#include <stdio.h> // printf, perror
#include <wait.h> // wait
#include <stdlib.h> // exit

int main()
{
    
    // create a sub-process
    pid_t pid = fork();
    
    // prints its own pid if the sub-process
    if (pid == 0)
    {
    
        printf("pid of the child process: %d\n", getpid());
        exit(0);
    }


    // Recycle sub-process resources by wait
    int status;
    pid = wait(&status);
    if (pid == -1) perror("wait");

    printf("pid of recycled sub-processes: %d\n", pid);

    return 0;
}

/********************************************************* Copyright  2022 Shengkai Liu. All rights reserved. FileName: waitpid.c Author: Shengkai Liu Date: 2022-05-31 ***********************************************************/

#include <unistd.h> // fork, getpid
#include <stdio.h> // printf, perror
#include <wait.h> // waitpid
#include <stdlib.h> // exit

int main()
{
    
    // create a sub-process
    pid_t pid = fork();
    
    
    if (pid == 0)
    {
    
        // prints its own pid if the sub-process
        printf("pid of the child process: %d\n", getpid());
        exit(0);
    }
    else if (pid > 0)
    {
    
        // Recycle sub-process resources by wait
        int status;
        pid = waitpid(pid, &status, 0);
        if (pid == -1) perror("wait");

        printf("pid of recycled sub-processes: %d\n", pid);
        exit(0);
    }
    return -1;
}