003_process_base

1. Functions associated with creating a process
    system : execute a shell command and it's a blocking function
            system("ls -l");
        Create an independent process,and it has independent code space and memory space.
        The "system" function will return after the process is finished.
    popen : pipe stream to or from a process
        Create a child process and also create a pipe between the parent process and the child process.
    exec : This is a family of functions
        execl/execlp : replace the code data of the current code space and the function itself do not create a new process
    fork : Create a child process and clone the code space of the parent process and it's location that the parent process is executed to.
        The "fork" is not called in the child process, so it return 0.
        The child process and the parent process are run at the same time.

//system

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
int main()
{
    printf("Process:%d ",getpid());
    sleep(3);
    return 99;
} // test

#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<sys/wait.h>
main()
{
    printf("system:%d ",getpid());
    int r = system("./test");
    // You can not get the return value directly
    printf("return value: %d ",r>>8&255);
    printf("return value: %d ",WEXITSTATUS(r));
}

//popen

#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<sys/wait.h>

main()
{
    char buf[1025];
    //FILE *f = popen("ls -l","r");
    FILE *f = popen("./test","r");
    int fd = fileno(f);

    int r;
    printf("begin===============================");
    while((r = read(fd,buf,1024))>0)
    {
        buf[r] = 0;
        printf("%s",buf);
    }
    printf("end=============================== ");
    close(fd);
    pclose(f);
}

//exec

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

int main()
{
    printf("main:%d ",getpid());
    //int r = execl("test","param1",NULL);
        //The following code is replaced by the program "test",so the following code will not be executed
        //We do not create a new process,we just replace the code space,so there is only one process all the time and printf the same ID.
    //int r = execl("/bin/ls","ls","-l",NULL); //we must use "/bin/ls"
    int r = execlp("ls","ls","-l",NULL); //we can use "ls", which is different from execl which use "/bin/ls"
        //execl : Only search in the current path
        //execlp : Search in the current path and PATH
    printf("end:%d",r);
}

//fork

#include<stdio.h>
#include<unistd.h>
int main()
{
    printf("Before creating a process! ");
    int pid = fork();
        // We create a new process and clone the code space of the parent process
    if(pid == 0) //child process
    {
        while(1)
        {
            printf("this is child process:%d ",getpid());
            sleep(1);
        }
    }
    else // parent process
    {
        while(1)
        {
            printf("this is parent process:%d ",getpid());
            sleep(1);
        }
    }
    printf("After creating a process: ");
    return 0;
}