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Process (below): process control, termination, waiting, replacement

2022-08-02 05:00:00 【RNGWGzZs】

--------------“Good things always come as promised”

(1)进程控制:

① 如何理解fork有两个返回值？

父:子 ==1:N; The parent process does not need to be represented,子进程需要标识.

Multiple child processes perform different tasks,父进程需要通过id Distinguish subprocesses.

因此调用fork时候,The kernel will do:

1.分配新的内存块和内核数据给子进程.

2.The code for the parent process Data is copied to the child process.

3.and add the child process to In the list of system process calls.

4.fork返回,and perform scheduling.

②写时拷贝:

通常情况下,父子代码共享,数据也是(Physical memory is the same).但是when one party tries to write,In order to ensure the independence of each process,此时会发生写时拷贝.

为什么要写时拷贝？

①保证进程的独立性.

Why not separate when created？

Because the child process does not necessarily need all the data of the parent process.

The child process does not necessarily use it immediately(按需分配);因此进行延时分配,本质是Efficient use of memory space.

fork调用失败原因？

1.Too many programs in the system

2.User process exceeded limit(OS进行的限制).

(2)进程终止:

①进程退出码:

mainTo whom the function returns the value？操作系统

main函数也是函数,So it must be called.The return value will be returned to操作系统.

为什么要有返回值？ ------------运行加载,完成工作,Need to know the degree of completion.

途径:

echo $?

退出码0 、!0:

#include<string.h>

strerror(size_t i); //错误码

each error code,have corresponding error meanings.But it's all man-made.

②进程退出:

1.exit:进程终止;and release the buffer's data

_exit:进程终止,Do not do the rest of the work

2.异常退出:At this point the exit code has no meaning anymore

3.进程终止,what the operating system will do? Must go to management！

释放内存数据结构 Dispatch queue removal.

(3)进程等待:父进程等待

①Waiting for necessity:回收子进程、Get its exit information.

②进程等待方法:

1.wait：Waiting for the received child process to returnpid

#include<sys/wait.h>

pid_t wait(int* status)

小结:父进程通过wait Read child process information,The zombie state will also change automatically.

在子进程运行期间(wait),父进程在做什么？什么也没做------> 阻塞等待！！！

2.waitpid:Specifies the waiting process

                    //Record the status of the child process
waitpid(pid_id pid,int* status,int options)
  

//List of monitoring scripts
while :; do ps ajx|head -1 && ps ajx | grep myproc |grep -v 'grep';echo "#########################" ; sleep 1 ;done;

等待成功！=子进程运行成功.

status:退出结果 ---------->查看子进程运行状态

注:进程异常终止,Essentially because of the received signal.Its exit code also has no value.

An exception occurred in this process,收到信号,此时Exit codes don't make any sense either.

实际中一般用 Macro detection:

WIFEXITED(status)    //进程信号 0 与 !0


WEXITSTATUS(status)  //进程退出码

Create and wait for multiple processes:

补充:阻塞与非阻塞

//若pid没有返回 That is, the child process is not executed,No more waiting for child processes 返回0 不予等待
// 若正常退出 则返回子进程pid

pid_t ret=waitpid(id,&status,WNOHANG);

So for the parent process,You need to keep waiting for the child process to finish executing.

(4)进程替换:

The child process executes the new process.

进程替换函数:

函数	....
int execl(const char path, const char arg, ...);	列表
int execlp(const char file, const char arg, ...);	列表+默认路径
int execle(const char path, const char arg, ...,char *const envp[]);	列表+自定义路径
int execv(const char path, char const argv[]);	数组
int execvp(const char file, char const argv[]);	数组+默认路径
int execve(const char path, char const argv[], char *const envp[]);	数组+自定义路径