如何在 C 中使用等待 [英] how to use wait in C

问题描述

我如何使用 wait ?它只是让我不知所措.我 fork 一个带有递归的 procs 树，现在孩子们在我运行 pstree 时必须暂停(等待/睡眠)，以便我可以打印 proc 树.

我应该使用

int 状态；等待(&状态);

或者更确切地说

wait(NULL)

我应该把它放在哪里?在父 if(pid > 0) 或子 if(pid==0) 中?也许在 ifs 的末尾，所以我将所有 pid 存储在数组中，然后对它们运行 for 并使用等待?

我的代码模板:

void ProcRec(int index){pid_t pid;int noChild = getNChild(index);国际我= 0;for(i = 0; i  0){/* 父进程 */}否则如果(pid == 0){/* 子进程.*/createProc(index+1);}别的{/* 错误 */退出(EXIT_FAILURE)；}}if(getpid() == root){睡眠(1)；pid = fork();如果(pid == 0)execl("/usr/bin/pstree", "pstree", getppid(), 0);}}

解决方案

wait 系统调用使进程进入睡眠状态并等待子进程结束.然后用子进程的退出代码填充参数(如果参数不是NULL).

所以如果在父进程中你有

int 状态；如果(等待(&状态)> = 0){如果(WEXITED(状态)){/* 子进程正常退出，通过 `return` 或 `exit` */printf("子进程退出，状态为 %d\n", WEXITSTATUS(status));}}

在子进程中，您执行例如exit(1)，那么上面的代码就会打印

<前>子进程以 1 状态退出

<小时>

还要注意等待所有子进程很重要.当父进程仍在运行时，您没有等待的子进程将处于所谓的僵尸状态，一旦父进程退出，子进程将被孤立并成为进程 1 的子进程.

How do i use wait ? It just baffles me to no end. I fork a tree of procs with recursion and now the children have to pause(wait/sleep) while I run pstree so I can print the proc tree.

Should i use

int status;
wait(&status);

or rather

wait(NULL)

and where should i put this? in the parent if(pid > 0) or in the children if(pid==0)? Maybe at the end of ifs, so I store all the pids in array and then run a for over them and use wait?

my code template:

void ProcRec(int index)
{
     pid_t pid;
     int noChild = getNChild(index);

     int i= 0;
     for(i = 0; i < noChild; i++)
     { 
          pid = fork();

        if (pid > 0)
        {
            /* parent process */
        }
        else if (pid == 0)
        {
            /* child process. */
            createProc(index+1);
        }
        else
        {
            /* error */
            exit(EXIT_FAILURE);
        }
    }

    if(getpid() == root)
    {
        sleep(1); 
        pid = fork();
        if(pid == 0)
          execl("/usr/bin/pstree", "pstree", getppid(), 0);    
    }
}

解决方案

The wait system-call puts the process to sleep and waits for a child-process to end. It then fills in the argument with the exit code of the child-process (if the argument is not NULL).

So if in the parent process you have

int status;
if (wait(&status) >= 0)
{
    if (WEXITED(status))
    {
        /* Child process exited normally, through `return` or `exit` */
        printf("Child process exited with %d status\n", WEXITSTATUS(status));
    }
}

And in the child process you do e.g. exit(1), then the above code will print

Child process exited with 1 status

---

Also note that it's important to wait for all child processes. Child processes that you don't wait for will be in a so-called zombie state while the parent process is still running, and once the parent process exits the child processes will be orphaned and made children of process 1.

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