c - 我在哪里放置 perror ("wait") 和 fork 代码

Question

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


int main(int argc, char **argv) {


    for(int i = 0; i < 10; i++) {
        pid_t pid = fork();
        if(pid == 0) {
            while(1) {
                pid_t pid2 = fork();
                wait(NULL);

            }
        }

    }
    wait(NULL);
    return(0);

}

基本上，该程序会运行多个 hello world 进程并使用 ctrl+C 关闭。我将如何处理等待错误？像错误(等待)。我认为我必须使用 int status 而不是 NULL，但不确定在涉及孤儿进程时如何去做。

给定的代码是

$ gcc -Wall above.c
$ ./a.out
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world
^C (until ctrl C is hit)
$

Answer 1

函数 perror 仅在您知道函数失败并且errno 已设置，因此您要打印一条错误消息。你通常写perror("something failed") 紧跟在可能失败的函数之后 and设置 errno(该函数的文档会告诉您是否函数设置 errno 失败)。

man perror

SYNOPSIS

   #include <stdio.h>

   void perror(const char *s);

   #include <errno.h>

DESCRIPTION

The perror() function produces a message on standard error describing the last error encountered during a call to a system or library function....

这与 wait 无关，它是参数，只有在wait 失败，您想要打印有关 wait 失败的错误消息。

man wait

SYNOPSIS

   #include <sys/types.h>
   #include <sys/wait.h>

   pid_t wait(int *wstatus);

   pid_t waitpid(pid_t pid, int *wstatus, int options);

DESCRIPTION

All of these system calls are used to wait for state changes in a child of the calling process... ...

wait() and waitpid()

The wait() system call suspends execution of the calling process until one of its children terminates. The call wait(&wstatus) is equivalent to:

       waitpid(-1, &wstatus, 0);

...

RETURN VALUE

wait(): on success, returns the process ID of the terminated child; on error, -1 is returned.

...

Each of these calls sets errno to an appropriate value in the case of an error.

如果你只是想等待一个 child 退出，你可以做wait(NULL)。但是，如果您想知道退出的 child 的状态，那么您有将指针传递给 int。

int wstatus;

pid_t wp = wait(&wstatus);

if(wp == -1)
{
    // here I use perror because if wait returns -1 then there
    // was an error and errno is set
    perror("could not wait for child\n");
    exit(1);
}

if(WIFEXITED(wstatus))
    printf("Child with pid %d exited normally with status %d\n", wp, WEXITSTATUS(wstatus));
else
    printf("Child with pid %d exited abnormally\n", wp);

我个人更喜欢 waitpid 而不是 wait，它可以让你更好地控制 child 你在等。

参见 man wait