如何分叉多个进程,当一个使用wait()函数? [英] How does one use the wait() function when forking multiple processes?
问题描述
学习使用的叉()命令和管道如何到父之间的数据和它的子女。我目前正在写一个简单的程序来测试叉和管道的功能是如何工作的。我的问题似乎是等待功能的正确使用/位置。我希望家长等待其两个孩子完成处理。这里是code我到目前为止有:
Learning to use the fork() command and how to pipe data between a parent and it's children. I am currently trying to write a simple program to test how the fork and pipe functions work. My problem seems to be the correct use/placement of the wait function. I want the parent to wait for both of its children to finish processing. Here is the code I have so far:
int main(void)
{
int n, fd1[2], fd2[2];
pid_t pid;
char line[100];
if (pipe(fd1) < 0 || pipe(fd2) < 0)
{
printf("Pipe error\n");
return 1;
}
// create the first child
pid = fork();
if (pid < 0)
printf("Fork Error\n");
else if (pid == 0) // child segment
{
close(fd1[1]); // close write end
read(fd1[0], line, 17); // read from pipe
printf("Child reads the message: %s", line);
return 0;
}
else // parent segment
{
close(fd1[0]); // close read end
write(fd1[1], "\nHello 1st World\n", 17); // write to pipe
// fork a second child
pid = fork();
if (pid < 0 )
printf("Fork Error\n");
else if (pid == 0) // child gets return value 0 and executes this block
// this code is processed by the child process only
{
close(fd2[1]); // close write end
read(fd2[0], line, 17); // read from pipe
printf("\nChild reads the message: %s", line);
}
else
{
close(fd2[0]); // close read end
write(fd2[1], "\nHello 2nd World\n", 17); // write to pipe
if (wait(0) != pid)
printf("Wait error\n");
}
if (wait(0) != pid)
printf("Wait error\n");
}
// code executed by both parent and child
return 0;
} // end main
目前我看起来输出线沿线的东西:
Currently my output looks something along the lines of:
./fork2
Child reads the message: Hello 1st World
Wait error
Child reads the message: Hello 2nd World
Wait error
哪里是合适的地方,使家长等待?
Where is the appropriate place to make the parent wait?
谢谢,
托梅克
推荐答案
这似乎大多是OK(我没有运行它,请注意)。你的逻辑错误是假设孩子会在某些特定的顺序结束;不检查的结果等待(0)
针对特定的PID,除非你确信你知道你要找回其中之一!
That seems mostly ok (I didn't run it, mind you). Your logic error is in assuming that the children will end in some particular order; don't check the results of wait(0)
against a particular pid unless you're sure you know which one you're going to get back!
编辑:
我跑你的程序;你必须至少有一个错误,你的第二个孩子进程调用等待()
,你可能根本不想做的事。我建议打破你的一些code的伸到功能,让你可以更清楚地看到你表演没有所有的混乱操作的顺序。
I ran your program; you do have at least one bug, your second child process calls wait()
, which you probably didn't want to do. I recommend breaking some of your code out into functions, so you can more clearly see the order of operations you're performing without all the clutter.
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