fork()和exec()两个子进程 [英] fork() and exec() Two Child Processes

问题描述

我两次调用fork()来创建两个子进程.我希望子进程A进行exec()调用，并且希望子进程B也进行exec()调用.给定代码的问题是，子进程A中的第一个exec()之后，下一个fork()似乎没有发生，程序退出了.我认为这与exec()如何覆盖父进程有关.我要完成的是从fork()创建的每个子进程中调用exec().

I am calling fork() twice to create two child processes. I want child process A to do an exec() call and child process B to also do an exec() call. The problem I am having with the given code is that after the first exec() from child process A, the next fork() does not seem to occur and the program exits. I think that it has to do with how exec() overlays the parent process. What I want to accomplish is to call exec() from each of the child processes created by fork().

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <iostream>

int main() {

pid_t cpid_a, cpid_b;

cpid_a = fork();
if(cpid_a < 0) {
    std::cout << "Fork failed." << '\n';
    return 1;
}
else if(cpid_a == 0) { // code for child process A
    execlp("/bin/ls", "ls", NULL);

    cpid_b = fork();
    if(cpid_b < 0) {
        std::cout << "Fork failed." << '\n';
        return 1;
    }
    else if(cpid_b == 0) { // code for child process B
        execlp("/bin/ls", "ls", NULL);
    }
}
else { // code for parent process
    while(wait(NULL) != -1);
}
return 0;
}

推荐答案

else if(cpid_a == 0) { // code for child process A
    execlp("/bin/ls", "ls", NULL);

如果此调用成功，则执行以下语句，之后将不执行任何操作.这就是exec()的工作方式.紧随其后的fork()永远不会发生.这就是exec()的工作方式.如果exec()成功，它将永不返回.替换过程将在其位置执行.

If this calls succeeds, the following statement, and nothing that follows will ever be executed. That's how exec() works. The immediately-following fork() never occurs. That's simply how exec() works. If exec() succeeds, it never returns. The replacement process gets executed in its place.

您甚至在上面添加了100％正确的注释:子进程A的代码". if()语句中的所有内容均为子进程A的代码"，并在fork()返回0时执行.

You even added the 100% correct comment, above: "code for child process A". Everything inside the if() statement is "code for child process A", and gets executed when fork() returns 0.

您还正确地声明了您希望父进程派生第二个进程.好吧，您需要让 代码明显地由父进程而不是子进程执行:

You also correctly stated that you want the parent process to fork a second process. Well, you need to have that code obviously get executed by the parent process, and not the child process:

else if(cpid_a == 0) { // code for child process A
    execlp("/bin/ls", "ls", NULL);
    exit(1);
} else {
    cpid_b = fork();

  // The rest of the code.

现在，父流程继续进行，第二遍fork()，继续执行您的其余计划.

Now, the parent process goes ahead and fork() a second time, proceeded on the rest of your plan.

P.S. exit()只是一个很好的衡量标准. exec()返回的唯一时间是exec()无法执行给定进程时. /bin/ls的可能性很小；如果丢失了，您还有更大的问题要担心.尽管如此，这在技术上是正确的，因为在那一点上继续执行将导致完全混乱.同样，如果缺少/bin/ls，那将是最少的问题，但是，例如，如果系统用完了内存并且由于该原因无法执行它，也可能会发生这种情况.在这种情况下，无需向火上添加燃料；但无论如何，进程都会死掉.

P.S. The exit() is just for a good measure. The only time exec() returns is when exec() fails to execute the given process. Highly unlikely, in the case of /bin/ls; if it's missing you have bigger problems to worry about. Still, that's the technically correct thing to do, since continuing execution at that point will result in complete chaos. Again, if /bin/ls is missing that's going to be the least of the problems, but this can also happen if, say, the system ran out of memory and can't execute it for that reason; in which case there's no need to add fuel to the fire; but rather have the process die anyway.

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