c++ - 为什么这段代码会失败？ child 不等

Question

好的，所以我一直在努力学习掌握子进程并正确地等待它们完成。我已经阅读了很多 Stack Overflow Q/A，但我似乎仍然无法按照我的意愿让它工作。我一直在阅读/搜索这本书(C++ Primer plus 第 6 版)- 我已经做了一些研究，但我仍然不能像我想的那样等待它。

所以我在 Stack Overflow 上查看了几件事以供引用:Checking the status of a child process in C++

到目前为止，这是我尝试过的:

using namespace std;
int main() {
int status;
pid_t child;
child = fork();

if ( child > 0 ) {
    cout << "\nChild #1 is in charge\n";
    execlp("ls", "ls", NULL);
}

else if ( child < 0 ) {
    cout << "\nSomething wen't wrong in the forking process\n";
}

else {

}

child = fork();
if ( child > 0 ) {
    cout << "\nSecond child is in charge\n";
    execlp("locate", "locate", "etc", NULL);
}

else if ( child < 0 ) {
    cout << "\nSomething went wrong in the forking of second child!\n";
}

else {

}

现在这会显示 Child #1 is in charge 和 Second child is in charge 然后它会混合这两个命令(我看到一些 ls 之间找到 etc).

我尝试的第二件事:

using namespace std;
int main() {
int status;
pid_t child;
pid_t ch_status = waitpid(child, &status, WNOHANG);

child = fork();
if ( child > 0 ) {
    cout << "\nChild is in charge\n";
    execlp("ls", "ls", NULL);
}

else if ( child < 0 ) {
    cout << "\nSomething wen't wrong in the forking process\n";
}

if ( ch_status == 0 ) {

}

else if ( ch_status == -1 ) {
    cout << "\nERROR IN CHILD #1\n";
}

else {

}

child = fork();
if ( child == 0 ) {
    cout << "\nSecond child is in charge\n";
    execlp("locate", "locate", "etc", NULL);
}

else if ( child < 0 ) {
    cout << "\nSomething went wrong in the forking of second child!\n";
}

if ( ch_status == 0 ) {

}

else if ( ch_status == -1 ) {
    cout << "\nERROR IN CHILD #1\n";
}

else {

}

child = fork();
if ( child > 0 ) {
    cout << "\nThird child is in charge!\n";
    execlp("echo", "echo", "herro", NULL);
}

else if ( child < 0 ) {
    cout << "\nForking of third child failed!\n";
}

if ( ch_status == 0 ) {

}

else if ( ch_status == -1 ) {
    cout << "\nERROR IN CHILD #2\n";
}

else {

}

return 0;
}

这更多是基于我提供的链接，它产生了与我进行的第一次测试运行相同的结果，除了它还会显示 ERROR IN CHILD #1/2。

这些命令是无关紧要的，我只是不想明白我在这里做错了什么……我也试过将它们嵌套在 else {//start second fork here } 中，但我这也无济于事。

根据我阅读waitpid(2) 手册后的理解，我应该使用WNOHANG； http://linux.die.net/man/2/waitpid

非常感谢任何建议/指示。

如果可能，请提交您将如何获得正确结果的示例代码(执行命令 1 -> 等待完成 -> 执行命令 2 -> 退出)。

期待回复。

Answer 1

您应该在 fork 后 在父级中调用 waitpid()。也就是说，它应该进入 child > 0 分支。

该函数的目的是“等待子状态发生变化”。但是，您在生成 child 之前调用它。

代码应该是这样的:

using namespace std;
int main() {

int status;
pid_t child;

child = fork();
if ( child == 0 ) {
    cout << "\nChild is in charge" << endl;
    execlp("ls", "ls", NULL);
} else if ( child < 0 ) {
    cout << "\nSomething wen't wrong in the forking process" << endl;
} else {
    cout << "Parent waiting" << endl;
    pid_t ch_status = waitpid(child, &status, WNOHANG);
    if (ch_status == -1) {
      cout << "\nERROR IN CHILD #1" << endl;
    }
}

child = fork();
//same procedure as above

}