TLPI-Ch24

fork()

In parent: returns process ID of child on success or -1 on error;
in successfully created child: always returns 0.
完成对fork()调用后将存在两个进程，且每个进程都会从fork()的返回处继续执行。

#include 
#include 


using namespace std;
int main() {
    cout << getpid() << endl;
    cout << "init process\n";
    pid_t childPid;
    switch(childPid = fork()) {
        case -1:
            cerr << "wrong\n";
            exit(1);
            break;
        case 0: // child preocess, fork() called by child return 0, while it called by parent returns process ID of children.
            cout << "From child, getpid\t" << getpid() << endl;
            cout << "From child, childPid\t" << childPid << endl;
            cout << "Parent\t" << getppid() << endl;
            break;
        default: // parent process
            cout << "From parent, getpid\t" << getpid() << endl;
            cout << "From parent, childPid\t" << childPid << endl;
            cout << "Parent\t " << getppid() << endl;
            break;
    }
    return 0;
}

out

init process
From parent, getpid 3312 # 父进程的ID
From parent, childPid   3313 # 子进程ID
Parent   1583    # 父进程的父进程是bash 
From child, getpid  3313 # 
From child, childPid    0 # 子进程中fork()返回直
Parent  989  # 子进程的父进程，因为没有使用wait()函数，父进程退出后，子进程的父进程便会交给/sbin/upstart --user