以python代码解释fork系统调用

import os

print('Process (%s) start...' % os.getpid())
# Only works on Unix/Linux/Mac:
pid = os.fork()
print ("haha")
if pid == 0:
    print('I am child process (%s) and my parent is %s.' % (os.getpid(), os.getppid()))
else:
    print('I (%s) just created a child process (%s).' % (os.getpid(), pid))

"""
haha
haha
Process (876) start...
I (876) just created a child process (877).
I am child process (877) and my parent is 876.
"""

执行到os.fork()时，发起fork系统调用，操作系统复制父进程得到子进程。fork系统调用完毕，返回给父进程子进程的id号，返回给子进程0，然后父子进城开始执行。我们可以看到haha被打印了两次，更加说明了子进程是复制父进程得来的

再举个例子，说明子金成是复制父进程:

print("This is console module")

from multiprocessing import Process
import os
import time
ppp=3
def run_proc(name):
    print('Run child process %s (%s)...' % (name, os.getpid()),ppp) #观察这句的执行结果
if __name__=='__main__':
    print('Parent process %s.' % os.getpid())
    p = Process(target=run_proc, args=('test',))
    print('Child process will start.')
    p.start()
    #p.join()
    print('Child process end.')

"""
Parent process 928.
Process will start.
Run child process test (929)...,3
Process end.
"""

子进程正常打印了ppp的变量，说明子金城不仅仅是复制了实际的执行函数run_proc，而是复制了父进程全部的代码，所以才能正常打印ppp变量