How many processes are created in this snippet using fork?

Can anyone help me in understanding how this will created 19 process?

Main()
{
    Fork()开发者_如何学C;
    Fork() && fork () || fork ();
    Fork ();
}

Try to draw a tree of the process creation and study/remember the following points:

P1. fork() returns the pid (which is bigger tan 0) to the current process and returns 0 in the child process.
P2. you will need to know how the expression A() && B() || C() is evaluated; for example if A() returns 0 (false) function B will not get evaluated because 0 && whatever is always 0.

Now, let's label the calls to for ease of reference:

Main()
{
    Fork() /*[1]*/;
    Fork() /*[2]*/ && fork ()/*[3]*/ || fork ()/*[4]*/;
    Fork ()/*[5]*/;
}

I will draw the first level of process creation (and a bit of second level):

                          [0]
                /      /     \      \
               [1]    [2]    [3]    [5]

            /  |  \
          [2] [3] [5]

The above tree means process [0] (the initial process) will execute the fork() function numbered 1, 2, 3, and 5. Why didn't process [0] run fork()[4]? Because fork()[2] && fork[3] evaluate to true already, so no point to evaluate fork()[4].

Apply similar concept to the process forked by fork[1] on the second level to see why process fork[4] was not called.

You can complete the process creation tree by applying P1 and P2 at each level of the process creation tree.

Remember that fork has a return value, either 0 or a PID (can't remember if the child gets the PID or the parent). So the && and || operators will evaluate to true when a PID is returned in which case some more processes will be forked. Hope that helps