了解UNIX中的fork系统调用

Question

I'm trying to understand both the execution order of a line of code given to me earlier and process creation using the fork() system call. It's in C language for running on UNIX.

I understand the main concept behind fork() , nevertheless, I want to understand the process tree creation derived from the following line of code:

x = fork() || !fork();

Any help will be greatly appreciated!

Answer 1

The initial parent-process who runs the code (let's say its is PID = 1000 ), executes the x = fork() part, and spawns a new child-process (let's say PID = 1001 ).

For process PID = 1000 , the value of the logical expression so far is non-zero, because fork(2) returns its child's pid . Thus, due to short circuit evaluation , the rest part of the logical expression isn't evaluated as it's not needed.

For process PID = 1001 , the value of the logical expression so far is 0 , because in the case of the new process created, fork(2) returns 0 ; so it has to evaluate the rest part of the logical expression too. It executes the !fork() part, spawning a new process (let's say PID = 1002 ).

The process tree derived is the following:

       1000
        |
        |
       1001
        |
        |
       1002

Answer 2

chrk's answer is great, in addition, fork() return 2 times, one is pid of child process in parent process, the other is 0 returned from child process. Both processes continue to process after fork() returns.