Fork() function in C programming

问题:

I just need to understand this statement:

if (fork() && !fork())

shouldn't it always be false? I mean, if I write:

if (a && !a)

It's always false so the first should always be false too, am I wrong? Of course I am, but I'm hoping someone can explain this strange thing to me.

I'm studying C for an exam and I had to resolve this code:

int main(){
if(fork && !fork()){
   printf("a\n");
}
else printf("b\n");
}

回答1:

Every calls to the unix process creation system call fork() returns twice. First it returns with the PID of the child to the parent(the process which called fork()). Second it returns to 0 to the newly created child.

from man pages:

Return Value

On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno is set appropriately.

in your case

if (fork() && !fork())

The statement inside if , calls fork twice. So what will happen is following :

A
|----------------B
|                |
|---C            |
|   |            |         

Now first call to fork() will return in both A and B. In A it will be nonzero and in B it will be zero.

Second call to fork() will be evoked only from A. because first fork returned 0 to B, it will not Evoke a second fork(). its because && short circuits the evaluation if first operand is found non zero. Thanks to Daniel for pointing this out.

So we can make a table out of this:

PID       fork()1      fork()2
------------------------------
A           >0          >0
B           =0          >0
C           >0          =0

So from the chart, Process C's if will be evaluated to TRUE

Its important to remember, fork()1 didn't returned to C . it got the copy of Already evaluated expression from its parent.

I hope this explains your question.

回答2:

First off, is a function. It may not always return the same value.

In this case specifically, fork is a function which creates another process. The original process gets a positive return value (of the child's pid) and the child process gets a return value of 0.

In your code, there end up being a total of three processes. The if statement will evaluate to true for 1 of them (process C below).

     A
     |__________B
     |          |
     |__C       |
     |  |       |       
     |  |       |        

回答3:

shouldn't be always false?

No.

Because it's not a variable, each call tofork() creates a new child process.

回答4:

Each call to fork() returns two values, one to each process. So for each decision, there's one process that takes each path.

回答5:

The fork() function call returns 0 to the child process and the process ID to the parent process. Basically, what this does is forks once. If the process is the parent, it jumps to next block, the child then forks again. The parent of this process jumps to the next block, and the child in this block executes the code in the if statement.