Question

The following code:

B() { 
   pid_t pid; 
   if ((pid=fork())!= 0) 
       waitpid(pid,NULL,0); 
   printf("2 "); 
   if (fork() == 0) 
      { printf("3 "); exit(0); } 
   printf("5 "); 
   exit(0); 
}

could have one of the outputs: and im not sure which one is the right output.

232553
235325
232355
235253
252533

these 2 lines mean if the pid is the parent, then wait for what?

if ((pid=fork())!= 0) 
           waitpid(pid,NULL,0); 

then if it is the child process (fork = 0), then print 3.. correct?

 if (fork() == 0) 
              { printf("3 "); exit(0); }

Answer 1

B() { 
   pid_t pid; 

   /* On success, fork() returns 0 to the child. It returns the child's process 
    * ID to the parent. So this block of code, executes a fork(), and has the
    * parent wait for the child.
    * Amusing sidenote: if the fork() fails, the parent will wait for *any* 
    * child process, since the return will be -1.
    */
   if ((pid=fork())!= 0) 
       waitpid(pid,NULL,0); 

   /* Both the parent and the child will print 2. The parent, will, of course,
    * only print 2 after the child exits, because of the waitpid above.
    */
   printf("2 "); 

   /* Now we fork again. Both the parent and the child from the previous fork will
    * fork again, although the parent will do it *after* the child exit. The resulting 
    * child process will print a single 3 and then exit.
    */
   if (fork() == 0) 
      { printf("3 "); exit(0); } 

   /* Now both the parent and the child print a 5 and exit */
   printf("5 "); 
   exit(0); 
}

As David Schwartz said, the output of this program will consists of some permutation of the digits 2, 3 and 5. No one output is correct, because the output depends on the order in which the processes execute, which is arbitrary.