while(wait(NULL) > 0) doubts with result - c

Question

I have a piece of code and I have to explain what is happening in the exit.
while(wait(NULL)>0)
The code is the following:
#include <stdio.h>
main() {
int n=1;
while(n<5) {
n=n+1;
if (fork() == 0)
n=n+2;
}
printf("%d %d %d\n", getpid(), getppid(), n);
while (wait(NULL) > 0);
}
When I execute the program the result shows 6 processes with 6 children and respective parents, while the condition while n < 5 has been met. If we cancel
while (wait(NULL) > 0);
then some children remain as zombies.
For example, when you are in the first child the output should be n = 4 instead I get n = 5 and outputs the results randomly without order.
I want to understand exactly the behaviour of while (wait (NULL)> 0)

Answer 1

If the current process have no child processes, wait(NULL) returns -1. Otherwise it waits until one of them exits, and returns it's process ID.
So while wait(NULL) > 0); loops until there are no more child processes, since when the last child exits, wait() will return -1 and the while loop terminates. (And wait() returning 0 should be an impossible condition too).

Answer 2

For example, when you are in the first child the output should be n =
4
Look at the code again: the last time the thread enters the while, n<5, but then it increments n in the while body before exiting the while loop and printing.
(UPDATE: I just noticed that in your sample code, the line n=n+1 is not indented. Maybe this is just an accident in entering the code, but it may be contributing to your confusion, since "n=n+1" does not look like it's happening in the while loop) Obviously the compiler doesn't care, but from the wording of your question, it sounds as if you do not realize that after the n=n+2, any thread where n<5 will loop back and increment n at least one more time. This is why no thread ever outputs 4 or less for n.

Answer 3

I want to understand exactly the behaviour of while (wait (NULL)> 0)
This is may not be direct answer to your problem, which is more related to fork than while(wait(NULL)>0), but might help other people (like me, who found your post when searching about this construct)
tl;dr: it waits for termination of all (direct) childs.
wait(ptr) on success returns (non-negative) pid of terminated child, and saves its status to int pointed by ptr (you can safely pass NULL, if you don't care). On "fail", -1 is returned and errno set appropriately.
In case of this loop, we don't consider no child processes as error, contrarily: it would be error if it didn't happen.
For extra error handling, one could write:
while(wait(NULL) != -1);
if(errno != ECHILD) {
perror("wait() error");
exit(1)
}