I am reading book The Linux Programming Interface, I see the following 2 paragraphs about zombie process sort of conflicting:
When the parent does perform a wait(), the kernel removes the zombie,
since the last remaining information about the child is no longer
required. On the other hand, if the parent terminates without doing a
wait(), then the init process adopts the child and automatically
performs a wait(), thus removing the zombie process from the system.
If a parent creates a child, but fails to perform a wait(), then an
entry for the zombie child will be maintained indefinitely in the
kernel’s process table. If a large number of such zombie children are
created, they will eventually fill the kernel process table,
preventing the creation of new processes.
The first paragraph says, even the parent process did not call wait() and terminated the long running child process, init would pick the zombie process and clear the child process off upon its termination.
The following paragraph says, if the parent process does not call wait(), then its long running child processes will become zombie and occupy kernel process table forever.
I am confused - how can the situation of 2nd paragraph happens if init takes care of picking up zombie process? Or in what situation will init miss picking up zombie?
[Update] Yes, I misread the text, thanks Barmar, these 2 paragraphs talk about different scenarios.
Actually if the init was not there then the situation of second paragraph would happen. The init runs periodically that takes care of the zombie process.
More can be found here
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Hello I am new to learning about system calls. I am currently learning about fork() and wait() system calls. I know that fork() creates a new child process. What confuses me is the wait() call.
This is what I understand so far:
(1) When a process dies, it goes into a 'Zombie State' i.e. it does not release its PID but waits for its parent to acknowledge that the child process has died and then the PID is released
(2) So we need a way to figure out when the child process has ended so that we don't leave any processes in the zombie state
I am confused with the following things:
(1) When running a C program where I fork a new child process, if I don't call wait() explicitly, is it done internally when the child process ends? Because you could still write a block of code in C where you run fork() without wait() and it seems to work fine?
(2) What does wait() do? I know it returns the PID of the child process that was terminated, but how is this helpful/related to releasing the PID of the terminated process?
I am sorry for such naive questions but this is something I was really curious about and I couldn't find any good resources online! Your help is much appreciated!
wait isn't about preventing zombie states. Zombie states are your friend.
POSIX more or less lets you do two things with pids: signal them with kill or reap them (and synchronize with them) with wait/waitpid/waittid.
The wait syscalls are primarily for waiting on a process to exit or die from a signal (though they can also be used to wait on other process status changes such as the child becoming stopped or the child waking up from being stopped).
Secondarily, they're about reaping exit/died statuses, thereby releasing (zombified) pids.
Until you release a pid with wait/waitpid/waittid, you can continue flogging the pid with requests for it to die (kill(pid,SIGTERM);) or with some other signal (other then SIGKILL) and you can rest assured the pid represents the process you've forked off and that you're not accidentally killing someone else's process.
But once you reap a zombified pid by waiting on it, then the pid is no longer yours and another process might take it (which typically happens after some time, as pids in the system typically increment and then wrap arround).
That's why auto-wait would be a bad idea (in some cases it isn't and then you can achieve it with globally with signal(SIGCHLD,SIG_IGN);) and why (short-lived) zombies states are your friend. They keep the child pid stable for you until you're ready to release it.
If you exit without releasing any of your children's pids, then you don't have to worry about zombie children anymore--your child processes will be reparented to the init process, which will wait on them for you when they die.
When you call fork(), a new process is created with you being its parent. When the child process finishes its running with a call to exit(), its process descriptor is still kept in the kernel's memory. It is your responsibility as its parent to collect its exit code, which is done with a call to wait() syscall. wait() blocks the parent process until one of its childrens is finished.
Zombie process is the name given to a process whose exit code was never collected by its parent.
Regarding to your first question - wait() is not called automatically as zombie processes wouldn't exist if it did. It is your responsibility as a programmer. Omitting the call to wait() will still work as you mentioned - but it is considered a bad practice.
Both this link and this link explains it good.
I have just started learning about fork and wait in Linux and came across this paragraph in the wait() manual page notes:
A child that terminates, but has not been waited for becomes a "zombie". The kernel maintains a minimal set of information about the zombie process (PID, termination status, resource usage information) in order to allow the parent to later perform a wait to obtain information about the child. As long as a zombie is not removed from the system via a wait, it will consume a slot in the kernel process table, and if this table fills, it will not be possible to create further processes. If a parent process terminates, then its "zombie" children (if any) are adopted by init(8), which automatically performs a wait to remove the zombies.
A question that came to mind after reading this:
Isn't the fact that not using wait() causes a resource waste until the parent terminates, a problem that amplifies when the parent process is meant to be a long lived process in the system?
Does this means I should always use wait() as soon as possible after using fork?
Isn't the fact that not using wait() will cause a resource waste until
the parent will terminate?
When a child process is running, there's no wastage of resource; it's still doing its task. The resource waste that your citation talks about is only when a child dies but it's parent hasn't reaped it yet i.e. not wait()ed on the child process.
a problem that amplifies when the parent process is meant to be a long
lived process in the system?
When your application runs for a very longtime and keeps forking children, there's a chance that the system might run out of resources when many child process are still running or the parent process didn't reap the exited children. It's the job of the application process to to optimally manage the resources on the system and reaping the child processes as soon as they might have done.
Does this means I should always use wait() as soon as possible after
using fork?
There's no straight "as early" or "as late" kind of answer to this. For example, parent process might want to carry on do something useful when the child is still running rather than waiting (It might be unnecessary to even check periodically if children status with WNOHANG when parent knows the children might have a long tasks to finish). So in this case, waiting as soon as forking a process might not be what you want. In general, parent should call wait() whenever it expects the child(ren) to have completed its task (or wants to know the stauts of children). The responsibility lies with the programmer to code correctly and call wait() at the most appropriate time.
Say I create a process with multiple child process and I call wait() in the main process. If one child terminates, its pid is returned. But what happens if a couple of child process terminate simultaneously? The call should return with one of them, and a second call should return with the other, right? Is there a particular order in which they will return (maybe there is a precedence to the child with lower pid)?
No.
SUSv4 leaves explicitly unspecified in which order (if any) child processes are reaped by one or several wait calls. There is also no "accidential" order that you could rely on, since different Linux kernel versions perform differently. (Source: M. Kerrisk, TLPI, 26.1.1, page 542).
Somewhat related trivia:
You might wonder why you can reliably wait on several child processes that terminate concurrently at all. If you think about how signals work, you might be inclined to believe that it is perfectly possible to lose child termination signals. Signals, a well-known fact, are not queued (except for realtime signals, but SIGCHLD isn't one!). Which means that if you go strictly by the letter of the book, then clearly several children terminating could cause some child termination signals becoming lost!
You can only call wait once at the same time, so you can at most consume one signal synchronously as it is generated, and have a second one made pending before your next call to wait. It appears that there is no way to account for any other signals that are generated in the mean time.
Luckily, that isn't the case. Waiting on child processes demonstrably works fine. Always, 100%. But why?
The reason is simple, the kernel is "cheating". If a child process exits while you are blocked in wait, there is no question as to what's happening. The signal is immediately delivered, the status information is being filled in, the parent unblocks, and the child process goes * poof *.
On the other hand, if a child is exiting and the parent isn't in a wait call (or if the parent is in a wait call reaping another child), the system converts the exiting process to a "zombie".
Now, when the parent process performs a wait, and there are any zombie processes, it won't block waiting for a signal to be generated (which may never happen if all the children have already exited!). Instead, it will just reap one of the zombies, pretending the signal was delivered just now.
Wikipedia basically gives all the possible information about zombie processes that I NEED to know but just a simple line on how it might be useful..in that a conflict in PIDs will not exist in the event the parent process creates another child process.
How is this then actually "useful"? Wouldn't the PID be then available if the named zombie process were to be removed instead of being kept there?
Or are there any other reasons as to why the zombie process should exist?
Zombie processes are actually really important and definitely need to exist. First it's important to understand how process creation works in Unix/Linux. The only way to create a new process is for an existing process to create a new child process via fork(). In this way, all of the processes on the system are arranged in a nice orderly tree heirarchy. Try running ps -Hu <your username> on a Linux system to see the heirarchy of processes that you own.
In many programs it is critically important for a parent process to be able to obtain basic information about its child processes that have exited. This basic information includes the exit status and resource usage of the child. When the parent is ready to get information about a dead child process it calls one of the wait() functions to wait for a child to exit and obtain exit status and resource usage info.
But what happens if a child process exits before the parent waits for it? This is where zombie processes become necessary. The operating system can't just discard the child process; the operation of the parent process may be dependent upon knowing the exit status or resource usage of the child. i.e. The parent process might need to know that the child exited abnormally, or it might be collecting CPU usage statistics for its children, etc. So, the only choice is to save off that information and make it available to the parent when it finally does call wait(). This information is what a zombie process is and it's a critical part of how process management works on Unix/Linux. Zombie processes allow the parent to be guaranteed to be able to retreive exit status, accounting information, and process id for child processes, regardless of whether the parent calls wait() before or after the child process exits.
This is why a zombie process is necessary.
Footnote: If the parent process never calls wait(), then the child process is reparented to the init process when the parent process dies, and init will wait() for the child.
The answer is on Wikipedia as well, which is:
This entry is still needed to allow the parent process to read its
child's exit status.
Zombie processes are useful.
Zombie processes allow the parent to be guaranteed to be able to retrieve exit status, accounting information, and process id of the child processes.
A process that doesn't clean up its child zombies isn't programmed properly.
How do zombie threads get formed in C/C++, and what do you need to make sure to do in order to prevent them from being created? I know they're just normal threads that didn't terminate properly, but I'm a little hazy on the specifics.
A zombie thread is a joinable thread which has terminated, but which
hasn't been joined. Normally, either a thread should be joined at some
time, or it should be detached. Otherwise, the OS maintains its state
for some possible future join, which takes resources.
Do you mean pthreads or zombie processes? A zombie process (not thread) gets created when a parent doesn't reap its child. It's because the OS keeps the return state of the process if the parent needs it later. If the parent dies, the child is given to the init thread which just sits and calls "wait" over and over again (reaping any children that die). So a zombie process can only be created when the parent is still alive and the child has terminated.
The same applies for pthreads. If you detach the thread, it will not keep that process termination state around after it finishes (similar to processes).