I have a while loop that reads data from a child process using blocking I/O by redirecting stdout of the child process to the parent process. Normally, as soon as the child process exits, a blocking read() in this case will return since the pipe that is read from is closed by the child process.
Now I have a case where the read() call does not exit for a child process that finishes. The child process ends up in a zombie state, since the operating system is waiting for my code to reap it, but instead my code is blocking on the read() call.
The child process itself does not have any child processes running at the time of the hang, and I do not see any file descriptors listed when looking in /proc/<child process PID>/fd. The child process did however fork two daemon processes, whose purpose seems to be to monitor the child process (the child process is a proprietary application I do not have any control over, so it is hard to say for sure).
When run from a terminal, the child process I try to read() from exits automatically, and in turn the daemon processes it forked terminate as well.
Linux version is 4.19.2.
What could be the reason of read() not returning in this case?
Follow-up: How to avoid read() from hanging in the following situation?
The child process did however fork two daemon processes ... What could be the reason of read() not returning in this case?
Forked processes still have the file descriptor open when the child terminates. Hence read call never returns 0.
Those daemon processes should close all file descriptors and open files for logging.
A possible reason (the most common) for read(2) blocking on a pipe with a dead child, is that the parent has not closed the writing side of the pipe, so there's still an open (for writing) descriptor for that pipe. Close the writing side of the pipe in the parent process before reading from it. The child is dead (you said zombie) so it cannot be the process with the writing side of the pipe open. And don't forget to wait(2) for the child in the parent, or you'll get a system full of zombies :)
Remember, you have to do two closes in your code:
One in the parent process, to close the writing side of the pipe, leaving the parent process with only a reading descriptor.
One in the child process (just before exec(2)ing) closing the reading side of the pipe, leaving the child process only with a writing descriptor.
In case you want to use the pipe(2) to send information to the child, change the reading for writing and viceversa in the above two points.
Related
If a parent and child process communicate via a pipe, what would happen if a parent reads from a pipe after a child process has written to it and died?
When the child dies, its end of the pipe is automatically closed. The parent will read EOF after reading everything that the child wrote before it died, just as if the client called close() explicitly.
Note that the parent can only read data that has actually been written to the pipe. If the child process is performing buffered output, as is the default when using stdio, all the data the application has written might not be in the pipe when it dies. Stdio buffers are automatically flushed when the process calls exit(), but if it dies due to a signal this will not be called.
My book on C applied to Linux, says that if a process creates a child with a fork(), then the pipe created between them follow this principle:
It is important to notice that both the parent process and the child process initially close their unused ends of the pipe
If both processes start with their pipe-end closed, how they know when the other is free to communicate? Maybe, is there an intermediate buffer between the processes?
Pipes on computers works very much like pipes in real life. There are two ends, you put something into one end and it comes out the other end.
Normally when using pipes in a program, you usually only want the input-end, where you write data, or you want the output-end, where data is read from. If the parent process only wants to write to the child process, and the child process only reads from the parent process, then the parent process could close the read end after the fork, and the child process can close the write end.
Pipe is an interprocess communication mechanism provided by the kernel. A process writing on the pipe need not worry whether there is some other process to read it. The communication is asynchronous. The kernel takes care of the data in transit.
What happens when the child process gets killed while the parent is blocked on read() from a pipe? How should I handle this scenario in parent process?
For clarification, parent process has two threads. Lets say thread1 was reading from the pipe when thread2 killed the child.
Will read() return -1?
Will appreciate any help here.
Pipe behavior has nothing to do with process relationships. The same rules apply regardless of whether the reader is the parent, child, sibling, or some other distant relation of the writer. Or even if the reader and writer are the same process.
The short answer is that death of a writing process is just an EOF from the reader's point of view, not an error, and this doesn't depend on whether the writing process voluntarily called _exit() or was killed by a signal.
The whole cause and effect chain goes like this:
Process X dies -> all of process X's file descriptors are closed.
One of process X's file descriptors was the write end of a pipe
A pipe write file descriptor is closed -> was it the last one?
3a. There are other write file descriptors on the same pipe (e.g. inherited by fork and still open in another process), nothing happens. Stop.
3b. There are no more write file descriptors for this pipe -> the pipe has hit EOF.
Pipe hits EOF -> readers notice.
4a. All read file descriptors for the pipe become readable, waking up any process that was blocking on select or poll or read or another similar syscall.
4b. If there is any leftover data in the pipe buffer (written before the last write file descriptor was closed), that data is returned to the reader(s).
4c. repeat 4b until the pipe buffer is empty
4d. Finally, read() returns 0, indicating EOF.
The exit status of a child process is returned to the parent by the wait family of syscalls, and you have to check that if you want to know when your child processes have been killed by a signal.
I do not know why the parent process needs to close both the file descriptors of a pipe before calling wait()?
I have a C program which does:
Parent creates child_a, which executes ls -l using execvp, and writes to the pipe (after closing read end of pipe).
Parent creates another child (without closing any file descriptor for pipe), called child_b, which executes 'wc' by reading from pipe.(after closing write end of pipe).
Parent waits for both children to complete by calling wait() twice.
I noticed that program is blocked if parent does not close both file descriptors of the pipe before calling the wait() syscall. Also after reading few questions already posted online it looks like this is the general rule and needs to be done. But I could not find the reason why this has to be done?
Why does wait() not return if the parent does not close the file descriptors of the pipe?
I was thinking that, in the worst case, if the parent does not close the file descriptor of pipe, then the only consequence would be that the pipe would keep existing (which is a waste of resource). But I never thought this would block the execution of child process (as can be seen because wait() does not return).
Also remember, parent is not using the pipe at all. It is child_a writing in the pipe, and child_b reading from the pipe.
If the parent process doesn't close the write ends of the pipes, the child processes never get EOF (zero bytes read) because there's a process that might (but won't) write to the pipe. The child process must also close the write end of the pipe for the same reason — if it doesn't, there's a process (itself) that might (but won't) write to the pipe, so the read won't return EOF.
If you duplicate one end of a pipe to standard output or standard error, you should close both ends of that pipe. It is a common mistake not to have enough calls to close() in multiprocess code using pipes. Occasionally, you get away with being sloppy, but the details vary by case and usually you don't.
I'am using fork to create child and parent processes and used pipe to send and receive messages between them.I'm running parent process with some inputs and it calls child process.If parent process is executed successfully, child process is getting killed automatically but If i press ctrl+c when parent process is running or if there is any segmentation fault, parent process is getting killed but child process is not getting killed.
Can anybody post me the logic to kill child process when parent process is abrupted ?
Since you already use a pipe for communication, this should be easy.
Pipe reads block until there's data to read and if parent process is the writer and died, you get EOF immediately. If your parent process never closes it's write end, then you have a reliable way to detect death.
Pipe writes hit SIGPIPE and return EPIPE from the call if the signal is ignored when there are no readers.
In the child, select (if you can block) on the fd of pipe and kill the process at appropriate time. There is no SIGCHLD equivalent to parent dying.
man 7 pipe for a good overview. Excerpt:
If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from
the pipe will see end-of-file (read(2) will return 0). If all file descriptors referring to the read end of a
pipe have been closed, then a write(2) will cause a SIGPIPE signal to be generated for the calling process. If
the calling process is ignoring this signal, then write(2) fails with the error EPIPE. An application that
uses pipe(2) and fork(2) should use suitable close(2) calls to close unnecessary duplicate file descriptors;
this ensures that end-of-file and SIGPIPE/EPIPE are delivered when appropriate.