Basic Questions about Pipes

Question

I have some basic questions about pipes I am unsure about.

a) What is the standard behavior if a process writing to a pipe gets killed (ie. SIGKILL SIGINT) Does it close the pipe? Does it flush the pipe? Or is the behavior undefined?

b) What is the standard behavior if a process returns normally? Is it guaranteed to flush the pipe and close the pipe? (without explicitly doing so of course).

I would like these answers to be as general as possible, but in reality if it depends entirely on the OS specs I can accept that! However, if there is a Posix standard or a current defined Windows behavior I would be very grateful to know.

Thanks.

Answer 1

a. What is the standard behavior if a process writing to a pipe gets killed (ie. SIGKILL SIGINT) Does it close the pipe? Does it flush the pipe? Or is the behavior undefined?

SIGKILL never allows any cleanup - the process dies, dead. With SIGINT, it depends on whether the process handles the signal. If so, it is likely to exit via exit(2), which flushes standard I/O file handles. The question is - was the pipe connected to standard output or via popen()? If so, outstanding buffered data may be flushed; if not, there is no buffered data so flushing is immaterial.

If there is unread data in the pipe, that data remains in the pipe, ready for the reader to collect - assuming there is a reader.

b. What is the standard behavior if a process returns normally? Is it guaranteed to flush the pipe and close the pipe? (without explicitly doing so of course).

It depends on whether the pipe was connected via standard I/O or not. If not, there is nothing pending. If so, then yes, any material in the buffers will be flushed as the standard I/O stream is closed.

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c. Thanks for the info on signals and the unread data, but I'm a little confused about the standard I/O pipe connection. After you mentioned popen() I looked it up and the man page says its return value identical to an I/O stream and the streams are fully buffered by default. I'm just not clear on the difference between the two nor do I understand where the difference comes from.

The basic system call for creating pipes is pipe(2). It creates two file descriptors, one for the read end of the pipe, one for the write end. If you do nothing else with them, then they remain as file descriptors, with unbuffered output (via write(2) and related system calls). If the process terminates, there is no buffering in the application; the pipe is closed.

If you use popen(3), then it does a whole lot more work for you. It still invokes pipe(2) to create the pipes, but it then does a fork(2). The child arranges the correct configuration of the pipes and launches the child process. The parent also closes the unused end of the pipe, and uses fdopen(3) to create a standard I/O file stream for the calling process to use.

With the file stream, if there is data in the I/O buffer, then a close or equivalent will ensure that the outstanding data is flushed and the file descriptor is closed.

Answer 2

The normal behaviour is that all file descriptors are closed when a process terminates. This means that a pipe, like any other open file descriptor, is closed normally.

One interesting thing about pipes, though: in POSIX, if a process writes to a pipe that has been closed, the writer will get a signal, SIGPIPE.

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Edit:

A caveat: The difference between s SIGx termination and a normal termination is that, like any other file write, you may loose data that has been buffered (via a FILE write) and not yet written to the file descriptor.