Shell – Send EOF to named pipe – cleaning up / drying up fifo

fifopipeshell-scripttail

If I have some random processes reading from a named pipe:

tail -f MYNAMEDPIPED 
cat MYNAMEDPIPE | someOtherProc

Elsewhere, I have a handle on MYNAMEDPIPED by name.
is there a safe and clean way to stop the tail process by deleting MYNAMEDPIPED or perhaps somehow "drying it up"?

In other words

MYNAMEDPIPED.noMoreDataIsComingThroughSoPleaseStopTailingThis()

🙂

From one of the comments, it says to send EOF to MYNAMEDPIPE. But I cannot figure out how to do this.

This shows the difficulty I am facing:

http://comp.os.linux.questions.narkive.com/2AW9g5yn/sending-an-eof-to-a-named-pipe

Best Answer

EOF is not a character nor an "event" and could not be sent through the pipe, or "fed" to its writing end, as some stubborn urban legend suggests.

The only way to generate an EOF on the reading end of a pipe/fifo (ie cause a read(2) on it to return 0) is to close all open handles to its writing end.

This will happen automatically if all the processes that had opened a named pipe in write mode and all the children that have inherited the file descriptors through fork() are terminated [1].

It's not possible for a read(2) on a named pipe to return 0 it that pipe was opened in read/write mode, eg. with

exec 7<>/path/to/fifo

because in that case there is a single file descriptor / handle for both ends of the pipe and closing the write end will also close the read end making impossible for a read(2) to return 0 (pipes do not support any kind of half-close as sockets do with shutdown(2)).

[1] And all processes that have received the file descriptor via SCM_RIGHTS ancillary message on a unix socket.

Notice that tail -f by definition won't terminate upon EOF, whether the file it's reading from is regular or special. One way to kill all processes that are holding a open handle to a file descriptor is with fuser(1):

tail -f /path/to/fifo
...
> /path/to/fifo  # let any blocking open(2) through
fuser -TERM -k /path/to/fifo

Beware that this will also kill processes that have (inadvertently) inherited an open handle to /path/to/fifo from their parents.

Related Solutions

Shell – Prevent automatic EOFs to a named pipe, and send an EOF when I want it

As others have indicated, the reader of a pipe receives EOF once there are no writers left. So the solution is to make sure there is always one writer holding it open. That writer doesn't have to send anything, just hold it open.

Since you're using a shell script, the simplest solution is to tell the shell to open the pipe for writing. And then close it when you're done.

#!/bin/sh
mkfifo P
exec 3>P # open file descriptor 3 writing to the pipe
program < P
# < P tail -n +1 -f | program
echo some stuff > P
cat more_stuff.txt > P
exec 3>&- # close file descriptor 3

Note that if you omit the last line, file descriptor 3 will be automatically closed (and thus the reader receive EOF) when the script exits. Aside from convenience, this also provides a safety of sorts if the script were to somehow terminate early.

Reading a named pipe: tail or cat

When you do:

cat fifo

Assuming no other process has opened the fifo for writing yet, cat will block on the open() system call. When another process opens the file for writing, a pipe will be instantiated and open() will return. cat will call read() in a loop and read() will block until some other process writes data to the pipe.

cat will see end-of-file (eof) when all the other writing processes have closed their file descriptor to the fifo. At which points cat terminates and the pipe is destroyed¹.

You'd need to run cat again to read what will be written after that to the fifo (but via a different pipe instance).

In:

tail -f file

Like cat, tail will wait for a process to open a file for writing. But here, since you didn't specify a -n +1 to copy from the beginning, tail will need to wait until eof to find out what the last 10 lines were, so you won't see anything until the writing end is closed.

After that, tail will not close its fd to the pipe which means the pipe instance won't be destroyed, and will still attempt to read from the pipe every second (on Linux, that polling can be avoided via the use of inotify and some versions of GNU tail do that there). That read() will return with eof (straight away, which is why you see 100% CPU with -s 0 (which with GNU tail means to not wait between read()s instead of waiting for one second)) until some other process opens the file again for writing.

Here instead, you may want to use cat, but make sure the pipe instance always stays around after it has been instantiated. For that, on most systems, you could do:

cat 0<> fifo # the 0 is needed for recent versions of ksh93 where the
             # default fd changed from 0 to 1 for the <> operator

cat's stdin will be open for both reading and writing which means cat will never see eof on it (it also instantiates the pipe straight away even if there's no other process opening the fifo for writing).

On systems where that doesn't work, you can do instead:

cat < fifo 3> fifo

That way, as soon as some other process opens the fifo for writing, the first read-only open() will return, at which point the shell will do the write-only open() before starting cat, which will prevent the pipe from ever being destroyed again.

So, to sum up:

compared to cat file, it would not stop after the first round.
compared to tail -n +1 -f file: it would not do a useless read() every second after the first round, there would never be eof on the one instance of the pipe, there would not be that up to one second delay when a second process opens the pipe for writing after the first one has closed it.
compared to tail -f file. In addition to the above, it would not have to wait for the first round to finish before outputting something (only the last 10 lines).
compared to cat file in a loop, there would be only one pipe instance. The race windows mentioned in ¹ would be avoided.

¹ at this point, in between the last read() that indicates eof and cat terminating and closing the reading end of the pipe, there is actually a small windows during which a process could open the fifo for writing again (and not be blocked as there's still a reading end). Then, if it writes something after cat has exited and before another process opens the fifo for reading, it would get killed with a SIGPIPE.

Best Answer

Related Solutions

Shell – Prevent automatic EOFs to a named pipe, and send an EOF when I want it

Reading a named pipe: tail or cat

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