Can close() block

fifofilespipesocket

For example, if you close a pipe or socket that still has data in its write buffer that hasn't been read by the process on the other end, does close() block until the data is read or the other process dies?

Best Answer

Yes, close can block:

If O_NONBLOCK is not set and there have been no signals posted for the STREAM, and if there is data on the module's write queue, close() shall wait for an unspecified time (for each module and driver) for any output to drain before dismantling the STREAM.

And:

If fildes refers to a socket, close() shall cause the socket to be destroyed. If the socket is in connection-mode, and the SO_LINGER option is set for the socket with non-zero linger time, and the socket has untransmitted data, then close() shall block for up to the current linger interval until all data is transmitted.

Related Solutions

Bash – Races when piping two commands to a named pipe

Why not just do:

{ echo foo; echo bar;} > /tmp/p

If you want your controlling script to leave the pipe open, you can do:

exec 3<> /tmp/p

Opening a named pipe in read-write mode is to avoid that to block if the pipe hasn't been opened yet. That would instantiate it if it wasn't yet. It works on Linux at least but is not guaranteed to by POSIX.

Alternatively (and portably):

: < /tmp/p & exec 3> /tmp/p

Then instead of having each process open the named pipe, you can also do:

cmd >&3

And in the end, you'd do:

exec 3>&-

To close the writing end to let readers know it's finished.

change all <s to >s and <s to >s if you need the logic to be the other way round.

Does named pipe modify the filesystem

The file object itself is created in the filesystem, but no data is stored in a file system. From the mkpipe(3) manpage:

   A  FIFO special file is similar to a pipe, except that it is created in
   a different way.  Instead of being an anonymous communications channel,
   a  FIFO  special  file  is  entered  into  the  file  system by calling
   mkfifo().

About the only time the data might be stored on disk is during hibernation when memory is written to the swap space, including buffers - however this an corner case.

Best Answer

Related Solutions

Bash – Races when piping two commands to a named pipe

Does named pipe modify the filesystem

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