05-17-2013
Processing a file list via named pipe
I have a ksh93 script I use that processes a file list in the order that they exist in the list. I would like to speed up processing of the list by having multiple processes handle it at once. I was thinking that perhaps a good way to handle this would be to write the list to a named pipe and some number of processes that I can specify would be created and read from the named pipe the file it could work on and when finished with that file it would again read the named pipe until nothing is left. Each process would read the named pipe after finishing processing a file. The files then would be processed close to the order they existed in the file list which I need done.
My only consideration is regarding how to stop the processes from then hanging waiting for more input from the named pipe when there are no more files left to process. Would the best way to handle that be to write several times some kind of exit flag to the named pipe so that the processes as they read the next file from the list will instead encounter the exit flag and know to exit or is there a better way?
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FIFO(4) Linux Programmer's Manual FIFO(4)
NAME
fifo - first-in first-out special file, named pipe
DESCRIPTION
A FIFO special file (a named pipe) is similar to a pipe, except that it is accessed as part of the file system. It can be opened by multi-
ple processes for reading or writing. When processes are exchanging data via the FIFO, the kernel passes all data internally without writ-
ing it to the file system. Thus, the FIFO special file has no contents on the file system, the file system entry merely serves as a refer-
ence point so that processes can access the pipe using a name in the file system.
The kernel maintains exactly one pipe object for each FIFO special file that is opened by at least one process. The FIFO must be opened on
both ends (reading and writing) before data can be passed. Normally, opening the FIFO blocks until the other end is opened also.
A process can open a FIFO in non-blocking mode. In this case, opening for read only will succeed even if noone has opened on the write side
yet; opening for write only will fail with ENXIO (no such device or address) unless the other end has already been opened.
Under Linux, opening a FIFO for read and write will succeed both in blocking and non-blocking mode. POSIX leaves this behaviour undefined.
This can be used to open a FIFO for writing while there are no readers available. A process that uses both ends of the connection in order
to communicate with itself should be very careful to avoid deadlocks.
NOTES
When a process tries to write to a FIFO that is not opened for read on the other side, the process is sent a SIGPIPE signal.
FIFO special files can be created by mkfifo(3), and are specially indicated in ls -l.
SEE ALSO
mkfifo(3), mkfifo(1), pipe(2), socketpair(2), open(2), signal(2), sigaction(2)
Linux Man Page 1999-06-20 FIFO(4)