fifo - first-in first-out special file, named pipe
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 multiple
processes for reading or writing. When processes are exchanging data
via the FIFO, the kernel passes all data internally without writing 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 reference 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.
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.
mkfifo(3), mkfifo(1), pipe(2), socketpair(2), open(2), signal(2),
Linux Man Page 1999-06-20 FIFO(4)
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