DESCRIPTION
A FIFO special file (a named pipe) is similar to a pipe, except that
it is accessed as part of the filesystem. 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 filesystem. Thus, the FIFO special file
has no contents on the filesystem; the filesystem entry merely serves
as a reference point so that processes can access the pipe using a
name in the filesystem.
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 nonblocking mode. In this case, opening
for read-only will succeed even if no-one 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 nonblocking mode. POSIX leaves this behavior 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
