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.