From Linux Man Pages
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 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 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 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 SIG-
PIPE signal.
FIFO special files can be created by mkfifo(3), and are specially indicated in ls -l.
RELATED
mkfifo(1), open(2), pipe(2), sigaction(2), signal(2), socketpair(2), mkfifo(3), pipe(7)
CATEGORY