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 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 succeeds even if no one has opened on the write side yet and opening
for write-only fails 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 deadlocks.
For details of the semantics of I/O on FIFOs, see pipe
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
FIFO special files can be created by mkfifo
(3), and are indicated by
with the file type 'p'.