Pipes and FIFOs (also known as named pipes)
provide a unidirectional interprocess communication channel.
A pipe has a
read end
and a
write end.
Data written to the write end of a pipe can be read
from the read end of the pipe.
A pipe is created using
pipe(2),
which creates a new pipe and returns two file descriptors,
one referring to the read end of the pipe,
the other referring to the write end.
Pipes can be used to create a communication channel between related
processes; see
pipe(2)
for an example.
A FIFO (short for First In First Out) has a name within the file
system (created using
mkfifo(3)),
and is opened using
open(2).
Any process may open a FIFO, assuming the file permissions allow it.
The read end is opened using the
O_RDONLY
flag; the write end is opened using the
O_WRONLY
flag.
See
fifo(7)
for further details.
Note:
although FIFOs have a pathname in the file system,
I/O on FIFOs does not involve operations on the underlying device
(if there is one).
I/O on Pipes and FIFOs
The only difference between pipes and FIFOs is the manner in which
they are created and opened.
Once these tasks have been accomplished,
I/O on pipes and FIFOs has exactly the same semantics.
If a process attempts to read from an empty pipe, then
read(2)
will block until data is available.
If a process attempts to write to a full pipe (see below), then
write(2)
blocks until sufficient data has been read from the pipe
to allow the write to complete.
Non-blocking I/O is possible by using the
fcntl(2)
F_SETFL
operation to enable the
O_NONBLOCK
open file status flag.
The communication channel provided by a pipe is a
byte stream:
there is no concept of message boundaries.
If all file descriptors referring to the write end of a pipe
have been closed, then an attempt to
read(2)
from the pipe will see end-of-file
(read(2)
will return 0).
If all file descriptors referring to the read end of a pipe
have been closed, then a
write(2)
will cause a
SIGPIPE
signal to be generated for the calling process.
If the calling process is ignoring this signal, then
write(2)
fails with the error
EPIPE.
An application that uses
pipe(2)
and
fork(2)
should use suitable
close(2)
calls to close unnecessary duplicate file descriptors;
this ensures that end-of-file and
SIGPIPE/EPIPE
are delivered when appropriate.
A pipe has a limited capacity.
If the pipe is full, then a
write(2)
will block or fail, depending on whether the
O_NONBLOCK
flag is set (see below).
Different implementations have different limits for the pipe capacity.
Applications should not rely on a particular capacity:
an application should be designed so that a reading process consumes data
as soon as it is available,
so that a writing process does not remain blocked.
In Linux versions before 2.6.11, the capacity of a pipe was the same as
the system page size (e.g., 4096 bytes on i386).
Since Linux 2.6.11, the pipe capacity is 65536 bytes.
PIPE_BUF
POSIX.1-2001 says that
write(2)s
of less than
PIPE_BUF
bytes must be atomic: the output data is written to the pipe as a
contiguous sequence.
Writes of more than
PIPE_BUF
bytes may be non-atomic: the kernel may interleave the data
with data written by other processes.
POSIX.1-2001 requires
PIPE_BUF
to be at least 512 bytes.
(On Linux,
PIPE_BUF
is 4096 bytes.)
The precise semantics depend on whether the file descriptor is non-blocking
(O_NONBLOCK),
whether there are multiple writers to the pipe, and on
n,
the number of bytes to be written:
O_NONBLOCK disabled, n <= PIPE_BUF
All
n
bytes are written atomically;
write(2)
may block if there is not room for
n
bytes to be written immediately
O_NONBLOCK enabled, n <= PIPE_BUF
If there is room to write
n
bytes to the pipe, then
write(2)
succeeds immediately, writing all
n
bytes; otherwise
write(2)
fails, with
errno
set to
EAGAIN.
O_NONBLOCK disabled, n > PIPE_BUF
The write is non-atomic: the data given to
write(2)
may be interleaved with
write(2)s
by other process;
the
write(2)
blocks until
n
bytes have been written.
O_NONBLOCK enabled, n > PIPE_BUF
If the pipe is full, then
write(2)
fails, with
errno
set to
EAGAIN.
Otherwise, from 1 to
n
bytes may be written (i.e., a "partial write" may occur;
the caller should check the return value from
write(2)
to see how many bytes were actually written),
and these bytes may be interleaved with writes by other processes.
Open File Status Flags
The only open file status flags that can be meaningfully applied to
a pipe or FIFO are
O_NONBLOCK
and
O_ASYNC.
Setting the
O_ASYNC
flag for the read end of a pipe causes a signal
(SIGIO
by default) to be generated when new input becomes available on the pipe
(see
fcntl(2)
for details).
On Linux,
O_ASYNC
is supported for pipes and FIFOs only since kernel 2.6.
Portability notes
On some systems (but not Linux), pipes are bidirectional:
data can be transmitted in both directions between the pipe ends.
According to POSIX.1-2001, pipes only need to be unidirectional.
Portable applications should avoid reliance on
bidirectional pipe semantics.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
