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pipe - overview of pipes and FIFOs
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 only allow communication
between related processes: one process creates the pipe, and then
allows another process to inherit duplicate file descriptors referring
to the pipe as a result of calling fork(2).
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(4) 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.
It is not possible to apply lseek(2) to a pipe.
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
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 x86). Since Linux 2.6.11,
the pipe capacity is 65536 bytes.
POSIX.1 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
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
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.
On some systems (but not Linux), pipes are bidirectional: data can be
transmitted in both directions between the pipe ends. According to
POSIX.1, pipes only need to be unidirectional. Portable applications
should avoid reliance on bidirectional pipe semantics.
dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socketpair(2),
stat(2), mkfifo(3), fifo(4), epoll(4)