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
. 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
), 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
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)
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. Nonblocking 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
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
signal to be generated for the calling process. If the calling
process is ignoring this signal, then write(2)
fails with the error
. An application that uses pipe(2)
should use suitable close(2)
calls to close unnecessary duplicate file
descriptors; this ensures that end-of-file and SIGPIPE
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)
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.
POSIX.1-2001 says that write(2)
s of less than PIPE_BUF
be atomic: the output data is written to the pipe as a contiguous sequence.
Writes of more than PIPE_BUF
bytes may be nonatomic: the kernel may
interleave the data with data written by other processes. POSIX.1-2001
to be at least 512 bytes. (On Linux, PIPE_BUF
is 4096 bytes.) The precise semantics depend on whether the file descriptor is
), whether there are multiple writers to the
pipe, and on n
, the number of bytes to be written:
- O_NONBLOCK disabled, n <=
- All n bytes are written atomically; write(2)
may block if there is not room for n bytes to be written
- O_NONBLOCK enabled, n <=
- 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
- O_NONBLOCK disabled, n >
- The write is nonatomic: 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 >
- 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
Open File Status Flags¶
The only open file status flags that can be meaningfully applied to a pipe or
FIFO are O_NONBLOCK
Setting the O_ASYNC
flag for the read end of a pipe causes a signal
by default) to be generated when new input becomes available on
the pipe (see fcntl(2)
for details). On Linux, O_ASYNC
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.1-2001, pipes only need to be unidirectional. Portable applications
should avoid reliance on bidirectional pipe semantics.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found