Section: Linux Programmer's Manual (7)
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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
Data written to the write end of a pipe can be read
from the read end of the pipe.
A pipe is created using
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
for an example.
A FIFO (short for First In First Out) has a name within the filesystem
and is opened using
Any process may open a FIFO, assuming the file permissions allow it.
The read end is opened using the
flag; the write end is opened using the
for further details.
although FIFOs have a pathname in the filesystem,
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
will block until data is available.
If a process attempts to write to a full pipe (see below), then
blocks until sufficient data has been read from the pipe
to allow the write to complete.
Nonblocking I/O is possible by using the
operation to enable the
open file status flag.
The communication channel provided by a pipe is a
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
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
will cause a
signal to be generated for the calling process.
If the calling process is ignoring this signal, then
fails with the error
An application that uses
should use suitable
calls to close unnecessary duplicate file descriptors;
this ensures that end-of-file and
are delivered when appropriate.
It is not possible to apply
to a pipe.
A pipe has a limited capacity.
If the pipe is full, then a
will block or fail, depending on whether the
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.
Since Linux 2.6.35, the default pipe capacity is 65536 bytes,
but the capacity can be queried and set using the
for more information.
POSIX.1-2001 says that
of less than
bytes must be atomic: the output data is written to the pipe as a
Writes of more than
bytes may be nonatomic: the kernel may interleave the data
with data written by other processes.
to be at least 512 bytes.
is 4096 bytes.)
The precise semantics depend on whether the file descriptor is nonblocking
whether there are multiple writers to the pipe, and on
the number of bytes to be written:
- O_NONBLOCK disabled, n <= PIPE_BUF
bytes are written atomically;
may block if there is not room for
bytes to be written immediately
- O_NONBLOCK enabled, n <= PIPE_BUF
If there is room to write
bytes to the pipe, then
succeeds immediately, writing all
- O_NONBLOCK disabled, n > PIPE_BUF
The write is nonatomic: the data given to
may be interleaved with
by other process;
bytes have been written.
- O_NONBLOCK enabled, n > PIPE_BUF
If the pipe is full, then
Otherwise, from 1 to
bytes may be written (i.e., a "partial write" may occur;
the caller should check the return value from
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
flag for the read end of a pipe causes a signal
by default) to be generated when new input becomes available on the pipe
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.1-2001, pipes only need to be unidirectional.
Portable applications should avoid reliance on
bidirectional pipe semantics.
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- I/O on pipes and FIFOs
- Pipe capacity
- Open file status flags
- Portability notes
- SEE ALSO