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NAME
pipe - overview of pipes and FIFOs
DESCRIPTION
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 filesystem (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 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 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. 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 (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.
Pipe capacity
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 nonatomic: 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 nonblocking (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 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 > 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.
SEE ALSO
dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socketpair(2), stat(2), mkfifo(3), epoll(7),
fifo(7)
COLOPHON
This page is part of release 3.54 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/.
Linux 2005-12-08 PIPE(7)