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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.
Since Linux 2.6.35, the default pipe capacity is 65536 bytes, but the capacity can be
queried and set using the fcntl(2) F_GETPIPE_SZ and F_SETPIPE_SZ operations. See fcntl(2)
for more information.
PIPE_BUF
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
nonatomic: 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 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. POSIX.1 requires only unidirectional pipes. 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), splice(2), stat(2),
mkfifo(3), epoll(7), fifo(7)
COLOPHON
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