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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
PIPE(7) Linux Programmer's Manual PIPE(7)
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 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 16 pages (i.e., 65,536 bytes in a system
with a page size of 4096 bytes). Since Linux 2.6.35, the default
pipe capacity is 16 pages, 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.
The following ioctl(2) operation, which can be applied to a file
descriptor that refers to either end of a pipe, places a count of the
number of unread bytes in the pipe in the int buffer pointed to by
the final argument of the call:
ioctl(fd, FIONREAD, &nbytes);
The FIONREAD operation is not specified in any standard, but is
provided on many implementations.
/proc files
On Linux, the following files control how much memory can be used for
pipes:
/proc/sys/fs/pipe-max-pages (only in Linux 2.6.34)
An upper limit, in pages, on the capacity that an unprivileged
user (one without the CAP_SYS_RESOURCE capability) can set for
a pipe.
The default value for this limit is 16 times the default pipe
capacity (see above); the lower limit is two pages.
This interface was removed in Linux 2.6.35, in favor of
/proc/sys/fs/pipe-max-size.
/proc/sys/fs/pipe-max-size (since Linux 2.6.35)
The maximum size (in bytes) of individual pipes that can be
set by users without the CAP_SYS_RESOURCE capability. The
value assigned to this file may be rounded upward, to reflect
the value actually employed for a convenient implementation.
To determine the rounded-up value, display the contents of
this file after assigning a value to it.
The default value for this file is 1048576 (1 MiB). The
minimum value that can be assigned to this file is the system
page size. Attempts to set a limit less than the page size
cause write(2) to fail with the error EINVAL.
Since Linux 4.9, the value on this file also acts as a ceiling
on the default capacity of a new pipe or newly opened FIFO.
/proc/sys/fs/pipe-user-pages-hard (since Linux 4.5)
The hard limit on the total size (in pages) of all pipes
created or set by a single unprivileged user (i.e., one with
neither the CAP_SYS_RESOURCE nor the CAP_SYS_ADMIN
capability). So long as the total number of pages allocated
to pipe buffers for this user is at this limit, attempts to
create new pipes will be denied, and attempts to increase a
pipe's capacity will be denied.
When the value of this limit is zero (which is the default),
no hard limit is applied.
/proc/sys/fs/pipe-user-pages-soft (since Linux 4.5)
The soft limit on the total size (in pages) of all pipes
created or set by a single unprivileged user (i.e., one with
neither the CAP_SYS_RESOURCE nor the CAP_SYS_ADMIN
capability). So long as the total number of pages allocated
to pipe buffers for this user is at this limit, individual
pipes created by a user will be limited to one page, and
attempts to increase a pipe's capacity will be denied.
When the value of this limit is zero, no soft limit is
applied. The default value for this file is 16384, which
permits creating up to 1024 pipes with the default capacity.
Before Linux 4.9, some bugs affected the handling of the pipe-user-
pages-soft and pipe-user-pages-hard limits; see BUGS.
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. The target for delivery of signals must be set using
the fcntl(2) F_SETOWN command. 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.
BUGS
Before Linux 4.9, some bugs affected the handling of the pipe-user-
pages-soft and pipe-user-pages-hard limits when using the fcntl(2)
F_SETPIPE_SZ operation to change a pipe's capacity:
(1) When increasing the pipe capacity, the checks against the soft
and hard limits were made against existing consumption, and
excluded the memory required for the increased pipe capacity.
The new increase in pipe capacity could then push the total
memory used by the user for pipes (possibly far) over a limit.
(This could also trigger the problem described next.)
Starting with Linux 4.9, the limit checking includes the memory
required for the new pipe capacity.
(2) The limit checks were performed even when the new pipe capacity
was less than the existing pipe capacity. This could lead to
problems if a user set a large pipe capacity, and then the
limits were lowered, with the result that the user could no
longer decrease the pipe capacity.
Starting with Linux 4.9, checks against the limits are performed
only when increasing a pipe's capacity; an unprivileged user can
always decrease a pipe's capacity.
(3) The accounting and checking against the limits were done as
follows:
(a) Test whether the user has exceeded the limit.
(b) Make the new pipe buffer allocation.
(c) Account new allocation against the limits.
This was racey. Multiple processes could pass point (a)
simultaneously, and then allocate pipe buffers that were
accounted for only in step (c), with the result that the user's
pipe buffer allocation could be pushed over the limit.
Starting with Linux 4.9, the accounting step is performed before
doing the allocation, and the operation fails if the limit would
be exceeded.
Before Linux 4.9, bugs similar to points (1) and (3) could also occur
when the kernel allocated memory for a new pipe buffer; that is, when
calling pipe(2) and when opening a previously unopened FIFO.
mkfifo(1), dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2),
socketpair(2), splice(2), stat(2), tee(2), vmsplice(2), mkfifo(3),
epoll(7), fifo(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-07-13 PIPE(7)
Pages that refer to this page: fcntl(2), intro(2), open(2), pipe(2), readv(2), splice(2), tee(2), vmsplice(2), write(2), proc(5), fifo(7), signal(7)