pipe(7) - Linux manual page (original) (raw)
pipe(7) Miscellaneous Information Manual pipe(7)
NAME top
pipe - overview of pipes and FIFOsDESCRIPTION top
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)](../man2/pipe.2.html), 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)](../man2/pipe.2.html) for an example.
A FIFO (short for First In First Out) has a name within the
filesystem (created using [mkfifo(3)](../man3/mkfifo.3.html)), and is opened using [open(2)](../man2/open.2.html).
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)](../man7/fifo.7.html) 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)](../man2/read.2.html)
will block until data is available. If a process attempts to
write to a full pipe (see below), then [write(2)](../man2/write.2.html) 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)](../man2/fcntl.2.html) **F_SETFL**
operation to enable the **O_NONBLOCK** open file status flag or by
opening a [fifo(7)](../man7/fifo.7.html) with **O_NONBLOCK**. If any process has the pipe
open for writing, reads fail with **EAGAIN**; otherwise—with no
potential writers—reads succeed and return empty.
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)](../man2/read.2.html) from the pipe will see
end-of-file ([read(2)](../man2/read.2.html) will return 0). If all file descriptors
referring to the read end of a pipe have been closed, then a
[write(2)](../man2/write.2.html) will cause a **SIGPIPE** signal to be generated for the
calling process. If the calling process is ignoring this signal,
then [write(2)](../man2/write.2.html) fails with the error **EPIPE**. An application that
uses [pipe(2)](../man2/pipe.2.html) and [fork(2)](../man2/fork.2.html) should use suitable [close(2)](../man2/close.2.html) 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)](../man2/lseek.2.html) 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.
Before Linux 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)](../man2/fcntl.2.html) **F_GETPIPE_SZ** and **F_SETPIPE_SZ** operations. See
[fcntl(2)](../man2/fcntl.2.html) for more information. Since Linux 4.5, the default pipe
capacity is lower than 16 pages when the _pipe-user-pages-soft_
limit is exceeded.
The following [ioctl(2)](../man2/ioctl.2.html) 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)](../man2/write.2.html) 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 two
pages (one page before Linux 5.14), 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 writes 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)](../man2/write.2.html) 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)](../man2/write.2.html) succeeds immediately, writing all _n_ bytes;
otherwise [write(2)](../man2/write.2.html) fails, with _[errno](../man3/errno.3.html)_ set to **EAGAIN**.
**O_NONBLOCK** disabled, _n_ > **PIPE_BUF**
The write is nonatomic: the data given to [write(2)](../man2/write.2.html) may be
interleaved with [write(2)](../man2/write.2.html)s by other process; the [write(2)](../man2/write.2.html)
blocks until _n_ bytes have been written.
**O_NONBLOCK** enabled, _n_ > **PIPE_BUF**
If the pipe is full, then [write(2)](../man2/write.2.html) fails, with _[errno](../man3/errno.3.html)_ 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)](../man2/write.2.html) 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)](../man2/fcntl.2.html) **F_SETOWN** command. On Linux, **O_ASYNC** is
supported for pipes and FIFOs only since Linux 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:
(a) 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.
(b) 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.
(c) The accounting and checking against the limits were done as
follows:
(1) Test whether the user has exceeded the limit.
(2) Make the new pipe buffer allocation.
(3) Account new allocation against the limits.
This was racey. Multiple processes could pass point (1)
simultaneously, and then allocate pipe buffers that were
accounted for only in step (3), 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 (a) and (c) could also
occur when the kernel allocated memory for a new pipe buffer; that
is, when calling [pipe(2)](../man2/pipe.2.html) and when opening a previously unopened
FIFO.SEE ALSO top
[mkfifo(1)](../man1/mkfifo.1.html), [dup(2)](../man2/dup.2.html), [fcntl(2)](../man2/fcntl.2.html), [open(2)](../man2/open.2.html), [pipe(2)](../man2/pipe.2.html), [poll(2)](../man2/poll.2.html), [select(2)](../man2/select.2.html),
[socketpair(2)](../man2/socketpair.2.html), [splice(2)](../man2/splice.2.html), [stat(2)](../man2/stat.2.html), [tee(2)](../man2/tee.2.html), [vmsplice(2)](../man2/vmsplice.2.html), [mkfifo(3)](../man3/mkfifo.3.html),
[epoll(7)](../man7/epoll.7.html), [fifo(7)](../man7/fifo.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-08-29 pipe(7)
Pages that refer to this page:pipesz(1), fcntl(2), intro(2), open(2), pipe(2), splice(2), tee(2), vmsplice(2), write(2), proc_sys_fs(5), fifo(7), signal(7)