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pipe(7)
PIPE(7) Linux Programmer's Manual PIPE(7)
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 commu‐
nication 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 opera‐
tions 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 capac‐
ity 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 pro‐
vided 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 capac‐
ity 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 descrip‐
tor 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 inter‐
leaved 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 writ‐
ten), 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 fol‐
lows:
(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) simulta‐
neously, 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.
SEE ALSO
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)
COLOPHON
This page is part of release 5.02 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 2017-09-15 PIPE(7)