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 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) 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.
