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).
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.
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.
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.
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.
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.
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.
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.