PIPE(7) (2017-09-15) 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 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
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PIPE(7) (2017-09-15) PIPE(7)
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 implementa-
tions have different limits for the pipe capacity. Applica-
tions should not rely on a particular capacity: an applica-
tion 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:
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htmlmanrefstart/proc/sys/fs/pipe-user-pages-hard(sinceLinux4.5) .}f 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.
htmlmanrefend/proc/sys/fs/pipe-user-pages-soft(sinceL
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