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 re-
       main 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 fc-
       ntl(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 de-
       scriptor 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 as-
              signed 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 de-
              nied, 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 in-
       terleave  the  data  with data written by other processes.  POSIX.1 re-
       quires 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 inter-
              leaved  with write(2)s by other process; the write(2) blocks un-
              til n bytes have been written.

       O_NONBLOCK enabled, n > PIPE_BUF
              If the pipe is full, then write(2) fails, with errno set to  EA-
              GAIN.   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 fc-
       ntl(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 in-
            crease 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  de-
            crease 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  al-
            location 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 4.16 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)

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