pipe

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 4.15 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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