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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 or by opening a fifo(7) with O_NONBLOCK.  If any  process
       has  the  pipe  open  for  writing,  reads  fail  with EAGAIN; otherwise—with no potential
       writers—reads succeed and return empty.

       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.

       Before Linux 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 Linux 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:

       (a)  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.

       (b)  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.

       (c)  The accounting and checking against the limits were done as follows:

            (1)  Test whether the user has exceeded the limit.
            (2)  Make the new pipe buffer allocation.
            (3)  Account new allocation against the limits.

            This  was  racey.   Multiple  processes could pass point (1) simultaneously, and then
            allocate pipe buffers that were accounted for only in step (3), 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 (a) and (c) 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)