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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)