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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.

       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 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 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 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 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 follows:

            (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)  simultaneously,  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

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       found at https://www.kernel.org/doc/man-pages/.