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), mkfifo(3), epoll(7), fifo(7)

COLOPHON
       This  page  is  part of release 4.10 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				  2017-03-13			       PIPE(7)