man > pipe(7)

pipe(7)                           Miscellaneous Information Manual                           pipe(7)

NAME
       pipe - overview of pipes and FIFOs

DESCRIPTION
       Pipes  and FIFOs (also known as named pipes) provide a unidirectional interprocess communica‐
       tion 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 exam‐
       ple.

       A  FIFO  (short  for  First In First Out) has a name within the filesystem (created using mk‐
       fifo(3)), and is opened using open(2).  Any process may open a FIFO, assuming the  file  per‐
       missions  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 path‐
       name 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  at‐
       tempt to read(2) from the pipe will see end-of-file (read(2) will return 0).  If all file de‐
       scriptors  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, de‐
       pending 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 capac‐
       ity: 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_SET‐
       PIPE_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 implementa‐
       tions.

   /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 as‐
              signed  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 writ‐
       ten 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_NON‐
       BLOCK 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 di‐
       rections between the pipe ends.  POSIX.1 requires only unidirectional pipes.  Portable appli‐
       cations 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 exist‐
            ing 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  allo‐
            cate  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 allo‐
       cated memory for a new pipe buffer; that is, when calling pipe(2) and when opening  a  previ‐
       ously 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)

Linux man-pages 6.7                          2023-10-31                                      pipe(7)