pipe

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
       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
       This page is part of release 5.03 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                             2017-09-15                           PIPE(7)