# pipe

## 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.  Since Linux 4.5, the default pipe
           capacity is lower than 16 pages when the _pipe-user-pages-soft_
           limit is exceeded.
    
           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 two
                  pages (one page before Linux 5.14), 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.
    


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