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 file system (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 file system, 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_NON-
       BLOCK  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 65536 bytes.

   PIPE_BUF
       POSIX.1-2001 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.1-2001 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 mul-
       tiple 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 (see fcntl(2) for details).  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.   According  to POSIX.1-2001, pipes only need to be unidirectional.  Portable applications should avoid reliance on
       bidirectional pipe semantics.

SEE ALSO
       dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socketpair(2), stat(2), mkfifo(3), epoll(7), fifo(7)

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2005-12-08                                                    PIPE(7)