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Linux 2.6 - man page for pipe (linux section 7)

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 communi-
       cation 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 descrip-
       tors, 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  under-
       lying 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_NON-
       BLOCK open file status flag.

       The communication channel provided by a pipe is a byte stream: there is no concept of mes-
       sage 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 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  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 (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 unidi-
       rectional.  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 3.55 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)


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