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PIPE(2)			   Linux Programmer's Manual		       PIPE(2)

NAME
       pipe, pipe2 - create pipe

SYNOPSIS
       #include	<unistd.h>

       int pipe(int pipefd[2]);

       #define _GNU_SOURCE	       /* See feature_test_macros(7) */
       #include	<fcntl.h>	       /* Obtain O_* constant definitions */
       #include	<unistd.h>

       int pipe2(int pipefd[2],	int flags);

DESCRIPTION
       pipe()  creates	a pipe,	a unidirectional data channel that can be used
       for interprocess	communication.	The array pipefd is used to return two
       file  descriptors  referring to the ends	of the pipe.  pipefd[0]	refers
       to the read end of the pipe.  pipefd[1] refers to the write end of  the
       pipe.   Data  written  to  the write end	of the pipe is buffered	by the
       kernel until it is read from the	read end of the	pipe.  For further de-
       tails, see pipe(7).

       If  flags is 0, then pipe2() is the same	as pipe().  The	following val-
       ues can be bitwise ORed in flags	to obtain different behavior:

       O_CLOEXEC
	      Set the close-on-exec (FD_CLOEXEC) flag on the two new file  de-
	      scriptors.   See the description of the same flag	in open(2) for
	      reasons why this may be useful.

       O_DIRECT	(since Linux 3.4)
	      Create a pipe that performs I/O in "packet" mode.	 Each write(2)
	      to  the  pipe  is	 dealt with as a separate packet, and read(2)s
	      from the pipe will read one packet at a time.  Note the  follow-
	      ing points:

	      *	 Writes	 of  greater than PIPE_BUF bytes (see pipe(7)) will be
		 split into multiple packets.  The constant  PIPE_BUF  is  de-
		 fined in _limits.h_.

	      *	 If a read(2) specifies	a buffer size that is smaller than the
		 next packet, then the requested number	of bytes are read, and
		 the  excess  bytes in the packet are discarded.  Specifying a
		 buffer	size of	 PIPE_BUF  will	 be  sufficient	 to  read  the
		 largest possible packets (see the previous point).

	      *	 Zero-length packets are not supported.	 (A read(2) that spec-
		 ifies a buffer	size of	zero is	a no-op, and returns 0.)

	      Older kernels that do not	support	this flag will	indicate  this
	      via an EINVAL error.

       O_NONBLOCK
	      Set the O_NONBLOCK file status flag on the two new open file de-
	      scriptions.  Using this flag saves extra calls  to  fcntl(2)  to
	      achieve the same result.

RETURN VALUE
       On  success,  zero is returned.	On error, -1 is	returned, and errno is
       set appropriately.

ERRORS
       EFAULT pipefd is	not valid.

       EINVAL (pipe2())	Invalid	value in flags.

       EMFILE Too many file descriptors	are in use by the process.

       ENFILE The system limit on the total number  of	open  files  has  been
	      reached.

VERSIONS
       pipe2()	was  added to Linux in version 2.6.27; glibc support is	avail-
       able starting with version 2.9.

CONFORMING TO
       pipe(): POSIX.1-2001.

       pipe2() is Linux-specific.

EXAMPLE
       The following program creates a pipe, and then  fork(2)s	 to  create  a
       child  process;	the child inherits a duplicate set of file descriptors
       that refer to the same pipe.  After the fork(2),	 each  process	closes
       the  descriptors	 that it doesn't need for the pipe (see	pipe(7)).  The
       parent then writes the string contained in the  program's  command-line
       argument	 to the	pipe, and the child reads this string a	byte at	a time
       from the	pipe and echoes	it on standard output.

   Program source
       #include	<sys/types.h>
       #include	<sys/wait.h>
       #include	<stdio.h>
       #include	<stdlib.h>
       #include	<unistd.h>
       #include	<string.h>

       int
       main(int	argc, char *argv[])
       {
	   int pipefd[2];
	   pid_t cpid;
	   char	buf;

	   if (argc != 2) {
	       fprintf(stderr, "Usage: %s <string>\n", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   if (pipe(pipefd) == -1) {
	       perror("pipe");
	       exit(EXIT_FAILURE);
	   }

	   cpid	= fork();
	   if (cpid == -1) {
	       perror("fork");
	       exit(EXIT_FAILURE);
	   }

	   if (cpid == 0) {    /* Child	reads from pipe	*/
	       close(pipefd[1]);	  /* Close unused write	end */

	       while (read(pipefd[0], &buf, 1) > 0)
		   write(STDOUT_FILENO,	&buf, 1);

	       write(STDOUT_FILENO, "\n", 1);
	       close(pipefd[0]);
	       _exit(EXIT_SUCCESS);

	   } else {	       /* Parent writes	argv[1]	to pipe	*/
	       close(pipefd[0]);	  /* Close unused read end */
	       write(pipefd[1],	argv[1], strlen(argv[1]));
	       close(pipefd[1]);	  /* Reader will see EOF */
	       wait(NULL);		  /* Wait for child */
	       exit(EXIT_SUCCESS);
	   }
       }

SEE ALSO
       fork(2),	read(2), socketpair(2),	write(2), popen(3), pipe(7)

COLOPHON
       This page is part of release 3.74 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
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-07-08			       PIPE(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | EXAMPLE | SEE ALSO | COLOPHON

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