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
       details, 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
	      descriptors.   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
		 defined 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
	      descriptions.  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.

       On Linux (and other systems), pipe() does not modify pipefd on failure.
       A  requirement  standardizing  this behavior was added in POSIX.1-2016.
       The Linux-specific pipe2() system call likewise does not modify	pipefd
       on failure.

ERRORS
       EFAULT pipefd is not valid.

       EINVAL (pipe2()) Invalid value in flags.

       EMFILE The per-process limit on the number of open file descriptors has
	      been reached.

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

       ENFILE The  user	 hard  limit on memory that can be allocated for pipes
	      has been reached and the caller is not privileged; see pipe(7).

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, POSIX.1-2008.

       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 file 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), splice(2), write(2), popen(3), pipe(7)

COLOPHON
       This  page  is  part of release 4.10 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				  2016-12-12			       PIPE(2)