pipe(2)



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.

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)

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