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  ker‐
       nel 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 values 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 following 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 specifies 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 available 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 argu‐
       ment 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)