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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
       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.

              *  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
       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>
", 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, "
", 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 3.65 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/.



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