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       pipe, pipe2 - create pipe


       #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);


       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:

              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.

              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.


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


       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


       pipe2() was  added  to  Linux  in  version  2.6.27;  glibc  support  is
       available starting with version 2.9.


       pipe(): POSIX.1-2001, POSIX.1-2008.

       pipe2() is Linux-specific.


       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>

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

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

           if (pipe(pipefd) == -1) {

           cpid = fork();
           if (cpid == -1) {

           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);

           } 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 */


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


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