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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.

              Since Linux 4.5, it is possible to change the  O_DIRECT  setting  of  a  pipe  file
              descriptor using fcntl(2).

              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.

       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.


       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).


       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 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>

       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),  tee(2),  vmsplice(2),  write(2), popen(3),


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