PIPE(2)                   (2020-06-09)                    PIPE(2)

     NAME
          pipe, pipe2 - create pipe

     SYNOPSIS
          #include <unistd.h>

          /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64; see NOTES */
          struct fd_pair {
              long fd[2];
          };
          struct fd_pair pipe();

          /* On all other architectures */
          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 fol-
          lowing values can be bitwise ORed in flags to obtain differ-
          ent 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>.

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               *  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 indi-
               cate 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).

          O_NONBLOCK
               Set the O_NONBLOCK file status flag on the open file
               descriptions referred to by the new file descriptors.
               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,
          errno is set appropriately, and pipefd is left unchanged.

          On Linux (and other systems), pipe() does not modify pipefd
          on failure.  A requirement standardizing this behavior was
          added in POSIX.1-2008 TC2.  The Linux-specific pipe2() sys-
          tem call likewise does not modify pipefd on failure.

     ERRORS
          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 privi-
               leged; see pipe(7).

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     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, POSIX.1-2008.

          pipe2() is Linux-specific.

     NOTES
          The System V ABI on some architectures allows the use of
          more than one register for returning multiple values; sev-
          eral architectures (namely, Alpha, IA-64, MIPS, SuperH, and
          SPARC/SPARC64) (ab)use this feature in order to implement
          the pipe() system call in a functional manner: the call
          doesn't take any arguments and returns a pair of file
          descriptors as the return value on success.  The glibc
          pipe() wrapper function transparently deals with this.  See
          syscall(2) for information regarding registers used for
          storing second file descriptor.

     EXAMPLES
          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>

          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) {

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                  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), splice(2), tee(2),
          vmsplice(2), write(2), popen(3), pipe(7)

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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