NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | EXAMPLE | SEE ALSO | COLOPHON

PIPE(2)                   Linux Programmer's Manual                  PIPE(2)

NAME         top

       pipe, pipe2 - create pipe

SYNOPSIS         top

       #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         top

       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         top

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

ERRORS         top

       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         top

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

CONFORMING TO         top

       pipe(): POSIX.1-2001.

       pipe2() is Linux-specific.

EXAMPLE         top

       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>\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         top

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

COLOPHON         top

       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/.

Linux                            2014-02-11                          PIPE(2)