pipe

PIPE(2)                    Linux Programmer's Manual                   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 de-
       tails, see pipe(7).

       If  flags is 0, then pipe2() is the same as pipe().  The following val-
       ues 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  de-
              scriptors.   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  follow-
              ing points:

              *  Writes  of  greater than PIPE_BUF bytes (see pipe(7)) will be
                 split into multiple packets.  The constant  PIPE_BUF  is  de-
                 fined 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 spec-
                 ifies 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).

       O_NONBLOCK
              Set the O_NONBLOCK file status flag on the  open  file  descrip-
              tions  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.1-2016.
       The  Linux-specific pipe2() system 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 privileged; see pipe(7).

VERSIONS
       pipe2()  was  added to Linux in version 2.6.27; glibc support is avail-
       able starting with version 2.9.

NOTES
       The SystemV ABI on some architectures allows the use of more  than  one
       register  for returning multiple values; several 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  regard-
       ing registers used for storing second file descriptor.

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

       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 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) {
               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 5.05 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/.

Linux                             2019-03-06                           PIPE(2)