pipe(2)

NAME

   pipe, pipe2 - create pipe

SYNOPSIS

   #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

   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.   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.

   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

   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.

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
   available starting with version 2.9.

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), write(2), popen(3), pipe(7)

COLOPHON

   This  page  is  part of release 4.09 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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