PIPE(3)                    POSIX Programmer's Manual                   PIPE(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       pipe -- create an interprocess channel

SYNOPSIS
       #include <unistd.h>

       int pipe(int fildes[2]);

DESCRIPTION
       The pipe() function shall create a pipe and place two file descriptors,
       one each into the arguments fildes[0] and fildes[1], that refer to the
       open file descriptions for the read and write ends of the pipe. The
       file descriptors shall be allocated as described in Section 2.14, File
       Descriptor Allocation.  The O_NONBLOCK and FD_CLOEXEC flags shall be
       clear on both file descriptors. (The fcntl() function can be used to
       set both these flags.)

       Data can be written to the file descriptor fildes[1] and read from the
       file descriptor fildes[0].  A read on the file descriptor fildes[0]
       shall access data written to the file descriptor fildes[1] on a first-
       in-first-out basis. It is unspecified whether fildes[0] is also open
       for writing and whether fildes[1] is also open for reading.

       A process has the pipe open for reading (correspondingly writing) if it
       has a file descriptor open that refers to the read end, fildes[0]
       (write end, fildes[1]).

       The pipe's user ID shall be set to the effective user ID of the calling
       process.

       The pipe's group ID shall be set to the effective group ID of the
       calling process.

       Upon successful completion, pipe() shall mark for update the last data
       access, last data modification, and last file status change timestamps
       of the pipe.

RETURN VALUE
       Upon successful completion, 0 shall be returned; otherwise, -1 shall be
       returned and errno set to indicate the error, no file descriptors shall
       be allocated and the contents of fildes shall be left unmodified.

ERRORS
       The pipe() function shall fail if:

       EMFILE All, or all but one, of the file descriptors available to the
              process are currently open.

       ENFILE The number of simultaneously open files in the system would
              exceed a system-imposed limit.

       The following sections are informative.

EXAMPLES
   Using aa Pipe to Pass Data Between aa Parent Process and aa Child Process
       The following example demonstrates the use of a pipe to transfer data
       between a parent process and a child process. Error handling is
       excluded, but otherwise this code demonstrates good practice when using
       pipes: after the fork() the two processes close the unused ends of the
       pipe before they commence transferring data.


           #include <stdlib.h>
           #include <unistd.h>
           ...

           int fildes[2];
           const int BSIZE = 100;
           char buf[BSIZE];
           ssize_t nbytes;
           int status;

           status = pipe(fildes);
           if (status == -1 ) {
               /* an error occurred */
               ...
           }

           switch (fork()) {
           case -1: /* Handle error */
               break;

           case 0:  /* Child - reads from pipe */
               close(fildes[1]);                       /* Write end is unused */
               nbytes = read(fildes[0], buf, BSIZE);   /* Get data from pipe */
               /* At this point, a further read would see end-of-file ... */
               close(fildes[0]);                       /* Finished with pipe */
               exit(EXIT_SUCCESS);

           default:  /* Parent - writes to pipe */
               close(fildes[0]);                       /* Read end is unused */
               write(fildes[1], "Hello world\n", 12);  /* Write data on pipe */
               close(fildes[1]);                       /* Child will see EOF */
               exit(EXIT_SUCCESS);
           }

APPLICATION USAGE
       None.

RATIONALE
       The wording carefully avoids using the verb ``to open'' in order to
       avoid any implication of use of open(); see also write().

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.14, File Descriptor Allocation, fcntl(), read(), write()

       The Base Definitions volume of POSIX.1-2017, <fcntl.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                              PIPE(3)
